Performance of Hydrotreated Vegetable Oil–Diesel Blends: Ignition and Combustion Insights

Hydrotreated vegetable oil (HVO) is a second-generation biofuel with physicochemical properties similar to conventional diesel. Composed mainly of n-paraffins, it offers favorable autoignition characteristics. Produced by hydrotreating vegetable oils or animal fats, including waste sources such as used cooking oil, HVO contributes to lower greenhouse gas emissions and waste utilization. Thanks to its similarity to diesel, it can be used directly or in blends without engine modifications. Blending reduces fossil fuel use and pollutant emissions while maintaining engine performance. This study investigates the autoignition behavior of diesel, neat HVO, and HVO–diesel blends containing 25%, 50%, and 75% HVO by volume. Experiments were conducted in a constant-volume combustion chamber at 550 °C and 650 °C to simulate engine-relevant conditions. Autoignition quality was assessed using ignition delay, combustion delay, average and maximum pressure rise rate, maximum pressure rise, apparent heat release rate, and derived cetane number. The results show that higher HVO content increases the sensitivity of ignition delay, combustion delay, and average pressure rise rate to lower chamber temperature. In addition, a linear increase in derived cetane number was observed with increasing HVO concentration, providing new insights into ignition and combustion behavior of renewable fuel blends.