Effect of exhaust gas recirculation (EGR) on diesel engine carbonaceous PM emissions

Effect of exhaust gas recirculation (EGR) on diesel engine carbonaceous PM emissions | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Effect of exhaust gas recirculation (EGR) on diesel engine carbonaceous PM emissions Xinling Li, Pengcheng Zhao This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5197899/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 19 May, 2025 Read the published version in Environmental Science and Pollution Research → Version 1 posted 5 You are reading this latest preprint version Abstract As one of the most effective ways of reducing nitrogen oxides (NOx) emission, exhaust gas recirculation (EGR) has been widely used in diesel engines. However, EGR generally shows adversely effective in particulate matter (PM) emissions. The chemical composition of the PM with the application of EGR is not well identified because few previous publications focus on this topic, especially for high EGR rate cases. In this paper, emission characteristics of organic carbon (OC, OC 1 -OC 4 ), elemental carbon (EC, EC 1 -EC 2 ), particulate semi-volatile organic compounds (SVOCs) including 18 n -alkanes and 20 polycyclic aromatic hydrocarbons (PAHs) for a common-rail diesel engine at mild and high EGR rate conditions (up to maximum achievable level while maintaining stable combustion) were analyzed at four steady-state conditions comprehensively. It can be clearly observed that EGR rate instead of load and speed significantly affects the EC emission under the experimental conditions. EC emission increase with increasing EGR rate, which is divided to two sections, i.e., slight increase from 0 to 30% (mild EGR rate) and sharp increase from 30–45% (high EGR rate). TC is dominated by OC 1 , OC 2 and EC 1 at low EGR rate, and the fraction of EC 1 evidently increases with increasing EGR rate. It is observed that TC is heavily dominated by EC 2 at highest EGR rate ranges, which corresponds to the lower heat release rates (lower HRR max at higher EGR rate) and lower air-fuel ratio at these conditions. All the target PAHs increase with increasing EGR rate at the four operation modes. The adverse effect of EGR on PAH emission is less significant than EC emission. Moreover, the effect of EGR rate on the PAH ring distribution is not significant. Both of total ∑C 16 -C 25 and ∑C 26 -C 33 emission rates evidently increase at high EGR rate condition in comparison with those at baseline and mild EGR condition cases, which indicates that both fuel-derived and oil-derived n-alkanes exhibit higher emission rate at high EGR conditions compared with those at baseline and mild EGR condition. The application of EGR helped with other controlling strategies (e.g., fuel injection, after-treatment device) is suggested to suppress the carbonaceous PM formation for the modern common-rail diesel engine. Diesel engine common-rail Mild and high EGR rates Carbonaceous PM emissions Particulate SVOCs Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1. Introduction Diesel engines have been widely used because of their benefits of perfect economical and dynamic property. However, the application of plenty of diesel engines in traffic and industry fields heavily worsens environment because of their particulate matter (PM) and nitrogen oxide (NOx) emissions. The reduction of PM and NOx is an urgent need to meet the more stringent emission regulation requirements (Desantes et al., 2000 ). As one of the most effective ways in reducing NOx emission, exhaust gas recirculation (EGR) has been widely used in diesel engine at various engine operating conditions, especially at lower temperature conditions (Sakaida et al., 2019 ). However, EGR generally shows adversely effective in PM emissions due to the decease of temperature and oxygen level during combustion (Fayad et al., 2023a ; Fayad et al., 2023b ). It is noted that the opposite influences of EGR on the PM emissions in low temperature combustion mode (using heavy EGR) and conventional diesel combustion mode (Akihama et al., 2001 ; Ogawa et al., 2006 ). Because the employment of EGR normally increases PM emissions, the diesel particle filter (DPF) is used to control PM emissions. In general, periodical regeneration of DPF by soot oxidation must be conducted to reduce backpressure to maintain trapping efficiency. The physical and chemical properties of PM significantly change due to EGR effect, which affects the PM oxidative reactivity (Al-Qurashi et al., 2011 ; Li et al., 2014 ), and engine after-treatment devices efficiency. Additionally, the application of EGR at lower temperature will bring potential EGR valve deposit problem in the EGR system. Therefore, understanding the chemical composition of the PM with the application of EGR is relevant to get the highest PM reduction effect and protect EGR system. The chemical compositions in the filter for collecting diesel exhaust PM could be classified into four species, i.e., soluble organic fraction (SOF), solid carbon fraction, insoluble fraction (ISF), and sulfate by a Soxhlet extraction method. Additionally, metal species in the PM could also be available through elemental analysis. Several studies indicated that the contribution of lubricant oil with the application of EGR during combustion process presented a considerable influence on the chemical compositions of exhaust PM (Kreso et al., 1998 ; Seong and Boehman, 2011 ; Jain et al., 2017 ). Kreso et al. ( 1998 ) reported that solid carbon portion of the exhaust PM increased and the SOF portion generally decreased with increasing EGR basing on the data collected at the baseline, 10% and 16% EGR rates at two United States Environmental Protection Agency (EPA) steady-state operating conditions. The EGR also increased the sulfate emission. Trace metals (i.e., Al, Cu, Fe and Zn) in PM samples collected at EGR rates up to 30% from the diesel engines were investigated by Jain et al. ( 2017 ). They reported that increasing EGR significantly reduced the particulate trace metal concentration because lower in-cylinder temperature suppressed lubricant oil pyrolysis. Similar result has been observed by Seong and Boehman ( 2011 ), and they investigated the impact of intake oxygen enrichment (from 21 to 27% O 2 in intake) on diesel engine exhaust PM properties. Diesel combustion characteristics under ultra-high EGR condition were studied by Ogawa et al. ( 2006 ). In their works, the specific mass emissions of ISF and SOF in PM at 0, 30% and 62% EGR rates were compared. They found that ISF increased significantly under low EGR condition, but it disappeared up to 62% EGR. Akihama et al. ( 2001 ) have demonstrated that the ratio of SOF to dry soot was higher in the smokeless combustion mode (by using ultra-high EGR) than that in the conventional combustion mode, which was attributed to the air-fuel ration and combustion temperature were adequately lowered in the smokeless combustion mode. Carbonaceous substances, i.e., elemental carbon (EC) and organic carbon (OC) mainly constitute the diesel exhaust PM (Lu et al., 2012 ; Yang et al., 2017 ). EC generates from fuel droplet pyrolysis in the fuel rich zone under high pressure and temperature, which includes char-EC and soot-EC based on ignition temperature. Char-EC originated from the fuel pyrolysis under relatively mild combustion condition. Unlike char-EC, soot-EC is formed by gas-to-particle conversion under extreme combustion condition. In contrast, most of OC species consist of semi-volatile organic compounds (SVOCs), such as n -alkanes, polycyclic aromatic hydrocarbons (PAHs), hopanes and steranes, which originate from unburned and incomplete burned fuel and lubricant oil. It is known that SVOCs are regarded as main contributors to secondary organic aerosol (SOA) in urban atmosphere (Gentner et al., 2012 ). Therefore, characterization of diesel exhaust SVOCs is important to understand the formation (Souza and Corrêa) and value the effects on atmosphere pollution and human health. Moreover, diesel engine exhaust PAHs exhibit carcinogenic and mutagenic (WHO. IARC, 2012 ). It is recognized that PAH components emitted by diesel engines mainly come from three different sources: fuel containing unburned PAH, unburned lubricant oil transported from the crankcase caused by crankcase oil jumping, and the pyrolysis synthesis of 2–3 ring PAHs also contributes to emission of 5-ring and larger PAHs (Williams et al., 1989 ; Yilmaz et al., 2022 ; Yilmaz et al., 2023a ). Despite less toxic than PAH, paraffins, e.g., alkanes, hopanes and steranes, containing 10–35 carbon atoms are identified as principal organic particulate constituents in urban environment (Cao et al., 2021 ). Carbonaceous substances including OC, EC, PAH, and non-PAH have been widely proved to depend strongly on fuel formulation (Arias et al., 2022 ; Yilmaz & Davis, 2022 ; Yilmaz et al., 2023b ; Yilmaz et al., 2023c ; Sahu et al., 2024 ) and engine configuration (Perrone et al., 2014 ; Lim et al., 2015 ; Yilmaz & Donaldson, 2022 ; Zeraati-Rezaei et al., 2020 ; McCaffery et al., 2022 ). There are only few studies on the effect of EGR on carbonaceous substances from diesel engines (Wang et al., 2023 ; An et al., 2016 ). The mass fraction of volatile organic decreased from 31.7–5.6% as EGR ratio increased from 0 to 20% with thermogravimetric analysis for the diesel PM samples from dual-fuel (gasoline and diesel) combustion experiment (Wang et al., 2023 ). PAH emissions from GDI gasoline engine with EGR were analyzed by An et al. ( 2016 ). They observed that concentrations of PAHs in gas and particulate phases were only reduced at an EGR rate of 2.5%, while almost no change of PAH was observed with EGR increasing from 2.5–10%. To reveal the effect mechanism of EGR on the PAH formation, PAH formation in premixed flame with the CO 2 addition was investigated by Liu et al. ( 2018 ) by using a laser-induced fluorescence (LIF) technology. They revealed that the chemical inhibition effect with CO 2 addition suppresses PAHs formation. Recently, Kinoshita et al. ( 2023 ) indicated that the formation of hard deposits in the EGR system was much dependent on the exhaust PAH concentrations. Therefore, the characterization of carbonaceous substances of exhaust PM with the application EGR is also critical to ensure the reliable operation of the diesel engine. Few reports on carbonaceous substance emission characteristics of the engine with the introduction of EGR described above have not quantitatively analyzed OC/EC and the profiles of SVOCs (i.e., n -alkanes and PAHs) in a large EGR ratio range. The characterization of the SVOC profiles of exhaust PM in detail will aid to the profound understanding of the exhaust PM formation and environmental impact with the application of EGR. Additionally, the elucidation of carbonaceous PM emission characteristics is beneficial to protect the EGR valve and ensure the reliable operation of the diesel engine. Therefore, quantitative analysis of OC/EC, as well as characterization of the SVOC detailed profiles of a diesel engine operated in a large EGR rate range have been performed in this study. To the authors’ knowledge, it is the first time to elucidate the chemical composition of the PM with the application of EGR along with the in-cylinder combustion parameters and regulation gaseous emissions from a common-rail diesel engine at mild and high EGR rate conditions. The result will fill the knowledge gap of variations of chemical composition of the PM depending on combustion parameters with the application of in-cylinder controlling technique. 