EGR对柴油机燃烧影响模拟

Modeling of the Effects of EGR on Combustion of a Diesel Engine

废气再循环(EGR)作为控制缸内NOx生成的一项技术已广泛应用在现代直喷柴油发动机上。但EGR对氮氧化合物(NOx)、碳烟(Soot)排放的影响原因尚未被完全理解。为了全面分析EGR的特性,建立了基于GT-POWER的柴油机仿真模型。根据柴油机的基本结构,该模型为带有EGR系统的增压直喷柴油机一维流体动力学循环仿真模型。在分别固定进气压力和空燃比两种情况下,对EGR影响柴油机燃烧的特性进行了研究。结果表明,在恒定进气压力和EGR温度的情况下,随着EGR率的升高,缸内压力升高率减小,最高缸内爆发压力降低,燃烧放热始点推迟,燃烧峰值放热率升高。EGR导致Soot升高燃油经济性降低。在恒定进气空燃比和EGR温度的情况下,随着EGR率的升高,缸内压力的升高使燃烧放热始点提前,废气的惰性气体特性延缓燃烧成为次要因素。EGR的加入使燃烧恶化放热率降低。缸内的燃烧温度降低,减少了NOx的生成。小EGR率可以改善Soot的排放情况。所以在不同的边界条件下引入EGR的作用不同,在EGR控制策略中,利用控制进气空燃比的EGR控制方法并没有完全利用EGR特性,应该形成分别控制空气质量流量和EGR率的气路控制策略。在恒定EGR率的情况下,EGR温度的升高缩短了燃烧滞燃期,燃烧始点提前放热率峰值降低。最终缸内气体温度升高,NOx排放升高,Soot有轻微的改善,表明为了更好控制EGR系统,应对EGR温度进行控制。

Exhaust Gas Recirculation （EGR） is a common way to control in-cylinder NOx production and is used in most modern direct injection diesel engines. However the influence of EGR on NOx and Soot is not perfectly understood. In order to analyze the characteristics of EGR, a model of diesel engine based on the GT-POWER is established. Based on the basic structure of diesel engine, the model is a 1D fluid-dynamic engine cycle simulation model of TDI diesel engine with EGR system. The influences of EGR on combustion of the diesel engine are investigated in a constant inlet boost pressure and constant inlet air fuel ratio condition. In the condition of constant inlet boost pressure and constant EGR temperature, along with the rise of the EGR ratio, the rise ratio of the in-cylinder pressure reduces, the highest in-cylinder pressure drops, the start of combustion delays, the peak rate of heat release becomes higher. The EGR makes the Soot lifts and the Brake Specific Fuel Consumption comes down. In the condition of constant air fuel ratio and the EGR temperature, in the wake of the rise of EGR ratio, the in-cylinder pressure becomes higher, which makes the start of heat release shifting to an early time. The delay of heat release due to EGR is compensated by the rise of in-cylinder pressure. The characteristics of the inert gases in the exhaust gases become a secondary factor. EGR reduces the combustion efficiency. The rate of heat release becomes lower. The combustion temperature of in-cylinder comes down. The small EGR ratio can improve the emission. So, the effect of EGR at different boundary conditions is different. The control strategy of EGR with air fuel ratio as the controlling target is defective. One should adopt the control strategy with both inlet air mass flow and EGR ratio as controlling targets. In the condition of constant EGR ratio, the rise of EGR temperature shortens the ignition delay. The start of combustion shifts to an early time, and the rate of heat release comes down. The incylinder temperature and the emission of NOx rise, however, the emission of Soot improves. Dynamic property and economical efficiency deteriorate. In order to make a perfect EGR control system, the EGR temperature should also be controlled.