Study on the combustion characteristics of a high compression ratio HCCI engine fueled with natural gas

Homogenous charge compression ignition (HCCI) is a hybrid combustion technology that can achieve high thermal efficiency, low fuel consumption, lower nitrogen oxide (NOx) and particulate matter (PM) emissions. However, it is difficult to control the combustion process in a successful HCCI engine. In order to put forward effective methods to control combustion process at different conditions, the combustion characteristics at different intake temperature (T-in) and excess air ratio (lambda) were studied at a HCCI engine fueled with natural gas (NG). The sensitivity analysis of start of combustion (SOC) to T-in and lambda was carried out at different speeds. It was determined that in-cylinder pressure and heat release rate (HRR) increased, SOC advanced and duration of combustion (DOC) shorted with the increase of Tin, while the in-cylinder pressure and HRR decreased, SOC retarded and DOC prolonged with the mixture getting leaner. The results show that SOC can be effectively controlled by changing fuel concentration and activation group concentration, such as adding EGR, stratification of mixture and changing fuel components at lower and higher speeds; and SOC can be effectively controlled by controlling the temperature and pressure in cylinder, such as adding EGR, adding water and changing inlet temperature at medium speed. The coordinated control of combustion boundary conditions and fuel chemistry is the core academic idea of HCCI combustion. In addition, the correlation between CA50 and brake thermal efficiency was proposed.