2. Experiments and methods 2.1 Apparatus and operation conditions The experimental system consists of diesel engine test bed, dilution and sampling apparatus, on-line and off-line analysis equipment (Fig. S1 ). A China V 2.8 L 4-cylinder high-speed direct injection light-duty diesel engine was used to perform EGR experiments. The test engine is fitted with a high-pressure common-rail injection system, which can provide a maximum injection pressure of 180 MPa. Detail specifications of diesel engine and experimental apparatus are given in Table S1 . An alternating current (AC) dynamometer was coupled with the engine to measure and adjust the engine operation parameters. With the application of coolant and lubricant oil conditioning system, the coolant and lubricant oil were kept at 85 ± 1°C and 90 ± 1°C, respectively. The apparent heat release rate (HRR) was calculated basing on 200-cycle averaged cylinder pressure data (per 0.1° crank angle). Commercially available diesel, which meet the Chinese Phase V Emission Regulations (fuel sulfur < 10 ppm), was used in the study. An open access electronic control unit (ECU) was used to precisely control fuel injection and EGR parameters. A control valve was installed in the EGR loop to control the flow rate of exhaust gas into the intake manifold. EGR was changed from 0 to maximum achievable level while maintaining stable combustion. The engine was operated at 1600 and 2200 rpm, and at 25% and 50% loads under each engine speed. The EGR wide sweeps from 0 to 45% at 5% or 10% for 5–6 intervals at the four steady-state conditions. The injection pressure and injection timing were remained at 120 MPa and − 6°CA after top dead center (-6°CA ATDC), respectively. The engine operation parameters are listed in Table S2. Parameters of engine combustion and regulated emissions are detailed in Table S3. The tests in this study were repeated three times and the experimental results were average values of three tests. 2.2 PM dilution and collection PM dilution and collection are detailed in our previous study (Li et al., 2024 ). Briefly, a full-flow dilution tunnel (Horiba CVS ONE-MV-HE) and a sampling system (Horiba DLS-ONE) were used for PM dilution and collection. PM was collected using a constant flow sampler as a fraction of exhaust gas pass through the dilution tunnel (dilution ratio, approximate 1:8). PM was collected by Quart fiber filters (47mm, QFFs, Whatman) with sampling time from 5 to 20 min depending on the operating conditions. The collected samples were stored in a freezer at -20°C to be analyzed within three days. 2.3 Chemical analysis PM samples were analyzed by a TD-GC/MS system according to the method described previously (Zhao et al., 2022 ; Li et al., 2024 ). Briefly, the system is consisted of a thermal desorber (TD, Markers, UK), a gas chromatograph mass spectrometer (Agilent GC 7890A MSD 5975C, USA). The QFFs loaded PM samples were spiked with 50 ng internal standard and cut into smaller portions to facilitate their insertion into the glass TD tube. The sampling tube was heated for 10 min at 320°C through thermal desorption with a flow rate of 50 mL/min, and the desorbed analytes were put in a cold trap (-10°C). The cold trap was heated to 300°C with a temperature increase rate of 100°C/min. The desorbed analytes were coupled with a DB-5MS capillary column. MS was operated in electron impact (EI, 70 eV) ionization mode and the selected ion monitoring (SIM) mode was selected to determine the target compounds. The SVOCs including 20 PAHs (from naphthalene to coronene), 18 n -alkanes(C 16 -C 33 ) in PM samples were identified and quantified by both retention time and characteristic ions of target species with external standard calibration (Accustandard, USA). The retention time and characteristic ions of target PAHs are shown in Table S4. The details identification and quantification processes of SVOCs, OC (OC 1 -OC 4 ), EC (EC 1 -EC 4 ) are identical to the descriptions in the previous literature Li et al. ( 2024 ). The average recoveries are 86–95% for PAHs and n -alkanes. Experimental results obtained from the various test conditions were compared by using the two-tailed Student’s t test to verify if they were significantly different from each other at 95% significance level. 3. Result and Discussion 3.1 OC and EC emissions Figure 1 shows the effect of EGR rate on the specific emissions and mass fractions of OC, EC at four different operation modes. The specific emissions of EC for the baseline at 1600 rpm/25%, 1600 rpm/50%, 2200 rpm/25% and 2200 rpm/50% are 16.2, 11.2, 29.8 and 34.1 mg/kWh, respectively, while the corresponding emissions of EC for the highest EGR are 728, 760, 1324 and 162 mg/kWh, respectively. It can be clearly observed that EGR rate instead of load and speed significantly affects the EC emission under the experimental conditions in this study. Both at 1600 rpm/25% and 2200 rpm/25%, EC emission increases with increasing EGR rate, which is divided into two sections. When the EGR rate ranges from 0 to 30% (mild EGR rate), the increase of EC emission is only 0.5 and 1.4 times at 1600 rpm/25% and 2200 rpm/25%, respectively. The corresponding combustion parameters and gaseous emission listed in Table S3 indicated that the combustion is dominated by the premixed control at mild EGR level at low engine load. Charge dilution slightly decreases the peak in-cylinder pressure due to the slightly higher heat capacity of the diluent gases as well as slightly slower reaction rates during the premixed combustion. For example, the variations of maximum in-cylinder pressure ( P max ), maximum heat release rate ( HRR max ), combustion duration (CD) and brake specific fuel consumption (BSFC) were in a limited range, i.e., 8.3–7.3 MPa, 213–230 kJ/m 3 deg, 10–17℃A and 254–258 g/kWh when EGR increases from 0–30% at 1600 rpm/25%. While EC emission increases rapidly as the EGR rate increases from 30–45% (high EGR rate), the corresponding increase in EC emission is 29.4 and 17.3 times at 1600 rpm/25% and 2200 rpm/25%, respectively. At 1600 rpm/50%, EC emission also exhibits two-section characteristics with increasing EGR rate, only 2.0 times increase in EC emission in front section (0 to 16%), while 21.3 times increase in EC emission in rear section (16–30%). Although two-section characteristics with increasing EGR rate at 2200 rpm/50% are less evident than those at the others, higher EC emission is also observed when the EGR rate exceeds 15%. The increase in EC with increasing EGR rate is identical to the extensive observations of dry soot (Akihama et al., 2001 ), Bosch smoke (Ogawa et al., 2006 ), solid carbon (Ogawa et al., 2006 ; Kreso et al., 1998 ), and EC emission (Li et al., 2014 ) with increasing EGR rate up to approximately 30%. The EC emission is the result of combination of soot generation and oxidation. Soot mainly generates in local high temperature and over-rich fuel zones, while high temperature and high oxygen concentration promote the soot oxidation. Because of the introduction of EGR reduces the in-cylinder temperature and oxygen concentration, the soot oxidation is inhibited. The results of lower heat release rates (lower HRR max at higher EGR rate) and lower air-fuel ratio (as seen in Table 3) in this study also support that explanation. It is observed that the promotion of the diffusion combustion duration evidently increases when EGR rate increases from 30–45%. For example, the combustion duration is 17, 27 and 32℃A for 30%, 40% and 45% EGR at 1600 rpm/25%, respectively. Because of the deterioration of the engine performance at high EGR level, it was necessary to inject more fuel to compensate the reduced fuel thermal efficiency. For example, the BSFC, for 30%, 40% and 45% EGR rate at 1600 rpm/25%, is 254, 262 and 269 g/kWh, respectively. Due to the reduction in oxygen concentration and lowering in-cylinder temperature, oxidation of the soot is suppressed. Consequently, soot emission increases when EGR is applied, the growth rate of soot is slowly when the EGR rate is less than 30%, while it is enhanced rapidly with the increase of EGR afterward. Similarly, carbon monoxide (CO) emission also a sharply grow with the decease of intake oxygen content (Table S3), when the EGR rate up to high level (i.e., EGR rate 45%), the low oxygen level in the cylinder deteriorates combustion process and leads to incomplete combustion and a large amount of CO emission. The specific emissions of OC for the baseline at 1600 rpm/25%, 1600 rpm/50%, 2200 rpm/25% and 2200 rpm/50% are 94.1, 63.1, 123 and 198 mg/kWh, respectively, while the corresponding emissions of OC for the highest EGR are 219, 167, 336 and 221 mg/kWh, respectively. Like EC, OC emission also exhibit two section characteristics with increasing EGR rate at different operating conditions except for 2200 rpm/50%. At 1600 rpm/25% and 2200 rpm/25%, OC emission hardly changes with increasing EGR rate from 0–30% (in the front section), while an evident increase in OC emission was observed with increasing EGR rate from 30–45% (in the rear section). At 1600 rpm/50%, the effect of increasing EGR is more obvious in the rear section (from 16–30%) than in the front section (from 0 to 16%). Carbonaceous PM emissions were dominated by OC in the low EGR range, i.e., approximately 70–85%, but it decreases to approximately 20% (45% EGR rate at 1600 rpm/25% and 2200 rpm/25%; 30% EGR rate at 1600 rpm/50%) and to 50% (30% EGR rate at 1600 rpm/25% and 2200 rpm/25%; 22% EGR rate at 1600 rpm/50%), respectively. OC mainly originates from incomplete combustion fuel and lubricant oil at the low temperature condition, and carbonaceous PM emissions are generally dominated by OC at low load. At low EGR rates, the in-cylinder oxygen level and combustion temperature are beneficial for OC emission. The oxygen level decreases with increasing EGR rate, leading to the increase of OC emission, as seen in Table S3. As the EGR rate exceeds 30%, the sharply decrease of in-cylinder oxygen level aggravates the combustion process, which leads to a large amount of OC emission resulting from the incomplete combustion. Additionally, the condensation of incomplete-combustion fuel and lubricant oil also contributes to OC emission, which is correlated with the condensation of HC on the PM during exhaust dilution and cooling. Therefore, the distinct OC emission of the change of EGR rate is the result of incomplete combustion fuel and lubricant oil and gas-particle phase partition (Guan et al., 2017 ). The mass fractions of OC 1 -OC 4 and EC 1 -EC 3 in total carbonaceous PM (TC, OC + EC) change significantly with the variations of EGR rate as shown in Fig. 1b. Similar general features and fractions of low volatility organic of OC 4 and high refractory EC 3 , are relatively lower compared with other carbon sub-fractions accounting for less than 5% in TC. TC is dominated by OC 1 , OC 2 and EC 1 at low EGR rate, and the fraction of EC 1 evidently increases with increasing EGR rate. It is observed that TC is heavily dominated by EC 2 at highest EGR rate ranges, and the fractions of EC 2 in TC are 61.8%, 53.4% and 61.5% for 45% EGR rate at 1600 rpm/25% and 2200 rpm/25%, and 30% EGR rate at 1600 rpm/50%, respectively. Previous studies indicate that EC 2 generally produced at high engine load instead of middle and low loads without EGR (Lu et al., 2012 ; Li et al., 2014 ; Yang et al., 2017 ). We explained that high EGR rate presents the relatively low in-cylinder temperature, and EC 2 oxidation is suppressed. Moreover, as the discussion by Dec (1997), soot precursors mainly form and grow with high temperature and fuel rich environment. The application of EGR significantly decreases contact probability of fuel and oxygen, which suppresses in-cylinder reaction reactivity and promotes PAH growth to form soot nuclei, which leads to a rapid enhancement of the soot with graphitic, rigid, and high aromatic structure (Zhang et al., 2022 ). Additionally, the diffusion combustion duration significantly increases with increasing EGR rate, which leads to the long residence time during the combustion and promotes longer graphene segments by coagulation and growth processes. As a result, EC 2 with graphitic, rigid and high aromatic structure is triggered at this high temperature and low oxygen availability condition. 