Experimental investigation of CAI combustion in a two-stroke poppet valve DI engine

Abstract

Due to their ability to simultaneously reduce fuel consumption and NOx emissions, Controlled Auto Ignition (CAI) and HCCI combustion processes have been extensively researched over the last decade and adopted on prototype gasoline engines. These combustion processes were initially achieved on conventional two-stroke ported gasoline engines, but there have been significantly fewer studies carried out on the CAI combustion in two-stroke engines. This is primarily due to the inherent problems associated with conventional two-stroke engine intake and exhaust ports. Meanwhile, engine downsizing has been actively researched and developed as an effective means to improve the vehicle’s fuel economy. This is achieved by operating the engine at higher load regions of lower fuel consumption and by reducing the number of cylinders. However, aggressive downsizing of the current 4-stroke gasoline engine is limited by the knocking combustion and high peak cylinder pressure. As an alternative approach to engine downsizing, boosted two-stroke operation is being researched. In this thesis, it has been shown that the CAI combustion in the two-stroke cycle could be readily achieved at part-load conditions with significant reductions in CO and uHC emissions when compared to typical SI combustion in a single cylinder gasoline direct injection camless engine. In addition, extensive engine experiments have been performed to determine the optimum boosting for minimum fuel consumption during the two-stroke operation. In order to minimise the air short-circuiting rate, the intake and exhaust valve timings were varied and optimised. It is shown that the lean operation under boosted condition can extend the range of CAI combustion and increase combustion and thermal efficiencies as well as producing much lower CO and HC emissions. By means of the cycle-resolved in-cylinder measurements and heat release analysis, the improvement in combustion and thermal efficiencies were attributed to the improved in-cylinder mixture, optimised autoignition, and combustion phases. Finally, in view of the increased use of ethanol in gasoline engines, E15 and E85 were used and their effect on engine performance, fuel economy and exhaust emissions were investigated.