In diesel engines, combustion is initiated by a two-staged autoignition that includes both low- and high-temperature chemistry. The location and timing of both stages of autoignition are important parameters that influence the development and stabilisation of the flame. In this study, a two-dimensional direct numerical simulation (DNS) is conducted to provide a fully resolved description of ignition at diesel engine-relevant conditions. The DNS is performed at a pressure of 40 atmospheres and at an ambient temperature of 900 K using dimethyl ether (DME) as the fuel, with a 30 species reduced chemical mechanism. At these conditions, similar to diesel fuel, DME exhibits two-stage ignition. The focus of this study is on the behaviour of the low-temperature chemistry (LTC) and the way in which it influences the high-temperature ignition. The results show that the LTC develops as a “spotty” first-stage autoignition in lean regions which transitions to a diffusively supported cool-flame and then propagates up the local mixture fraction gradient towards richer regions. The cool-flame speed is much faster than can be attributed to spatial gradients in first-stage ignition delay time in homogeneous reactors. The cool-flame causes a shortening of the second-stage ignition delay times compared to a homogeneous reactor and the shortening becomes more pronounced at richer mixtures. Multiple high-temperature ignition kernels are observed over a range of rich mixtures that are much richer than the homogeneous most reactive mixture and most kernels form much earlier than suggested by the homogeneous ignition delay time of the corresponding local mixture. Overall, the results suggest that LTC can strongly influence both the timing and location in composition space of the high-temperature ignition.

在柴油发动机中，燃烧是由两阶段的自燃引发的，该阶段包括低温和高温化学物质。自燃两个阶段的位置和时间是影响火焰发展和稳定的重要参数。在这项研究中，进行了二维直接数值模拟（DNS），以提供完全解析的柴油机相关条件下点火的描述。使用二甲醚（DME）作为燃料，在40个大气压的压力和900 K的环境温度下执行DNS，化学机理降低了30种。在这些条件下，类似于柴油燃料，DME表现出两阶段点火。这项研究的重点是低温化学（LTC）的行为及其影响高温着火的方式。结果表明，LTC在贫油区发展为“斑点”的第一阶段自燃，然后过渡到扩散支撑的冷火焰，然后沿局部混合比梯度向富油区传播。冷焰速度比均质反应堆中第一阶段点火延迟时间的空间梯度要快得多。与均相反应器相比，冷火焰缩短了第二阶段的点火延迟时间，并且在更浓的混合物中，这种缩短变得更加明显。在一系列浓混合气中观察到多个高温点火核，这些浓混合气比均质的最具反应性的混合物浓得多，大多数核的形成要早于相应局部混合物的均质点火延迟时间所建议的时间。全面的，