3.2 Particulate PAH emissions Figure 2 shows the effect of EGR rate on particulate PAH emissions at four different operation modes. It is observed that PAHs with different emission levels present similar emission profiles. Pyrene (Pyr), phenanthrene (Phe) and fluoranthene (Flu) are the three predominant compounds. 20 target PAHs are classified by the number of aromatic rings and the contributions of different PAHs are provided in Fig. 3. Total PAH emissions are dominated by the 3–4 ring PAHs, which account for approximately 75–85% of total PAH emissions. While the lowest and highest ring number PAHs (2-ring and 5–7 rings) present the low abundance. The findings agree with studies of the diesel engine exhaust compounds despite of distinct fuel configurations (Yang et al., 2017 ) and engine features (Magara-Gomez et al., 2012 ; Zhang et al., 2019 ; Cui et al., 2021 ). However, the mass fraction of 5–6 rings in total PAHs is over 50% in gasoline engine exhaust (Rogge et al., 1993 ; Mi et al., 1996 ; Zheng et al., 2018 ) with various emission standard categories, and the fraction of several high molecular weight species, i.e., Benzo[a]pyrene (BaP) and Benzo[a]anthracene (DaA), was significantly higher than this study. The distinction of PAH profiles between diesel and gasoline engines is beneficial to clarify the source properties between diesel and gasoline emissions. Some researchers (Collier et al.,1995; Mi et al.,2000; Souza and Corrêa, 2016 ) suggested that 2–3 ring low molecular PAHs are evidently influenced by the PAHs level in the fuel, which is survival from incomplete combustion fuel and lubricant oil. Collier et al. ( 1995 ) analyzed the 3–4 ring PAH recoveries (percent of fuel PAH) for the diesel engine exhaust. They found that PAH recoveries were highest at low load and progressively declined with increase of load. Similarly, Mi et al. ( 2000 ) reported that high temperature at high speed and high load was beneficial to more PAH mass decomposition and resulted in a lower ratio of PAH in the fuel and PAH emissions. High molecular weight PAHs derived from thermal synthesis have been identified through the fuel pyrolysis experiment, and a large fraction of 5-ring PAH in the total PAH was observed during alkylbenzene pyrolysis critical temperature where ∑PAH mass concentration was highest basing on flow reactor experiment (Zhao et al., 2023 ). It is shown that the engine, especially with the application of EGR, generated higher 2–7 ring PAH species at four operation modes, which suggests that stable high molecular weight PAHs generally originated from lower molecular weight hydrocarbon compounds via pyrolysis and pyrosynthesis during combustion processes. In addition, the unburned fuel and lubricant oil might also be the source of these emissions Brandenberger et al. ( 2005 ). It is shown that almost all the target PAHs increase with increasing EGR rate at the four operation modes. In comparison with the baseline case (0 EGR rate), the increase of the maximum EGR case in total ∑ 20 PAH emission rate is 4.1, 7.5, 4.7, and 3.5 times at 1600 rpm/25%, 1600 rpm/50%, 2200 rpm/25% and 2200/50%, respectively, as shown in Fig. 3. In comparison with the baseline case, the increase in PAHs with different rings (from 2-ring to 6–7 rings) for the maximum EGR case for the four operation modes is 3.4–13.4, 3.4–6.9, 2.1–8.3, 3.3–12.5 and 4.7–6.7 times, respectively. While the corresponding increase in EC emission is up to 29.4 time as shown in Fig. 1. Obviously, the adverse effect of EGR on PAH emission is less significant than EC emission. Which probably results from the intensity transition from PAH to soot at high EGR rate condition. Whereas, previous studies indicate that EGR has only small impact on particulate PAH emissions from the diesel engine Kreso et al. ( 1998 ) and the gasoline engine (An et al., 2016 ). Kreso et al. ( 1998 ) examined the three PAH compounds (Flu, Pyr and B[a]A) in the exhaust from a diesel engine operated at the baseline, 10% and 16% EGR rates at two engine loads (25% and 75% loads). At 25% load, there was no change for the three PAH compounds concentration in the particle phase between 10% and 16% EGR rates, while higher PAH emissions occurred at the higher EGR rate at 75% load, but only for the most volatile compound, i.e., Flu and Pyr. An et al. ( 2016 ) measured PAH emissions from a GDI gasoline engine with a low EGR range (up to 10%). They reported that approximately 40% reduced in individual particulate PAH emissions as EGR rate increased from 0 to 2.5%, while almost no change of particulate-phase PAHs was observed with increasing EGR as the EGR rate exceeds 2.5%, which could be attributed to their distinct EGR range, the in-cylinder combustion parameters derived from the operation conditions for the engines with different configurations. Low EGR operating condition was performed by Kreso et al. ( 1998 ) and An et al. (2019), i.e., up to 16% and 10% EGR rate, respectively. In this study, a high EGR rate (up to 45%) is achieved and the oxygen concentration reaches its limit of ECU controlling or reaches the engine state of unstable combustion. As mentioned above, there are two major sources of PAHs in diesel engine exhaust PM. The first is pyrosynthesis process of fuel and lubricant oil during combustion producing a large number of PAHs with stable structure, and the second is survival of PAHs from the unburned fuel and lubricant oil. The impact of EGR on particulate PAHs is the result of the relative contribution of those two PAH emission pathways, namely pryrosynthesis and survival. According to Collier et al. ( 1995 ), fuel impinged on the piston bowl walls tends to survive at low in-cylinder combustion temperatures. Because of the higher heat capacity of the diluent gases and slower reaction rates during the premixed combustion, charge dilution with the EGR leads to low in-cylinder temperature and results in the increase of survival of unburned PAH compounds (Huestis et al., 2007 ). Yilmaz et al. (2005) also reported that wetstacking appeared to occur at low engine loads because of the existence of PAH production/unburned fuel at very lean combustion conditions and low exhaust temperatures leading to condensation of unburned fuel and PAHs. Liu et al. ( 2018 ) have detected that PAHs fluorescence signals in the premixed C 2 H 4 flame. They found that signals almost linearly increased with equivalence ratio. The result was also reproduced by KM2 and ABF mechanisms and it was attributed to a higher carbon content at high equivalence ratio condition. Therefore, the increase in PAH emissions with EGR could be explained as follows: simultaneously decrease of oxygen content and temperature leads to the weak oxidation of PAH, and the effect of weak oxidation of PAH dominates the increase in PAH emission with EGR. 3.3 Particulate n -alkane emissions Figure 4 shows the effects of EGR rate on the emissions of particulate n -alkanes at four different operation modes. The n -alkane distributions for C 16 -C 33 exhibit a clear unimodal pattern with the most abundance at C 20 -C 22 , except for 45% EGR rate at 2200 rpm/25%. Such a distribution is like the popular patterns of n -alkanes in diesel exhaust PM (Schauer et al., 1999 ; Fujitani et al., 2012 ). However, carbon number distribution of particulate n -alkane emissions emitted from the gasoline engine shows a bimodal pattern, i.e., peaking at C 20 -C 22 and C 24 -C 27 . The great contribution of low molecular n -alkanes (< C 22 ) in diesel engine PM reflects the relatively higher survivability of unburned fuel, while the particle n -alkanes emitted from the gasoline engines are speculated to originate from the unburned lubricant oil (Rogge et al., 1993 ). It is shown that an evident bimodal pattern was observed for 45% EGR rate at 2200 rpm/25%, which reflects the relatively higher survival of lubricant oil and unburned diesel fuel at low load, high speed and high EGR rate conditions, while total alkanes quantified in diesel exhaust PM (< 1µm) also exhibit a bimodal distribution and peak between C 19 –C 25 as observed by Lim et al. ( 2015 ). Therefore, engine configurations, the operation conditions and fuel components employed could influence the alkane emissions, which indicates that the categorization of diesel and gasoline engine emissions basing on alkane profile patterns should be employed conditionally and may not be generalized. In the mild EGR rate range, the influence of EGR rate on the n -alkane emissions is unsignificant, while extraordinarily high n -alkane emissions appear at maximum EGR conditions, i.e., 45% EGR rate at 1600 rpm/25%, 30% EGR rate at 1600 rpm/50%, 45% EGR rate at 2200 rpm/25%, and 20% EGR rate at 2200 rpm/50%. As shown in Fig. 4, in comparison with the baseline case (0 EGR rate), the increase of the maximum EGR case in total ∑ 18 n -alkanes emission rate is 3.0, 5.6, 3.9 and 2.0 times at 1600 rpm/25%, 1600 rpm/50%, 2200 rpm/25% and 2200/50%, respectively. At an extremely high EGR rate, the excess air coefficient decreases significantly and the high-temperature oil-rich areas increase, leading to a tremendous amount of carbon nuclei generation. It is postulated that volatile materials could be adsorbed on surface of PM (Guan et al., 2017 ). Therefore, the high emissions of n -alkanes along with high soot emission are the result of condensation of unburned fuel. Additionally, under high EGR rate, the temperature and the oxygen concentration are lower, which weakens the oxidation ability of diesel fuel, resulting in significant increase in n -alkane emissions. According to Brandenberger et al. ( 2005 ), diesel and lubricant oil contribution to the paraffins emissions are different due to their distinct molecular mass. Typical diesel fuels exhibit low boiling points (280–360℃) and low molecular hydrocarbon mixtures (C 10 to C 25 ), while final boiling point for lubricant oil is usually over 550°C and consists of high molecular hydrocarbon (up to C 45 ). Therefore, as shown in Fig. 5, the n -alkanes are classified into high boiling (C 16 -C 25 ) and low boiling components (C 26 -C 33 ) based on carbon number, corresponding to the fuel-derived and oil-derived n -alkanes, respectively. In comparison with the baseline case (0 EGR rate), the increase of the maximum EGR case in total ∑C 16 -C 25 emission rate is 2.0, 4.8, 2.0 and 1.1 time at 1600 rpm/25%, 1600 rpm/50%, 2200 rpm/25% and 2200/50%, respectively, while the corresponding increase of emission rate of total ∑C 26 -C 33 is 2.0, 3.1, 10.6 and 0.5 time, respectively. Which indicates that both of fuel-derived and oil-derived n -alkanes exhibit higher emission rate at high EGR conditions compared with those at baseline and mild EGR condition. Generally, the influence of EGR rate on the fuel and lubricating oil contribution to n -alkane emissions is unsignificant, i.e., the common fractions of the emission rate of ∑C 26 -C 33 in total ∑C 16 -C 33 are less than 15%, while evident high lubricant oil contribution (over 30%) is observed for 45% EGR rate at 2200 rpm/25%, which could be attributed to the enhancement of lubricant oil consumption or the suppress of the oxidation for the long chain n -alkane during combustion at high speed, low load and high EGR rate condition. 