Investigation of Valve Timings on Lean Boost CAI Operation in a Two-stroke Poppet Valve DI Engine

摘要

Controlled Auto Ignition (CAI), also known as Homogeneous Charge Compression Ignition (HCCI), is one of the most promising combustion technologies to reduce the fuel consumption and NO_x emissions. In order to take advantage of the inherent ability to retain a large and varied amount of residual at part-load condition and its potential to achieve extreme engine downsizing of a poppet valve engine running in the 2-stroke cycle, a single cylinder 4-valves camless direct injection gasoline engine has been developed and employed to investigate the CAI combustion process in the 2-stroke cycle mode. The CAI combustion is initiated by trapped residual gases from the adjustable scavenging process enabled by the variable intake and exhaust valve timings. In addition, the boosted intake air is used to provide the in-cylinder air/fuel mixture for maximum combustion efficiency. In this study, the lean boost operation has been implemented and results show that CAI combustion can be operated between a speed range from 800rpm to 3000rpm (limited by the speed of the camless system) and an IMEP range from 1.1bar to 7.8bar. In particular, effects of Exhaust Valve Opening (EVO) and Intake Valve Closure (IVC) timings on the gas scavenging process and substantial combustion process and emissions were investigated at some different load points. The results show that EVO mainly affects the thermodynamic efficiency through effective expansion ratio. IVC has strong effect on CAI combustion phase and duration through effective compression ratio.