Effect of combustion period on carbon crystallite size of soot particles emitted from a diesel engine with diesel and hydrogenated vegetable oil fuels

• Crystallite size of soot particles with HVO is smaller than that with diesel fuel. • Crystallite size shows a positive correlation with high-temperature residence time. • Aromatic content in fuel has a significant impact on crystallite size. This study investigates the relationship between the carbon crystallite size of soot particles emitted from a diesel engine and the combustion characteristics as influenced by operating conditions and fuel type. Soot particles were collected from a single-cylinder direct-injection diesel engine operated under varying fuel injection timings, injection pressures, engine loads, and engine speeds. The crystallite size of soot was evaluated using laser Raman spectroscopy and high-resolution transmission electron microscopy. Three fuels were evaluated: conventional diesel, hydrogenated vegetable oil (HVO) as a next-generation biodiesel, and HVO blended with 30 wt% toluene as an aromatic additive. Across all tested conditions, the soot generated from HVO demonstrated crystallite sizes at least 0.096 nm smaller than those from diesel fuel, resulting in accelerated oxidation at lower ambient temperatures. The qualitative trends in crystallite size variation with respect to engine parameters were consistent between diesel fuel and HVO. When high-temperature residence time was calculated based on crank-angle-resolved combustion duration, a strong positive correlation was observed between the residence time and crystallite size, with a coefficient of determination exceeding 0.9. In contrast, including the expansion stroke in the residence time calculation significantly reduced the coefficient of determination. These findings indicate that the crystallite size of soot particles is primarily established during the late combustion phase, with only minor modifications occurring during the expansion stroke. For the HVO–toluene blended fuel, the crystallite size was slightly larger than that of diesel fuel, indicating that aromatic components promote the graphitization of soot particles.