4. Conclusions In this study, emissions of OC, EC, PAHs and n -alkane in PM from a common-rail diesel engine at mild and high EGR rates were examined. The results are summarized as follows: EGR rate instead of load and speed significantly affects the EC emission under the experimental conditions. EC emission increase with increasing EGR rate, which is divided to two sections. When the EGR rate ranges from 0 to 30% (mild EGR rate), the increase in EC emission is slight, while the corresponding increase in EC emission is sharply as the EGR rate increases from 30–45% (high EGR rate). TC is dominated by OC1, OC2 and EC1 at low EGR rate, and the fraction of EC1 evidently increases with increasing EGR rate. It is observed that TC is heavily dominated by EC2 at highest EGR rate ranges. Total PAH emissions are dominated by the 3–4 ring PAHs, which account for approximately 75–85% in total PAH emissions. While the lowest and highest ring number PAHs (2-ring and 5–7 rings) present the low abundance. All the target PAHs increase with increasing EGR rate at the four operation modes. The adverse effect of EGR on PAH emission is less significant than EC emission. Which probably results from the intensity transition from PAH to soot at high EGR rate condition. Moreover, the effect of EGR rate on the PAH ring distribution is not significant.The n -alkane homologs for C 16 -C 33 exhibit a clear unimodal distribution pattern with the most abundance at C 20 -C 22 , except for 45% EGR rate at 2200 rpm/25%, while an evident bimodal pattern is observed for 45% EGR rate at 2200 rpm/25%.which reflects the relatively higher lubricant oil consumption or the suppress of the oxidation for the long chain n -alkane at low load, high speed and high EGR rate condition. Both of ∑C 16 -C 25 and ∑C 26 -C 33 significantly increase with increasing EGR rate, which indicates that both of fuel-derived and oil-derived n -alkanes exhibit higher emission rate at high EGR conditions compared with those at baseline and mild EGR condition. The high EC, 2–7 ring PAH and n -alkane emission under high EGR rate conditions indicated that the application of EGR helped with other controlling strategies (e.g., fuel injection, aftertreatment device) is demanded to suppress the carbonaceous PM formation for the modern common-rail diesel engine. Therefore, the carbonaceous PM exhaust characteristics with the application of EGR coupled with injection strategies, i.e., under low temperature combustion mode should be performed in the next work. Declarations Acknowledgments The authors would like to thank the National Natural Science Foundation of China (No. 51976119) for financial support of this study. Acknowledgments The authors would like to thank the National Natural Science Foundation of China (No. 51976119) for financial support of this study. Funding The authors would like to thank the National Natural Science Foundation of China (No. 51976119) for financial support of this study. Authors’ Contributions Xinling Li: Writing – original draft, Writing – review & editing, Methodology, Investigation. Pengcheng Zhao: Methodology, Data curation. Ethical Approval Not applicable. Consent to Participate Not applicable. Consent to Publish Not applicable. Competing Interests The authors have no relevant financial or nonfinancial interests to disclose. Data Availability Statement The data used to support the findings of this study are available from the corresponding author upon request. References An YZ, Teng SP, Pei YQ, Qin J, Li X, Zhao H (2016) An experimental study of polycyclic aromatic hydrocarbons and soot emissions from a GDI engine fueled with commercial gasoline. Fuel 164:160–171 Akihama K, Takatori Y, Inagaki K, Sasaki S, Dean AM (2001) Mechanism of the smokeless rich diesel combustion reducing temperature. SAE Tech. 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J Clean Prod 185:797–804 Supplementary Files SupportingInformations.docx Cite Share Download PDF Status: Published Journal Publication published 19 May, 2025 Read the published version in Environmental Science and Pollution Research → Version 1 posted Editorial decision: Accept 05 May, 2025 Reviewers agreed at journal 28 Apr, 2025 Reviewers invited by journal 28 Apr, 2025 Editor assigned by journal 14 Apr, 2025 First submitted to journal 10 Apr, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5197899","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":449252635,"identity":"dba0bf32-6b11-4e1f-8a95-deb5c5abc392","order_by":0,"name":"Xinling Li","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAtUlEQVRIiWNgGAWjYBACPgbmxgcMPGC2AXFa2BgYmw0YeAxI09ImAVVNrBaJxLbKHzJ/EhvYm7dJMNTcIUILz8G22zw8BokNPMfKJBiOPSNCC3tj220GkBaJHDMJxobDRGhhZmwr/AHSIv+GWC1AWxjADpPgIVYLz8FmaR4eY+M2nrRii4RjRGjhl0g++PFnj5xsP/vhjTc+1BChBQwYe0ARBAQJRGoAgh/EKx0Fo2AUjIIRCAD6CTCd27ZZwgAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0001-8538-2332","institution":"Shanghai Jiao Tong University","correspondingAuthor":true,"prefix":"","firstName":"Xinling","middleName":"","lastName":"Li","suffix":""},{"id":449252636,"identity":"9e0c7d37-0235-4b0d-9ebd-f80750652e78","order_by":1,"name":"Pengcheng Zhao","email":"","orcid":"","institution":"Shanghai Jiao Tong University","correspondingAuthor":false,"prefix":"","firstName":"Pengcheng","middleName":"","lastName":"Zhao","suffix":""}],"badges":[],"createdAt":"2024-10-03 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16:04:31","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1308148,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5197899/v1/559fc2d6-9ca3-4357-8588-1029ac418d3d.pdf"},{"id":81651335,"identity":"6c5464a3-b6d0-4c43-9839-f36956fc2689","added_by":"auto","created_at":"2025-04-29 16:17:47","extension":"docx","order_by":6,"title":"","display":"","copyAsset":false,"role":"supplement","size":467146,"visible":true,"origin":"","legend":"","description":"","filename":"SupportingInformations.docx","url":"https://assets-eu.researchsquare.com/files/rs-5197899/v1/5ee23983573efd2cc4cb8e2c.docx"}],"financialInterests":"","formattedTitle":"Effect of exhaust gas recirculation (EGR) on diesel engine carbonaceous PM emissions","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eDiesel engines have been widely used because of their benefits of perfect economical and dynamic property. However, the application of plenty of diesel engines in traffic and industry fields heavily worsens environment because of their particulate matter (PM) and nitrogen oxide (NOx) emissions. The reduction of PM and NOx is an urgent need to meet the more stringent emission regulation requirements (Desantes et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2000\u003c/span\u003e). As one of the most effective ways in reducing NOx emission, exhaust gas recirculation (EGR) has been widely used in diesel engine at various engine operating conditions, especially at lower temperature conditions (Sakaida et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). However, EGR generally shows adversely effective in PM emissions due to the decease of temperature and oxygen level during combustion (Fayad et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2023a\u003c/span\u003e; Fayad et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2023b\u003c/span\u003e). It is noted that the opposite influences of EGR on the PM emissions in low temperature combustion mode (using heavy EGR) and conventional diesel combustion mode (Akihama et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Ogawa et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). Because the employment of EGR normally increases PM emissions, the diesel particle filter (DPF) is used to control PM emissions. In general, periodical regeneration of DPF by soot oxidation must be conducted to reduce backpressure to maintain trapping efficiency. The physical and chemical properties of PM significantly change due to EGR effect, which affects the PM oxidative reactivity (Al-Qurashi et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Li et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2014\u003c/span\u003e), and engine after-treatment devices efficiency. Additionally, the application of EGR at lower temperature will bring potential EGR valve deposit problem in the EGR system. Therefore, understanding the chemical composition of the PM with the application of EGR is relevant to get the highest PM reduction effect and protect EGR system.\u003c/p\u003e \u003cp\u003eThe chemical compositions in the filter for collecting diesel exhaust PM could be classified into four species, i.e., soluble organic fraction (SOF), solid carbon fraction, insoluble fraction (ISF), and sulfate by a Soxhlet extraction method. Additionally, metal species in the PM could also be available through elemental analysis. Several studies indicated that the contribution of lubricant oil with the application of EGR during combustion process presented a considerable influence on the chemical compositions of exhaust PM (Kreso et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e1998\u003c/span\u003e; Seong and Boehman, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Jain et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Kreso et al. (\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e1998\u003c/span\u003e) reported that solid carbon portion of the exhaust PM increased and the SOF portion generally decreased with increasing EGR basing on the data collected at the baseline, 10% and 16% EGR rates at two United States Environmental Protection Agency (EPA) steady-state operating conditions. The EGR also increased the sulfate emission. Trace metals (i.e., Al, Cu, Fe and Zn) in PM samples collected at EGR rates up to 30% from the diesel engines were investigated by Jain et al. (\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). They reported that increasing EGR significantly reduced the particulate trace metal concentration because lower in-cylinder temperature suppressed lubricant oil pyrolysis. Similar result has been observed by Seong and Boehman (\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2011\u003c/span\u003e), and they investigated the impact of intake oxygen enrichment (from 21 to 27% O\u003csub\u003e2\u003c/sub\u003e in intake) on diesel engine exhaust PM properties. Diesel combustion characteristics under ultra-high EGR condition were studied by Ogawa et al. (\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). In their works, the specific mass emissions of ISF and SOF in PM at 0, 30% and 62% EGR rates were compared. They found that ISF increased significantly under low EGR condition, but it disappeared up to 62% EGR. Akihama et al. (\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2001\u003c/span\u003e) have demonstrated that the ratio of SOF to dry soot was higher in the smokeless combustion mode (by using ultra-high EGR) than that in the conventional combustion mode, which was attributed to the air-fuel ration and combustion temperature were adequately lowered in the smokeless combustion mode.