Amid rising demands for fuel efficiency and emissions reduction, enhancing the thermal efficiency of gasoline engines has become imperative, which requires higher efficiency combustion strategies and integrated optimized design to maximize the work output from fuel. In gasoline engine, both increasing the compression ratio and using lean burn mode improve the ratio of useful work output to the energy input effectively, which resulting in higher thermal efficiency. Although there is limited scope for increasing the compression ratio due to the higher sensitivity to knocking, especially under stoichiometric conditions, reduced sensitivity could be got with leaner mixture fill into cylinder, which can further increase the specific heat ratio and thermal efficiency. However, realizing the efficiency benefits of lean burn in gasoline engines necessitates overcoming critical challenges like ensuring robust ignition process and accelerating burning rates to achieve short, stable combustion durations. To overcome these problems, a kind of pre-chamber ignition concept has been developed, which integrate a dedicate injector and spark plug into one assembly on cylinder head. In this paper, the focus is on the development of such a system and the combustion characteristics on gasoline engine with it.
Primary experimental data is obtained by an ultra-lean single cylinder research gasoline engine equipped with pre-chamber concept. Key structure parameters such as pre-chamber volumetric fraction and jet hole have been comparatively investigated. Experimental study also be performed on a multi-cylinder engine equipped with the same pre-chamber concept in order to verify the maximum Gross-ITE and BTE simultaneously. Results indicated that optimized pre-chamber structure design could make much contribute to the thermal efficiency under ultra-lean mode with lambda up to 2.1. Under this condition, combustion stability was also significantly improved compared to spark ignition with faster and knock-free combustion process. Due to the lower temperature of ultra-lean combustion, Nitrogen Oxides emissions can be significantly reduced. Furthermore, for a given load, it was shown that maximum GITE of 50% on SCE was measured at 2500rpm-11.5bar IMEP together with the maximum BTE of 46.3% on MCE under the same operating condition.