气门间隙对发动机性能的影响研究

某四缸自然吸气汽油发动机怠速时燃烧不稳定,经热力学性能仿真计算发现怠速时气门重叠角偏大,导致残余废气率较高,从而引起燃烧不稳定及循环变动大。可通过调整进气凸轮轴安装相位或增大气门间隙来推迟进气门开启时刻,减小气门重叠角。本文对加大气门间隙方案做换气过程分析计算,加大气门间隙,气门重叠角减小。在低速时,小的气门重叠角有利于减小废气倒流,提高进气量;但在高速时进气惯性大,由于加大气门间隙,减小了进气迟闭角,造成高速时进气量减小。试验结果验证仿真分析的正确性。试验结果表明,减小气门重叠角可显著改善怠速工况的循环变动和排放,中低速工况的扭矩也有所提升,但高速段功率稍有下降。

Investigation of the Effects of Valve Clearance on Engine Performance

Unstable combustion occurred on a 4 - cylinder natural aspirated gasoline engine at idle speed. The results from thermodynamics simulation illustrated that too large valve overlap resulted in higher residual gas in combustion chamber. Consequently, unstable combustion and higher coefficiency of variation （COV） would occur. Two methods were adopted to reduce valve overlap angle and delay intake valve timing. In this paper the simulation of complete gas charge process was performed for the latter solution. The simulation results showed that at low speed less valve overlap led to less exhaust back flow and more fresh air was sucked into the cylinder. At high speed the inertia of air flow was dramatic. However, the intake valve close timing was ad- vanced. As a result, the amount of fresh air flow was decreased. Subsequently engine tests were carried out to validate the simulation results. The test results indicated that less valve overlap can significantly improve COV and emissions at idle and also the output torque at low and middle speed was considerably elevated, but rated power was slightly dropped.