Numerical Study Of The Stratified Flame-induced Auto-ignition(SFI) Combustion Processes Of Highly-diluted Mixture With Exhaust In A Two-stroke Gasoline Engine

The internal combustion engines with two strokes can effectively increase the specific power of theirs counterparts with the same displacement and hence improve fuel economy.However,high residual gases in the cylinders of two stroke gasoline engines result in unstable combustion and slow heat release.To solve these problems of the two stroke engines,a stratified flame induced-auto-ignition(SFI)combustion mode is proposed to increase the burning rate of the highly-diluted mixture by exhaust in the cylinder and improve the thermal efficiency.To this end,the effect of double fuel injection strategies,spark timing,engine speed and the intake valve timing on the mixture formation and SFI combustion in a poppet-valve two-stroke direct injection gasoline engine was numerically simulated through a three-dimensional code at partial loads in a fixed amount of fuel burned in a cycle.The results can be drawn as follows:With the increase of ratio in the second fuel injection,fuel-air equivalence ratio around the spark plug increases and it decreases from the cylinder center to cylinder wall compared to the single fuel injection.The ignition timing advances slightly and then delays with the second fuel injection ratio at a fixed spark timing while combustion duration observes an opposite trend.In the meantime,indicated mean effective pressure,the maximum pressure rise rate and the peak pressure in the cylinder first increase and then decrease.Double fuel injection facilitates SFI combustion.Among all the cases,the flame propagation is the highest and the auto-ignition duration is the shortest at the second injection ratio of 30%.At a fixed second injection ratio,the auto-ignition timing is delayed and the required heat released by the flame propagation to induce the auto-ignition is decreased with the delay of sparking timing.The SFI combustion processes are affected by engine speed.At 1000 r/min,larger proportion of mixture is consumed by the flame propagation,due to proper mixture pattern in the cylinder caused by longer mixing time.At 1500 r/min,the mixture in the cylinder distributes from relatively rich in the center to lean in the periphery region.At 2000 r/min,the heating effect from residual gases and compression results in increased temperature in the end gases and advanced auto-ignition timing.In the meantime,more mixture in front of the flame is consumed by the auto-ignition.With the delay of the intake valve closing timing(IVC),the heating effect from the residuals in the cylinder decreases and the temperature of the mixture in the periphery region decreases and lean fuel appears in the periphery region.At IVC =-114°CA ATDC,the flame propagation speed is faster at the early stage of the heat release.Thus the auto-ignition in the end gases requires less heat released by flame propagation.As the IVC is retarded,the mixture with low temperature and reaction in the end gases increases.More heat released by the flame propagation is required to induce the auto-ignition in the cylinder.The auto-ignition timing is delayed and the indicated mean effective pressure decreases.The auto-ignition can happen at different IVCs.However,less proportion of the end gases in the cylinder is consumed by the auto-ignition while most fuel is burned by flame propagation at IVC =-94°CA ATDC.