\u003c/p\u003e \u003cp\u003eCarbonaceous substances, i.e., elemental carbon (EC) and organic carbon (OC) mainly constitute the diesel exhaust PM (Lu et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Yang et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). EC generates from fuel droplet pyrolysis in the fuel rich zone under high pressure and temperature, which includes char-EC and soot-EC based on ignition temperature. Char-EC originated from the fuel pyrolysis under relatively mild combustion condition. Unlike char-EC, soot-EC is formed by gas-to-particle conversion under extreme combustion condition. In contrast, most of OC species consist of semi-volatile organic compounds (SVOCs), such as \u003cem\u003en\u003c/em\u003e-alkanes, polycyclic aromatic hydrocarbons (PAHs), hopanes and steranes, which originate from unburned and incomplete burned fuel and lubricant oil. It is known that SVOCs are regarded as main contributors to secondary organic aerosol (SOA) in urban atmosphere (Gentner et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Therefore, characterization of diesel exhaust SVOCs is important to understand the formation (Souza and Corr\u0026ecirc;a) and value the effects on atmosphere pollution and human health. Moreover, diesel engine exhaust PAHs exhibit carcinogenic and mutagenic (WHO. IARC, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). It is recognized that PAH components emitted by diesel engines mainly come from three different sources: fuel containing unburned PAH, unburned lubricant oil transported from the crankcase caused by crankcase oil jumping, and the pyrolysis synthesis of 2\u0026ndash;3 ring PAHs also contributes to emission of 5-ring and larger PAHs (Williams et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e1989\u003c/span\u003e; Yilmaz et al., \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Yilmaz et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2023a\u003c/span\u003e). Despite less toxic than PAH, paraffins, e.g., alkanes, hopanes and steranes, containing 10\u0026ndash;35 carbon atoms are identified as principal organic particulate constituents in urban environment (Cao et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Carbonaceous substances including OC, EC, PAH, and non-PAH have been widely proved to depend strongly on fuel formulation (Arias et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Yilmaz \u0026amp; Davis, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Yilmaz et al., \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2023b\u003c/span\u003e; Yilmaz et al., \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2023c\u003c/span\u003e; Sahu et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) and engine configuration (Perrone et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Lim et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Yilmaz \u0026amp; Donaldson, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Zeraati-Rezaei et al., \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; McCaffery et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). There are only few studies on the effect of EGR on carbonaceous substances from diesel engines (Wang et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; An et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). The mass fraction of volatile organic decreased from 31.7\u0026ndash;5.6% as EGR ratio increased from 0 to 20% with thermogravimetric analysis for the diesel PM samples from dual-fuel (gasoline and diesel) combustion experiment (Wang et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). PAH emissions from GDI gasoline engine with EGR were analyzed by An et al. (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). They observed that concentrations of PAHs in gas and particulate phases were only reduced at an EGR rate of 2.5%, while almost no change of PAH was observed with EGR increasing from 2.5\u0026ndash;10%. To reveal the effect mechanism of EGR on the PAH formation, PAH formation in premixed flame with the CO\u003csub\u003e2\u003c/sub\u003e addition was investigated by Liu et al. (\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) by using a laser-induced fluorescence (LIF) technology. They revealed that the chemical inhibition effect with CO\u003csub\u003e2\u003c/sub\u003e addition suppresses PAHs formation. Recently, Kinoshita et al. (\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) indicated that the formation of hard deposits in the EGR system was much dependent on the exhaust PAH concentrations. Therefore, the characterization of carbonaceous substances of exhaust PM with the application EGR is also critical to ensure the reliable operation of the diesel engine.\u003c/p\u003e \u003cp\u003eFew reports on carbonaceous substance emission characteristics of the engine with the introduction of EGR described above have not quantitatively analyzed OC/EC and the profiles of SVOCs (i.e., \u003cem\u003en\u003c/em\u003e-alkanes and PAHs) in a large EGR ratio range. The characterization of the SVOC profiles of exhaust PM in detail will aid to the profound understanding of the exhaust PM formation and environmental impact with the application of EGR. Additionally, the elucidation of carbonaceous PM emission characteristics is beneficial to protect the EGR valve and ensure the reliable operation of the diesel engine. Therefore, quantitative analysis of OC/EC, as well as characterization of the SVOC detailed profiles of a diesel engine operated in a large EGR rate range have been performed in this study. To the authors\u0026rsquo; knowledge, it is the first time to elucidate the chemical composition of the PM with the application of EGR along with the in-cylinder combustion parameters and regulation gaseous emissions from a common-rail diesel engine at mild and high EGR rate conditions. The result will fill the knowledge gap of variations of chemical composition of the PM depending on combustion parameters with the application of in-cylinder controlling technique.\u003c/p\u003e"},{"header":"2. Experiments and methods","content":"\u003cp\u003e2.1 Apparatus and operation conditions\u003c/p\u003e \u003cp\u003eThe experimental system consists of diesel engine test bed, dilution and sampling apparatus, on-line and off-line analysis equipment (Fig.\u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). A China V 2.8 L 4-cylinder high-speed direct injection light-duty diesel engine was used to perform EGR experiments. The test engine is fitted with a high-pressure common-rail injection system, which can provide a maximum injection pressure of 180 MPa. Detail specifications of diesel engine and experimental apparatus are given in Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e. An alternating current (AC) dynamometer was coupled with the engine to measure and adjust the engine operation parameters. With the application of coolant and lubricant oil conditioning system, the coolant and lubricant oil were kept at 85\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u0026deg;C and 90\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u0026deg;C, respectively. The apparent heat release rate (HRR) was calculated basing on 200-cycle averaged cylinder pressure data (per 0.1\u0026deg; crank angle). Commercially available diesel, which meet the Chinese Phase V Emission Regulations (fuel sulfur\u0026thinsp;\u0026lt;\u0026thinsp;10 ppm), was used in the study.\u003c/p\u003e \u003cp\u003eAn open access electronic control unit (ECU) was used to precisely control fuel injection and EGR parameters. A control valve was installed in the EGR loop to control the flow rate of exhaust gas into the intake manifold. EGR was changed from 0 to maximum achievable level while maintaining stable combustion. The engine was operated at 1600 and 2200 rpm, and at 25% and 50% loads under each engine speed. The EGR wide sweeps from 0 to 45% at 5% or 10% for 5\u0026ndash;6 intervals at the four steady-state conditions. The injection pressure and injection timing were remained at 120 MPa and \u0026minus;\u0026thinsp;6\u0026deg;CA after top dead center (-6\u0026deg;CA ATDC), respectively. The engine operation parameters are listed in Table S2. Parameters of engine combustion and regulated emissions are detailed in Table S3. The tests in this study were repeated three times and the experimental results were average values of three tests.\u003c/p\u003e \u003cp\u003e2.2 PM dilution and collection\u003c/p\u003e \u003cp\u003ePM dilution and collection are detailed in our previous study (Li et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Briefly, a full-flow dilution tunnel (Horiba CVS ONE-MV-HE) and a sampling system (Horiba DLS-ONE) were used for PM dilution and collection. PM was collected using a constant flow sampler as a fraction of exhaust gas pass through the dilution tunnel (dilution ratio, approximate 1:8). PM was collected by Quart fiber filters (47mm, QFFs, Whatman) with sampling time from 5 to 20 min depending on the operating conditions. The collected samples were stored in a freezer at -20\u0026deg;C to be analyzed within three days.\u003c/p\u003e \u003cp\u003e2.3 Chemical analysis\u003c/p\u003e \u003cp\u003ePM samples were analyzed by a TD-GC/MS system according to the method described previously (Zhao et al., \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Li et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Briefly, the system is consisted of a thermal desorber (TD, Markers, UK), a gas chromatograph mass spectrometer (Agilent GC 7890A MSD 5975C, USA). The QFFs loaded PM samples were spiked with 50 ng internal standard and cut into smaller portions to facilitate their insertion into the glass TD tube. The sampling tube was heated for 10 min at 320\u0026deg;C through thermal desorption with a flow rate of 50 mL/min, and the desorbed analytes were put in a cold trap (-10\u0026deg;C). The cold trap was heated to 300\u0026deg;C with a temperature increase rate of 100\u0026deg;C/min. The desorbed analytes were coupled with a DB-5MS capillary column. MS was operated in electron impact (EI, 70 eV) ionization mode and the selected ion monitoring (SIM) mode was selected to determine the target compounds. The SVOCs including 20 PAHs (from naphthalene to coronene), 18 \u003cem\u003en\u003c/em\u003e-alkanes(C\u003csub\u003e16\u003c/sub\u003e-C\u003csub\u003e33\u003c/sub\u003e) in PM samples were identified and quantified by both retention time and characteristic ions of target species with external standard calibration (Accustandard, USA). The retention time and characteristic ions of target PAHs are shown in Table S4. The details identification and quantification processes of SVOCs, OC (OC\u003csub\u003e1\u003c/sub\u003e-OC\u003csub\u003e4\u003c/sub\u003e), EC (EC\u003csub\u003e1\u003c/sub\u003e-EC\u003csub\u003e4\u003c/sub\u003e) are identical to the descriptions in the previous literature Li et al. (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). The average recoveries are 86\u0026ndash;95% for PAHs and \u003cem\u003en\u003c/em\u003e-alkanes. Experimental results obtained from the various test conditions were compared by using the two-tailed Student\u0026rsquo;s t test to verify if they were significantly different from each other at 95% significance level.