10. Study on High Efficiency Gasoline HCCI Lean Combustion Engines

The aim of this paper is to study the way of maximizing the thermal efficiency of gasoline engine. On the one hand, through the study of exergy loss in the combustion process, the thermodynamic parameters of low exergy loss in the combustion process were proposed, including low exergy loss equivalence ratio, low exergy loss initial combustion temperature, moderate EGR, high burst pressure, etc. On the other hand, the influence of engine thermodynamic cycle parameters on thermal efficiency, including engine compression ratio and specific heat ratio of charge, was studied. On the basis of the studies, the basic characteristics of high efficiency engine cycle, including lean burn, EGR, boosting and high compression ratio, were proposed. Then an experimental study on homogeneous compression combustion of gasoline with low exergy loss was carried out on a single cylinder engine. In order to suppress rough combustion, the equivalence ratio increases with the increase of load, and the boost pressure and combustion pressure must increase correspondingly. Appropriate increase of EGR can inhibit combustion rate, reduce heat loss and improve thermal efficiency. However, excessive EGR not only leads to the increase of incomplete combustion loss, but also reduces the thermal efficiency due to the decrease of specific heat ratio. What is interesting is that both the compression ratio and EGR can regulate the cycle temperature, but a change in the compression ratio does not change the specific heat ratio or the oxygen concentration. Therefore, thermodynamic parameters of low exergy loss and cycle parameters of high thermal efficiency are the approaches to achieve ultra-high thermal efficiency, whose limitation is the maximum burst pressure.