Study On Controlled Auto-Ignition Combustion And Its Control In A 4-Stroke Gasoline Engine

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...