Study on Ignition Mechanism of Non-Uniform Mixture

为了描述柴油机喷雾着火延迟期，提出了一个考虑喷雾吸热过程的随机模型。该模型考虑了湍流的产生和燃料与空气的湍流混合。对于吸热过程，由于蒸发和热解的热吸收速率被假定为恒定。在点火延迟期的化学反应表示的一步全球机制和点火被假定发生时，活性物种的浓度超过一个阈值。结果表明，该模型可以重现着火延迟的柴油喷雾和压力的历史，在快速压缩机的各种条件。此外，该方法还可用于预测不同喷射率（包括预喷）下柴油机的初始燃烧过程，在较高的初始温度下，着火延迟取决于喷雾的形成，而在较低的初始温度下，着火延迟取决于化学反应。在该模型中，燃料浓度和温度的不均匀性用概率密度函数表示，由于碰撞引起的热量和质量交换，初始偏析随时间的推移趋于形成不均匀性较小的混合物，最大放热速率与预测结果通过改变延迟机制相对应。试验结果还表明，在高喷射压力和小喷嘴直径的情况下，预喷射能有效地保证燃烧的平稳性。当给定适当的吸热常数作为喷射压力、喷嘴直径、环境温度和压力的函数时，点火延迟期被成功地预测为宽范围的喷射条件。根据预测结果，讨论了高压喷射对点火过程的影响。

To describe the ignition delay of diesel sprays, a stochastic model taking account of the endothermic process in the spray is proposed. In this model, generation of turbulence and turbulent mixing of fuel and air are taken into consideration. For the endothermic process, the rate of heat absorption due to vaporization and pyrolysis is assumed to be constant. Chemical reactions during the ignition delay period are expressed by a one-step global mechanism and ignition is assumed to occur when concentrations of active species exceed a threshold. It is shown that this model may reproduce the ignition delay of a diesel spray and pressure history in a rapid compression machine for a variety of conditions. Also, the ignition delay is dependent on spray formation at a high initial temperature, but on chemical reaction at a low initial temperature.Furthermore, this approach is applied to predict the initial combustion in diesel engines for various injection rates including pilot injection. In this model, heterogeneity of fuel concentration and temperature is expressed by probability density functions Due to the heat and mass exchange associated with the collision, the initial segregation tends to form a mixture of less heterogeneity as time goes on. Maximum rates of heat release correspond to the predicted results by varying delay mechanism. It is also shown that when injection pressure is high and nozzle diameter is small, pilot injection efficiently gives a smooth burning. When an appropriate constant of heat absorption is given as a function of injection pressure, nozzle diameter, ambient temperature and pressure, ignition delay periods are successfully predicted for a wide range of injection conditions. Based on the predicted results, a discussion is made about the effect of high-pressure injection on the ignition process.