Mapping of an HCCI engine using negative valve overlap strategy

In order to control combustion phasing and extend operation range of homogeneous charged compression ignition (HCCI) engines, several techniques such as controlling inlet air temperature and pressure, using alternative fuels, variable compression ratio, supercharging, etc. have been applied. Among them, variable valve mechanism received much attention to extend HCCI operating range and control autoignition. Mode switching between spark plug ignition (SI) to HCCI has been started to use widely because of limited operation range of the HCCI mode. To control of an engine switching combustion modes between SI and HCCI, embedded system needs parametrical control maps of the engine. In this study, operation range, air excess ratio, torque and thermal efficiency maps of an HCCI engine using negative valve overlap (NVO) strategy were obtained and additionally the effects of NVO on HCCI combustion in view of in-cylinder temperature, heat release rate, effective thermal efficiency, combustion phasing, and exhaust emissions were investigated. Also, variation of CO, HC, and NOx emissions were evaluated at different valve overlaps. Three different valve overlap including +8, −8, and −16 CA were used in the experiments. Test results showed that in-cylinder pressure and heat release decreased with NVO. The test engine could run without knocking by application of NVO. It was seen that operating range of HCCI extended with NVO. It was also found that effective thermal efficiency increased with −16ºCA valve overlap compared to other valve timings.