【作者】 秦静；

【导师】 赵华； 谢辉；

【作者基本信息】 天津大学 ， 动力机械及工程， 2006， 博士

【摘要】 可控自燃(Controlled Auto Ignition,CAI)汽油机因具有低氮氧化物排放和高热效率的特点而受到内燃机界的广泛关注,但CAI汽油机面临着火时刻控制和负荷扩展的难题。本文通过试验和模拟计算深入研究了汽油机CAI燃烧过程,分析了影响汽油机CAI燃烧的因素及其作用机理,通过敏感性分析探询这些影响因素的主要控制手段及关键控制参数,进而通过协同各控制参数改变燃烧的初始条件和边界条件以达到汽油机CAI燃烧控制的目的。在Ricardo单缸试验机上利用两对小升程进、排气凸轮轴,通过废气捕捉的方法实现了稳定的汽油机CAI燃烧。试验结果发现,残余废气率和空燃比是影响汽油机CAI燃烧过程的关键因素。其中,残余废气不仅提供了CAI燃烧所必需的热能,还起到了控制着火时刻、负荷大小和放热速度的作用。基于以上试验结果,本文提出了一个双阶段放热率公式来描述汽油机CAI燃烧过程,并结合一维发动机数值模拟方法,研究各控制参数对残余废气率和CAI燃烧过程的控制作用。计算结果表明,空燃比对着火时刻的影响最为明显,尤其是在混合气过稀时。而排气门定时决定了缸内混合气中残余废气和新鲜空气的比例,是控制CAI汽油机着火时刻和负荷的重要手段。为研究气门运动特性对汽油机CAI燃烧过程的控制作用,在Simulink环境中建立了带有进/排气门升程及定时都可控的四变量气门执行机构(4-variable valve actuating system, 4VVAS)的汽油机仿真试验平台,对其性能和气门参数控制特点进行了预测和分析,并在4VVAS-CAI发动机试验台架上进行了试验研究。仿真和试验结果都表明,可变气门机构能够有效地控制汽油机CAI燃烧过程。其中,排气门定时及升程能够控制缸内残余废气率,从而控制了混合气特性和压缩初始温度,进气门定时和升程能够调节发动机的有效压缩比和进气回流,以改变压缩过程中混合气的温度历程。不同气门升程下的发动机运行负荷范围不同,4VVAS系统通过使用不同升程的气门型线能够拓展低转速小负荷和高转速大负荷的运行范围。气门升程和定时的灵活可控使得4VVAS-CAI汽油机可采用不同的气门定时和升程组合方式获得相同的运行工况点,方便了各工况点的优化。此外,4VVAS机构能够实现CAI汽油机的负荷过渡,其中,同时调整气门定时和升程能够加快负荷过渡过程。更多还原

【Abstract】 CAI (Controlled Auto Ignition) gasoline engines have been received a lot of attention from the world-wide automotive field due to their ultra-low NOx emissions and high thermal efficiency. However, they are confronted with the problems in ignition timing control and operation range extension. In this paper, the combustion process of a CAI gasoline engine was investigated by experiment and simulation. Following the studies of the combustion-related factors, sensitivity analysis was carried out to obtain corresponding control methods and the key control parameters about these factors. With combinations of these control parameters, various initial and boundary conditions could be obtained to control the combustion process of CAI gasoline engines.Stable CAI combustion was achieved on a modified Ricardo single-cylinder experimental engine by using two pairs of camshafts with reduced valve lifts to trap residual gas during exhaust stroke. Experimental results show that CAI combustion is notably influenced by residual gas fraction and air-fuel ratio. The residual gas not only provides necessary heat energy for the mixture, but also influences the ignition timing, heat release rate and load.A piecewise heat release correlation was developed to characterize the combustion process of the CAI gasoline engines based on the experimental data. And a one-dimensional engine simulation program was implemented to study the effects of the control parameters on residual gas fraction and combustion process. It is found that air-fuel ratio affects the ignition timing remarkably, especially when the mixture is ultra-lean. Exhaust valve timings vary the fraction of in-cylinder residuals and fresh charge, and therefore can be utilized as one of the most effective ways to control ignition timings and engine loads.In order to study the effect of valve events on the combustion process of CAI gasoline engines, a gasoline engine simulation panel with 4-variable valve actuating system (4VVAS) was established based on Matlab-Simulink. The performances and valve control characteristics of the 4VVAS-CAI engine were predicted. And experiments were also carried on a 4VVAS-CAI test-bed. Both simulation and experimental results show that the variable valve system can effectively control the CAI combustion process. On the one hand, variations in exhaust valve lift and timing can alter the in-cylinder residual gas fraction and consequently determine the mixture更多还原