\u003c/p\u003e "},{"header":"3. Result and Discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003cp\u003e3.1 OC and EC emissions\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;1 shows the effect of EGR rate on the specific emissions and mass fractions of OC, EC at four different operation modes. The specific emissions of EC for the baseline at 1600 rpm/25%, 1600 rpm/50%, 2200 rpm/25% and 2200 rpm/50% are 16.2, 11.2, 29.8 and 34.1 mg/kWh, respectively, while the corresponding emissions of EC for the highest EGR are 728, 760, 1324 and 162 mg/kWh, respectively. It can be clearly observed that EGR rate instead of load and speed significantly affects the EC emission under the experimental conditions in this study. Both at 1600 rpm/25% and 2200 rpm/25%, EC emission increases with increasing EGR rate, which is divided into two sections. When the EGR rate ranges from 0 to 30% (mild EGR rate), the increase of EC emission is only 0.5 and 1.4 times at 1600 rpm/25% and 2200 rpm/25%, respectively. The corresponding combustion parameters and gaseous emission listed in Table S3 indicated that the combustion is dominated by the premixed control at mild EGR level at low engine load. Charge dilution slightly decreases the peak in-cylinder pressure due to the slightly higher heat capacity of the diluent gases as well as slightly slower reaction rates during the premixed combustion. For example, the variations of maximum in-cylinder pressure (\u003cem\u003eP\u003c/em\u003e\u003csub\u003emax\u003c/sub\u003e), maximum heat release rate (\u003cem\u003eHRR\u003c/em\u003e\u003csub\u003emax\u003c/sub\u003e), combustion duration (CD) and brake specific fuel consumption (BSFC) were in a limited range, i.e., 8.3\u0026ndash;7.3 MPa, 213\u0026ndash;230 kJ/m\u003csup\u003e3\u003c/sup\u003e deg, 10\u0026ndash;17℃A and 254\u0026ndash;258 g/kWh when EGR increases from 0\u0026ndash;30% at 1600 rpm/25%. While EC emission increases rapidly as the EGR rate increases from 30\u0026ndash;45% (high EGR rate), the corresponding increase in EC emission is 29.4 and 17.3 times at 1600 rpm/25% and 2200 rpm/25%, respectively. At 1600 rpm/50%, EC emission also exhibits two-section characteristics with increasing EGR rate, only 2.0 times increase in EC emission in front section (0 to 16%), while 21.3 times increase in EC emission in rear section (16\u0026ndash;30%). Although two-section characteristics with increasing EGR rate at 2200 rpm/50% are less evident than those at the others, higher EC emission is also observed when the EGR rate exceeds 15%. The increase in EC with increasing EGR rate is identical to the extensive observations of dry soot (Akihama et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2001\u003c/span\u003e), Bosch smoke (Ogawa et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2006\u003c/span\u003e), solid carbon (Ogawa et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Kreso et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e1998\u003c/span\u003e), and EC emission (Li et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2014\u003c/span\u003e) with increasing EGR rate up to approximately 30%. The EC emission is the result of combination of soot generation and oxidation. Soot mainly generates in local high temperature and over-rich fuel zones, while high temperature and high oxygen concentration promote the soot oxidation. Because of the introduction of EGR reduces the in-cylinder temperature and oxygen concentration, the soot oxidation is inhibited. The results of lower heat release rates (lower \u003cem\u003eHRR\u003c/em\u003e\u003csub\u003emax\u003c/sub\u003e at higher EGR rate) and lower air-fuel ratio (as seen in Table\u0026nbsp;3) in this study also support that explanation. It is observed that the promotion of the diffusion combustion duration evidently increases when EGR rate increases from 30\u0026ndash;45%. For example, the combustion duration is 17, 27 and 32℃A for 30%, 40% and 45% EGR at 1600 rpm/25%, respectively. Because of the deterioration of the engine performance at high EGR level, it was necessary to inject more fuel to compensate the reduced fuel thermal efficiency. For example, the BSFC, for 30%, 40% and 45% EGR rate at 1600 rpm/25%, is 254, 262 and 269 g/kWh, respectively. Due to the reduction in oxygen concentration and lowering in-cylinder temperature, oxidation of the soot is suppressed. Consequently, soot emission increases when EGR is applied, the growth rate of soot is slowly when the EGR rate is less than 30%, while it is enhanced rapidly with the increase of EGR afterward. Similarly, carbon monoxide (CO) emission also a sharply grow with the decease of intake oxygen content (Table S3), when the EGR rate up to high level (i.e., EGR rate 45%), the low oxygen level in the cylinder deteriorates combustion process and leads to incomplete combustion and a large amount of CO emission.\u003c/p\u003e \u003cp\u003eThe specific emissions of OC for the baseline at 1600 rpm/25%, 1600 rpm/50%, 2200 rpm/25% and 2200 rpm/50% are 94.1, 63.1, 123 and 198 mg/kWh, respectively, while the corresponding emissions of OC for the highest EGR are 219, 167, 336 and 221 mg/kWh, respectively. Like EC, OC emission also exhibit two section characteristics with increasing EGR rate at different operating conditions except for 2200 rpm/50%. At 1600 rpm/25% and 2200 rpm/25%, OC emission hardly changes with increasing EGR rate from 0\u0026ndash;30% (in the front section), while an evident increase in OC emission was observed with increasing EGR rate from 30\u0026ndash;45% (in the rear section). At 1600 rpm/50%, the effect of increasing EGR is more obvious in the rear section (from 16\u0026ndash;30%) than in the front section (from 0 to 16%). Carbonaceous PM emissions were dominated by OC in the low EGR range, i.e., approximately 70\u0026ndash;85%, but it decreases to approximately 20% (45% EGR rate at 1600 rpm/25% and 2200 rpm/25%; 30% EGR rate at 1600 rpm/50%) and to 50% (30% EGR rate at 1600 rpm/25% and 2200 rpm/25%; 22% EGR rate at 1600 rpm/50%), respectively. OC mainly originates from incomplete combustion fuel and lubricant oil at the low temperature condition, and carbonaceous PM emissions are generally dominated by OC at low load. At low EGR rates, the in-cylinder oxygen level and combustion temperature are beneficial for OC emission. The oxygen level decreases with increasing EGR rate, leading to the increase of OC emission, as seen in Table S3. As the EGR rate exceeds 30%, the sharply decrease of in-cylinder oxygen level aggravates the combustion process, which leads to a large amount of OC emission resulting from the incomplete combustion. Additionally, the condensation of incomplete-combustion fuel and lubricant oil also contributes to OC emission, which is correlated with the condensation of HC on the PM during exhaust dilution and cooling. Therefore, the distinct OC emission of the change of EGR rate is the result of incomplete combustion fuel and lubricant oil and gas-particle phase partition (Guan et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2017\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe mass fractions of OC\u003csub\u003e1\u003c/sub\u003e-OC\u003csub\u003e4\u003c/sub\u003e and EC\u003csub\u003e1\u003c/sub\u003e-EC\u003csub\u003e3\u003c/sub\u003e in total carbonaceous PM (TC, OC\u0026thinsp;+\u0026thinsp;EC) change significantly with the variations of EGR rate as shown in Fig.\u0026nbsp;1b. Similar general features and fractions of low volatility organic of OC\u003csub\u003e4\u003c/sub\u003e and high refractory EC\u003csub\u003e3\u003c/sub\u003e, are relatively lower compared with other carbon sub-fractions accounting for less than 5% in TC. TC is dominated by OC\u003csub\u003e1\u003c/sub\u003e, OC\u003csub\u003e2\u003c/sub\u003e and EC\u003csub\u003e1\u003c/sub\u003e at low EGR rate, and the fraction of EC\u003csub\u003e1\u003c/sub\u003e evidently increases with increasing EGR rate. It is observed that TC is heavily dominated by EC\u003csub\u003e2\u003c/sub\u003e at highest EGR rate ranges, and the fractions of EC\u003csub\u003e2\u003c/sub\u003e in TC are 61.8%, 53.4% and 61.5% for 45% EGR rate at 1600 rpm/25% and 2200 rpm/25%, and 30% EGR rate at 1600 rpm/50%, respectively. Previous studies indicate that EC\u003csub\u003e2\u003c/sub\u003e generally produced at high engine load instead of middle and low loads without EGR (Lu et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Li et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Yang et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). We explained that high EGR rate presents the relatively low in-cylinder temperature, and EC\u003csub\u003e2\u003c/sub\u003e oxidation is suppressed. Moreover, as the discussion by Dec (1997), soot precursors mainly form and grow with high temperature and fuel rich environment. The application of EGR significantly decreases contact probability of fuel and oxygen, which suppresses in-cylinder reaction reactivity and promotes PAH growth to form soot nuclei, which leads to a rapid enhancement of the soot with graphitic, rigid, and high aromatic structure (Zhang et al., \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Additionally, the diffusion combustion duration significantly increases with increasing EGR rate, which leads to the long residence time during the combustion and promotes longer graphene segments by coagulation and growth processes. As a result, EC\u003csub\u003e2\u003c/sub\u003e with graphitic, rigid and high aromatic structure is triggered at this high temperature and low oxygen availability condition.\u003c/p\u003e \u003cp\u003e3.2 Particulate PAH emissions\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;2 shows the effect of EGR rate on particulate PAH emissions at four different operation modes. It is observed that PAHs with different emission levels present similar emission profiles. Pyrene (Pyr), phenanthrene (Phe) and fluoranthene (Flu) are the three predominant compounds. 20 target PAHs are classified by the number of aromatic rings and the contributions of different PAHs are provided in Fig.