Investigation of combustion, performance and emission characteristics of 2-stroke and 4-stroke spark ignition and CAI/HCCI operations in a DI gasoline

In order to develop more efficient and cleaner gasoline engines, a number of new engine operating strategies have been proposed and researched on different engines, including the spark ignition (SI) and controlled autoignition (CAI) or HCCI in both 2-stroke and 4-stroke cycles in a poppet valve engine. In this work, a single cylinder direct injection gasoline engine equipped with an electro-hydraulic valve-train system has been commissioned and used to achieve seven different operating modes, including: 4-stroke throttle-controlled SI, 4-stroke intake valve throttled SI, 4-stroke positive valve overlap SI, 4-stroke negative valve overlap CAI, 4-stroke exhaust rebreathing CAI, 2-stroke CAI and 2-stroke SI. Their performance and emission characteristics were analysed and compared at a typical engine calibration operating condition of 1500rpm and 3.6bar IMEP in 4-stroke or 1.8bar IMEP in 2-stroke. Results show that 4-stroke positive valve overlap SI, 4-stroke NVO CAI and exhaust rebreathing CAI modes have better fuel economy and lower NOx emissions than the conventional throttled 4-stroke SI operation. The 2-stroke CAI operation was found to produce higher combustion efficiency and lower ISFC but lower brake efficiency than the 4-s-stroke operations at the same power output due to the supercharger’s efficiency. But, at the same IMEP as the 4-stroke operation, the 2-stroke CAI operation results in 29% reduction in BSFC, indicating its potential synergy with highly downsized direct injection gasoline engines for much better fuel economy and performance.