米勒循环及低压废气再循环技术对汽油机性能的影响研究

在某2.0L涡轮增压商用车缸内直喷汽油机概念设计阶段,运用GT-Power软件进行了一维热力学计算分析,并着重研究了米勒循环技术与低压废气再循环(exhaust gas recirculation,EGR)技术对燃油经济性的影响。仿真结果显示,长米勒循环和短米勒循环都存在使燃油经济性达到最佳的可变气门正时角度。在中高转速范围内,因为质量流量和流速过高导致长米勒循环的缸内空气无法顺利回流,米勒效应减弱,所以短米勒循环的燃油经济性更好;另外长米勒循环具有两次泵气过程,致使其泵气损失也大于短米勒循环。在低转速范围内,短米勒循环的增压压力需求更大,导致其泵气损失大于长米勒循环,所以长米勒循环的燃油经济性更好。对于试验用汽油机来说,短米勒循环具有明显的优势。低压EGR技术能够使缸内最高温度降低,减轻爆震倾向,因此燃烧重心可以适当提前,从而改善燃油经济性。另外,EGR和新鲜空气总量的增加也使最高燃烧压力增大,有利于燃油经济性的提高。不同工况下,缸内最高温度均随EGR率的增加而降低,因此在一定范围内EGR率越高燃油经济性越好,涡前排温越低,但EGR冷却器需要的散热功率也增大。在发动机燃烧开发阶段,推荐采用短米勒循环技术并结合8%的EGR率。为了验证仿真结果的准确性,开展了性能试验,试验结果与仿真结果在趋势上有很好的一致性。

Effects of Miller Cycle and Low-Pressure Exhaust Gas Recirculation on Gasoline Engine Performance

In the conceptual design stage of a gasoline engine (2.0TGDI) of a commercial vehicle, one-dimensional thermodynamic calculation and analysis is carried out using GT-POWER software, and the influence of Miller cycle technology and low-pressure EGR technology on fuel economy is studied. The simulation results show that both long Miller cycle and short Miller cycle has the optimal VVT angles for fuel economy. At medium-high-speed region, fuel economy of short Miller cycle is better because in long Miller cycle, reversal of air flow direction in cylinder is not possible due to high mass flow and velocity, and Miller effect is reduced. In addition, long Miller cycle has two pumping processes, resulting in more pumping loss than short Miller cycle. At low-speed region, fuel economy of long Miller cycle is better because boost pressure of short Miller cycle is higher, resulting in more pumping loss than long Miller cycle. For this gasoline engine, short Miller cycle has obvious advantages. Low pressure EGR technology can decrease maximum temperature in cylinder and limit knock tendency, hence CA50 can be advanced properly to optimize fuel economy. Furthermore, increase of total amount of EGR and fresh air also led to an increase in cylinder peak pressure, contributing to better fuel economy. On different operating conditions, the maximum temperature in cylinder decrease with the growing of EGR rate. Therefore, within a certain range, the higher EGR rate, the better fuel economy, and the lower turbine inlet temperature. However, in the meantime more cooling power is required by the EGR cooler. As a result, short Miller cycle technology is applied for engine combustion development process, and the recommended EGR rate is 8%. In order to verify the simulation results, the engine performance test is carried out, and the experimental results are in good agreement with the simulation results.