\u0026nbsp;3. Total PAH emissions are dominated by the 3\u0026ndash;4 ring PAHs, which account for approximately 75\u0026ndash;85% of total PAH emissions. While the lowest and highest ring number PAHs (2-ring and 5\u0026ndash;7 rings) present the low abundance. The findings agree with studies of the diesel engine exhaust compounds despite of distinct fuel configurations (Yang et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2017\u003c/span\u003e) and engine features (Magara-Gomez et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Zhang et al., \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Cui et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). However, the mass fraction of 5\u0026ndash;6 rings in total PAHs is over 50% in gasoline engine exhaust (Rogge et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e1993\u003c/span\u003e; Mi et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e1996\u003c/span\u003e; Zheng et al., \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) with various emission standard categories, and the fraction of several high molecular weight species, i.e., Benzo[a]pyrene (BaP) and Benzo[a]anthracene (DaA), was significantly higher than this study. The distinction of PAH profiles between diesel and gasoline engines is beneficial to clarify the source properties between diesel and gasoline emissions. Some researchers (Collier et al.,1995; Mi et al.,2000; Souza and Corr\u0026ecirc;a, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) suggested that 2\u0026ndash;3 ring low molecular PAHs are evidently influenced by the PAHs level in the fuel, which is survival from incomplete combustion fuel and lubricant oil. Collier et al. (\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e1995\u003c/span\u003e) analyzed the 3\u0026ndash;4 ring PAH recoveries (percent of fuel PAH) for the diesel engine exhaust. They found that PAH recoveries were highest at low load and progressively declined with increase of load. Similarly, Mi et al. (\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2000\u003c/span\u003e) reported that high temperature at high speed and high load was beneficial to more PAH mass decomposition and resulted in a lower ratio of PAH in the fuel and PAH emissions. High molecular weight PAHs derived from thermal synthesis have been identified through the fuel pyrolysis experiment, and a large fraction of 5-ring PAH in the total PAH was observed during alkylbenzene pyrolysis critical temperature where \u0026sum;PAH mass concentration was highest basing on flow reactor experiment (Zhao et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). It is shown that the engine, especially with the application of EGR, generated higher 2\u0026ndash;7 ring PAH species at four operation modes, which suggests that stable high molecular weight PAHs generally originated from lower molecular weight hydrocarbon compounds via pyrolysis and pyrosynthesis during combustion processes. In addition, the unburned fuel and lubricant oil might also be the source of these emissions Brandenberger et al. (\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2005\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIt is shown that almost all the target PAHs increase with increasing EGR rate at the four operation modes. In comparison with the baseline case (0 EGR rate), the increase of the maximum EGR case in total \u0026sum;\u003csub\u003e20\u003c/sub\u003ePAH emission rate is 4.1, 7.5, 4.7, and 3.5 times at 1600 rpm/25%, 1600 rpm/50%, 2200 rpm/25% and 2200/50%, respectively, as shown in Fig.\u0026nbsp;3. In comparison with the baseline case, the increase in PAHs with different rings (from 2-ring to 6\u0026ndash;7 rings) for the maximum EGR case for the four operation modes is 3.4\u0026ndash;13.4, 3.4\u0026ndash;6.9, 2.1\u0026ndash;8.3, 3.3\u0026ndash;12.5 and 4.7\u0026ndash;6.7 times, respectively. While the corresponding increase in EC emission is up to 29.4 time as shown in Fig.\u0026nbsp;1. Obviously, the adverse effect of EGR on PAH emission is less significant than EC emission. Which probably results from the intensity transition from PAH to soot at high EGR rate condition. Whereas, previous studies indicate that EGR has only small impact on particulate PAH emissions from the diesel engine Kreso et al. (\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e1998\u003c/span\u003e) and the gasoline engine (An et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Kreso et al. (\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e1998\u003c/span\u003e) examined the three PAH compounds (Flu, Pyr and B[a]A) in the exhaust from a diesel engine operated at the baseline, 10% and 16% EGR rates at two engine loads (25% and 75% loads). At 25% load, there was no change for the three PAH compounds concentration in the particle phase between 10% and 16% EGR rates, while higher PAH emissions occurred at the higher EGR rate at 75% load, but only for the most volatile compound, i.e., Flu and Pyr. An et al. (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) measured PAH emissions from a GDI gasoline engine with a low EGR range (up to 10%). They reported that approximately 40% reduced in individual particulate PAH emissions as EGR rate increased from 0 to 2.5%, while almost no change of particulate-phase PAHs was observed with increasing EGR as the EGR rate exceeds 2.5%, which could be attributed to their distinct EGR range, the in-cylinder combustion parameters derived from the operation conditions for the engines with different configurations. Low EGR operating condition was performed by Kreso et al. (\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e1998\u003c/span\u003e) and An et al. (2019), i.e., up to 16% and 10% EGR rate, respectively. In this study, a high EGR rate (up to 45%) is achieved and the oxygen concentration reaches its limit of ECU controlling or reaches the engine state of unstable combustion. As mentioned above, there are two major sources of PAHs in diesel engine exhaust PM. The first is pyrosynthesis process of fuel and lubricant oil during combustion producing a large number of PAHs with stable structure, and the second is survival of PAHs from the unburned fuel and lubricant oil. The impact of EGR on particulate PAHs is the result of the relative contribution of those two PAH emission pathways, namely pryrosynthesis and survival. According to Collier et al. (\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e1995\u003c/span\u003e), fuel impinged on the piston bowl walls tends to survive at low in-cylinder combustion temperatures. Because of the higher heat capacity of the diluent gases and slower reaction rates during the premixed combustion, charge dilution with the EGR leads to low in-cylinder temperature and results in the increase of survival of unburned PAH compounds (Huestis et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). Yilmaz et al. (2005) also reported that wetstacking appeared to occur at low engine loads because of the existence of PAH production/unburned fuel at very lean combustion conditions and low exhaust temperatures leading to condensation of unburned fuel and PAHs. Liu et al. (\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) have detected that PAHs fluorescence signals in the premixed C\u003csub\u003e2\u003c/sub\u003eH\u003csub\u003e4\u003c/sub\u003e flame. They found that signals almost linearly increased with equivalence ratio. The result was also reproduced by KM2 and ABF mechanisms and it was attributed to a higher carbon content at high equivalence ratio condition. Therefore, the increase in PAH emissions with EGR could be explained as follows: simultaneously decrease of oxygen content and temperature leads to the weak oxidation of PAH, and the effect of weak oxidation of PAH dominates the increase in PAH emission with EGR.\u003c/p\u003e \u003cp\u003e3.3 Particulate \u003cem\u003en\u003c/em\u003e-alkane emissions\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;4 shows the effects of EGR rate on the emissions of particulate \u003cem\u003en\u003c/em\u003e-alkanes at four different operation modes. The \u003cem\u003en\u003c/em\u003e-alkane distributions for C\u003csub\u003e16\u003c/sub\u003e-C\u003csub\u003e33\u003c/sub\u003e exhibit a clear unimodal pattern with the most abundance at C\u003csub\u003e20\u003c/sub\u003e-C\u003csub\u003e22\u003c/sub\u003e, except for 45% EGR rate at 2200 rpm/25%. Such a distribution is like the popular patterns of \u003cem\u003en\u003c/em\u003e-alkanes in diesel exhaust PM (Schauer et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Fujitani et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). However, carbon number distribution of particulate \u003cem\u003en\u003c/em\u003e-alkane emissions emitted from the gasoline engine shows a bimodal pattern, i.e., peaking at C\u003csub\u003e20\u003c/sub\u003e-C\u003csub\u003e22\u003c/sub\u003e and C\u003csub\u003e24\u003c/sub\u003e-C\u003csub\u003e27\u003c/sub\u003e. The great contribution of low molecular \u003cem\u003en\u003c/em\u003e-alkanes (\u0026lt;\u0026thinsp;C\u003csub\u003e22\u003c/sub\u003e) in diesel engine PM reflects the relatively higher survivability of unburned fuel, while the particle \u003cem\u003en\u003c/em\u003e-alkanes emitted from the gasoline engines are speculated to originate from the unburned lubricant oil (Rogge et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e1993\u003c/span\u003e). It is shown that an evident bimodal pattern was observed for 45% EGR rate at 2200 rpm/25%, which reflects the relatively higher survival of lubricant oil and unburned diesel fuel at low load, high speed and high EGR rate conditions, while total alkanes quantified in diesel exhaust PM (\u0026lt;\u0026thinsp;1\u0026micro;m) also exhibit a bimodal distribution and peak between C\u003csub\u003e19\u003c/sub\u003e\u0026ndash;C\u003csub\u003e25\u003c/sub\u003e as observed by Lim et al. (\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Therefore, engine configurations, the operation conditions and fuel components employed could influence the alkane emissions, which indicates that the categorization of diesel and gasoline engine emissions basing on alkane profile patterns should be employed conditionally and may not be generalized.\u003c/p\u003e \u003cp\u003eIn the mild EGR rate range, the influence of EGR rate on the \u003cem\u003en\u003c/em\u003e-alkane emissions is unsignificant, while extraordinarily high \u003cem\u003en\u003c/em\u003e-alkane emissions appear at maximum EGR conditions, i.e., 45% EGR rate at 1600 rpm/25%, 30% EGR rate at 1600 rpm/50%, 45% EGR rate at 2200 rpm/25%, and 20% EGR rate at 2200 rpm/50%. As shown in Fig.\u0026nbsp;4, in comparison with the baseline case (0 EGR rate), the increase of the maximum EGR case in total \u0026sum;\u003csub\u003e18\u003c/sub\u003e\u003cem\u003en\u003c/em\u003e-alkanes emission rate is 3.0, 5.6, 3.9 and 2.0 times at 1600 rpm/25%, 1600 rpm/50%, 2200 rpm/25% and 2200/50%, respectively. At an extremely high EGR rate, the excess air coefficient decreases significantly and the high-temperature oil-rich areas increase, leading to a tremendous amount of carbon nuclei generation. It is postulated that volatile materials could be adsorbed on surface of PM (Guan et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Therefore, the high emissions of \u003cem\u003en\u003c/em\u003e-alkanes along with high soot emission are the result of condensation of unburned fuel. Additionally, under high EGR rate, the temperature and the oxygen concentration are lower, which weakens the oxidation ability of diesel fuel, resulting in significant increase in \u003cem\u003en\u003c/em\u003e-alkane emissions.\u003c/p\u003e \u003cp\u003eAccording to Brandenberger et al. (\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2005\u003c/span\u003e), diesel and lubricant oil contribution to the paraffins emissions are different due to their distinct molecular mass. Typical diesel fuels exhibit low boiling points (280\u0026ndash;360℃) and low molecular hydrocarbon mixtures (C\u003csub\u003e10\u003c/sub\u003e to C\u003csub\u003e25\u003c/sub\u003e), while final boiling point for lubricant oil is usually over 550\u0026deg;C and consists of high molecular hydrocarbon (up to C\u003csub\u003e45\u003c/sub\u003e). Therefore, as shown in Fig.