Exploration of chemical composition effects on the autoignition of two commercial diesels: Rapid compression machine experiments and model simulation

Chemical composition difference widely exists in real fuels, and the composition difference will affect the fuel autoignition and heat release in HCCI-based advanced engines. This study aims to explore the composition effects on autoignition by comparing the autoignition characteristics of two commercial diesels (China Stage-V and Stage-VI). The main composition difference is that Stage-V diesel has a higher paraffin content and a lower naphthene content than Stage-VI diesel. Ignition delay times (IDTs) of the two diesels were measured in a heated rapid compression machine at equivalence ratios of 0.37–1.25, pressures of 10–20 bar, and temperatures of 687–865 K. It is found that the difference in the total IDTs of the two diesels varies with the temperature range, and the first-stage IDTs of Stage-V diesel are much shorter than those of Stage-VI diesel. The IDT discrepancies were appropriately explained using the composition difference between the two diesels. Model simulation was carried out using a ternary and a five-component diesel surrogate coupled with an updated kinetic model. Simulation results show that the composition effects on the autoignition of the two diesels can be well captured by the two surrogates, where the ternary and five-component surrogates agree well with Stage-V and Stage-VI diesels, respectively. To further reveal the intrinsic mechanism of the composition effects, low-temperature reactivity difference between the two surrogates was interpreted from a kinetic perspective. Rate of production (ROP) analysis on OH radical confirms that the addition of decalin in the five-component surrogate is primarily responsible for the longer first-stage IDT compared to the ternary surrogate. Since the two surrogates capture the composition difference and autoignition characteristics of the two diesels, the conclusion from the kinetic analysis will help understand the composition effects on the autoignition of the diesels.