Study on Ignition Mechanism of Non-Uniform Mixture

为了描述柴油喷雾的着火延迟，提出了一个考虑喷雾吸热过程的随机模型。在该模型中，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，假定汽化和热解的吸热速率是恒定的。着火延迟期的化学反应用一步全局机制表示，当活性物质的浓度超过阈值时，就会发生着火。结果表明，该模型可以在不同条件下再现柴油机喷雾着火延迟期和压力历程。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。此外，将该方法应用于包括预喷在内的不同喷油率的柴油机初始燃烧预测。在这个模型中，燃料浓度和温度的不均匀用概率密度函数来表示，由于与碰撞相关的热量和质量交换，初始的偏析随着时间的推移趋向于形成一个不那么不均匀的混合物。最大放热率与变延迟机制的预测结果相吻合。在喷油压力较高、喷嘴直径较小的情况下，预喷能有效地实现平稳燃烧。当给出合适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，成功地预测了较大范围的喷射条件下的点火延迟时间。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.