\u0026nbsp;5, the \u003cem\u003en\u003c/em\u003e-alkanes are classified into high boiling (C\u003csub\u003e16\u003c/sub\u003e-C\u003csub\u003e25\u003c/sub\u003e) and low boiling components (C\u003csub\u003e26\u003c/sub\u003e-C\u003csub\u003e33\u003c/sub\u003e) based on carbon number, corresponding to the fuel-derived and oil-derived \u003cem\u003en\u003c/em\u003e-alkanes, respectively. In comparison with the baseline case (0 EGR rate), the increase of the maximum EGR case in total \u0026sum;C\u003csub\u003e16\u003c/sub\u003e-C\u003csub\u003e25\u003c/sub\u003e emission rate is 2.0, 4.8, 2.0 and 1.1 time at 1600 rpm/25%, 1600 rpm/50%, 2200 rpm/25% and 2200/50%, respectively, while the corresponding increase of emission rate of total \u0026sum;C\u003csub\u003e26\u003c/sub\u003e-C\u003csub\u003e33\u003c/sub\u003e is 2.0, 3.1, 10.6 and 0.5 time, respectively. Which indicates that both of fuel-derived and oil-derived \u003cem\u003en\u003c/em\u003e-alkanes exhibit higher emission rate at high EGR conditions compared with those at baseline and mild EGR condition. Generally, the influence of EGR rate on the fuel and lubricating oil contribution to \u003cem\u003en\u003c/em\u003e-alkane emissions is unsignificant, i.e., the common fractions of the emission rate of \u0026sum;C\u003csub\u003e26\u003c/sub\u003e-C\u003csub\u003e33\u003c/sub\u003e in total \u0026sum;C\u003csub\u003e16\u003c/sub\u003e-C\u003csub\u003e33\u003c/sub\u003e are less than 15%, while evident high lubricant oil contribution (over 30%) is observed for 45% EGR rate at 2200 rpm/25%, which could be attributed to the enhancement of lubricant oil consumption or the suppress of the oxidation for the long chain \u003cem\u003en\u003c/em\u003e-alkane during combustion at high speed, low load and high EGR rate condition.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eIn this study, emissions of OC, EC, PAHs and \u003cem\u003en\u003c/em\u003e-alkane in PM from a common-rail diesel engine at mild and high EGR rates were examined. The results are summarized as follows:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eEGR rate instead of load and speed significantly affects the EC emission under the experimental conditions. EC emission increase with increasing EGR rate, which is divided to two sections. When the EGR rate ranges from 0 to 30% (mild EGR rate), the increase in EC emission is slight, while the corresponding increase in EC emission is sharply as the EGR rate increases from 30\u0026ndash;45% (high EGR rate). TC is dominated by OC1, OC2 and EC1 at low EGR rate, and the fraction of EC1 evidently increases with increasing EGR rate. It is observed that TC is heavily dominated by EC2 at highest EGR rate ranges. Total PAH emissions are dominated by the 3\u0026ndash;4 ring PAHs, which account for approximately 75\u0026ndash;85% in total PAH emissions. While the lowest and highest ring number PAHs (2-ring and 5\u0026ndash;7 rings) present the low abundance. All the target PAHs increase with increasing EGR rate at the four operation modes. The adverse effect of EGR on PAH emission is less significant than EC emission. Which probably results from the intensity transition from PAH to soot at high EGR rate condition. Moreover, the effect of EGR rate on the PAH ring distribution is not significant.The \u003cem\u003en\u003c/em\u003e-alkane homologs for C\u003csub\u003e16\u003c/sub\u003e-C\u003csub\u003e33\u003c/sub\u003e exhibit a clear unimodal distribution pattern with the most abundance at C\u003csub\u003e20\u003c/sub\u003e-C\u003csub\u003e22\u003c/sub\u003e, except for 45% EGR rate at 2200 rpm/25%, while an evident bimodal pattern is observed for 45% EGR rate at 2200 rpm/25%.which reflects the relatively higher lubricant oil consumption or the suppress of the oxidation for the long chain \u003cem\u003en\u003c/em\u003e-alkane at low load, high speed and high EGR rate condition. Both of \u0026sum;C\u003csub\u003e16\u003c/sub\u003e-C\u003csub\u003e25\u003c/sub\u003e and \u0026sum;C\u003csub\u003e26\u003c/sub\u003e-C\u003csub\u003e33\u003c/sub\u003e significantly increase with increasing EGR rate, which indicates that both of fuel-derived and oil-derived \u003cem\u003en\u003c/em\u003e-alkanes exhibit higher emission rate at high EGR conditions compared with those at baseline and mild EGR condition.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e \u003cp\u003eThe high EC, 2\u0026ndash;7 ring PAH and \u003cem\u003en\u003c/em\u003e-alkane emission under high EGR rate conditions indicated that the application of EGR helped with other controlling strategies (e.g., fuel injection, aftertreatment device) is demanded to suppress the carbonaceous PM formation for the modern common-rail diesel engine. Therefore, the carbonaceous PM exhaust characteristics with the application of EGR coupled with injection strategies, i.e., under low temperature combustion mode should be performed in the next work.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors would like to thank the National Natural Science Foundation of China (No. 51976119) for financial support of this study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors would like to thank the National Natural Science Foundation of China (No. 51976119) for financial support of this study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u0026nbsp; The authors would like to thank the National Natural Science Foundation of China (No. 51976119) for financial support of this study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026rsquo; Contributions\u003c/strong\u003e Xinling Li: Writing \u0026ndash; original draft, Writing \u0026ndash; review \u0026amp; editing, Methodology, Investigation. Pengcheng Zhao: Methodology, Data curation.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical Approval\u003c/strong\u003e Not applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to Participate\u003c/strong\u003e Not applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to Publish\u0026nbsp;\u003c/strong\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests\u0026nbsp;\u003c/strong\u003eThe authors have no relevant financial or nonfinancial interests to disclose.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability Statement\u0026nbsp;\u003c/strong\u003eThe data used to support the findings of this study are available from the corresponding author upon request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAn YZ, Teng SP, Pei YQ, Qin J, Li X, Zhao H (2016) An experimental study of polycyclic aromatic hydrocarbons and soot emissions from a GDI engine fueled with commercial gasoline. 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J Clean Prod 185:797\u0026ndash;804\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"environmental-science-and-pollution-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"espr","sideBox":"Learn more about [Environmental Science and Pollution Research](https://www.springer.com/journal/11356)","snPcode":"11356","submissionUrl":"https://submission.nature.com/new-submission/11356/3","title":"Environmental Science and Pollution Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Diesel engine, common-rail, Mild and high EGR rates, Carbonaceous PM emissions, Particulate SVOCs","lastPublishedDoi":"10.21203/rs.3.rs-5197899/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5197899/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAs one of the most effective ways of reducing nitrogen oxides (NOx) emission, exhaust gas recirculation (EGR) has been widely used in diesel engines. However, EGR generally shows adversely effective in particulate matter (PM) emissions. The chemical composition of the PM with the application of EGR is not well identified because few previous publications focus on this topic, especially for high EGR rate cases. In this paper, emission characteristics of organic carbon (OC, OC\u003csub\u003e1\u003c/sub\u003e-OC\u003csub\u003e4\u003c/sub\u003e), elemental carbon (EC, EC\u003csub\u003e1\u003c/sub\u003e-EC\u003csub\u003e2\u003c/sub\u003e), particulate semi-volatile organic compounds (SVOCs) including 18 \u003cem\u003en\u003c/em\u003e-alkanes and 20 polycyclic aromatic hydrocarbons (PAHs) for a common-rail diesel engine at mild and high EGR rate conditions (up to maximum achievable level while maintaining stable combustion) were analyzed at four steady-state conditions comprehensively. It can be clearly observed that EGR rate instead of load and speed significantly affects the EC emission under the experimental conditions. EC emission increase with increasing EGR rate, which is divided to two sections, i.e., slight increase from 0 to 30% (mild EGR rate) and sharp increase from 30\u0026ndash;45% (high EGR rate). TC is dominated by OC\u003csub\u003e1\u003c/sub\u003e, OC\u003csub\u003e2\u003c/sub\u003e and EC\u003csub\u003e1\u003c/sub\u003e at low EGR rate, and the fraction of EC\u003csub\u003e1\u003c/sub\u003e evidently increases with increasing EGR rate. It is observed that TC is heavily dominated by EC\u003csub\u003e2\u003c/sub\u003e at highest EGR rate ranges, which corresponds to the lower heat release rates (lower \u003cem\u003eHRR\u003c/em\u003e\u003csub\u003emax\u003c/sub\u003e at higher EGR rate) and lower air-fuel ratio at these conditions. All the target PAHs increase with increasing EGR rate at the four operation modes. The adverse effect of EGR on PAH emission is less significant than EC emission. Moreover, the effect of EGR rate on the PAH ring distribution is not significant. Both of total \u0026sum;C\u003csub\u003e16\u003c/sub\u003e-C\u003csub\u003e25\u003c/sub\u003e and \u0026sum;C\u003csub\u003e26\u003c/sub\u003e-C\u003csub\u003e33\u003c/sub\u003e emission rates evidently increase at high EGR rate condition in comparison with those at baseline and mild EGR condition cases, which indicates that both fuel-derived and oil-derived n-alkanes exhibit higher emission rate at high EGR conditions compared with those at baseline and mild EGR condition. The application of EGR helped with other controlling strategies (e.g., fuel injection, after-treatment device) is suggested to suppress the carbonaceous PM formation for the modern common-rail diesel engine.\u003c/p\u003e","manuscriptTitle":"Effect of exhaust gas recirculation (EGR) on diesel engine carbonaceous PM emissions","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-29 16:17:42","doi":"10.21203/rs.3.rs-5197899/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Accept","date":"2025-05-05T17:49:23+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2025-04-28T17:13:31+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-04-28T16:53:22+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-04-14T04:21:07+00:00","index":"","fulltext":""},{"type":"submitted","content":"Environmental Science and Pollution Research","date":"2025-04-11T02:53:06+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"environmental-science-and-pollution-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"espr","sideBox":"Learn more about [Environmental Science and Pollution Research](https://www.springer.com/journal/11356)","snPcode":"11356","submissionUrl":"https://submission.nature.com/new-submission/11356/3","title":"Environmental Science and Pollution Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"f544efc0-d9aa-4741-a837-8b052a8135d1","owner":[],"postedDate":"April 29th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-05-26T15:59:14+00:00","versionOfRecord":{"articleIdentity":"rs-5197899","link":"https://doi.org/10.1007/s11356-025-36511-3","journal":{"identity":"environmental-science-and-pollution-research","isVorOnly":false,"title":"Environmental Science and Pollution Research"},"publishedOn":"2025-05-19 15:56:55","publishedOnDateReadable":"May 19th, 2025"},"versionCreatedAt":"2025-04-29 16:17:42","video":"","vorDoi":"10.1007/s11356-025-36511-3","vorDoiUrl":"https://doi.org/10.1007/s11356-025-36511-3","workflowStages":[]},"version":"v1","identity":"rs-5197899","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5197899","identity":"rs-5197899","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}