Experimental and modeling study of the autoignition characteristics of commercial diesel under engine-relevant conditions

Knowledge of the autoignition characteristics of diesel fuels is of great importance for understanding the combustion performance in engines and developing surrogate fuels. Here ignition delays of China's stage 6 diesel, a commercial fuel, were measured in a heated rapid compression machine (RCM) under engine-relevant conditions. Gas-phase autoignition experiments were carried out at equivalence ratios ranging from 0.37 to 1.0, under compressed pressures of 10, 15, and 20?bar, and within a temperature range of 685–865?K. In all investigated conditions, negative temperature coefficient (NTC) behavior of the total ignition delays is observed. The autoignition of the diesel fuel exhibits pronounced two-stage characteristics with strong low-temperature reactivity. Experimental results indicate that the total ignition delays shorten with increasing compressed pressure, oxygen mole fraction and fuel mole fraction. The first-stage ignition delays are mainly controlled by compressed temperature and also affected by oxygen mole fraction and compressed pressure but show a very weak dependence on fuel mole fraction. Correlations describing the first-stage ignition delay and the total ignition delay were proposed to further clarify the ignition delay dependence on the multiple factors. Additionally, it is found that the newly measured ignition delays well coincide with and complement the diesel ignition data in the literature. A recently developed diesel mechanism was used to simulate the diesel autoignition on the RCM. The simulation results are found to agree well the experimental measurements over the whole temperature ranges. Species concentration analysis and brute force sensitivity analysis were also conducted to identify the crucial species and reactions controlling the autoignition of the diesel fuel.