Part-load performance and emissions analysis of SI combustion with EIVC and throttled operation and CAI combustion

ABSTRACT Direct Injection (DI) gasoline engines are staging a come-back because of its potential for improved fuel economy through principally the engine down-sizing by boosting, stratified charge combustion or possibly Controlled Auto Ignition (CAI) at part load operations. The problem with the Spark Ignition (SI) engine is its inherent low part-load efficiency. This problem arises due to the pumping loses that occur when the throttle closes or partially opens. One way of decreasing the pumping losses is to operate the engine lean or by adding residual gases. It is not possible to operate the engine unthrottled at very low loads due to misfire. However, the load can also be controlled by changing the valve closing timing – either early or late intake valve closing. Both strategies reduce the pumping loses and hence increase the efficiency. However the early intake valve closure (EIVC) can be used as mode transition from SI to CAI combustion. In order to investigate and develop a more efficient DI gasoline engine to overcome the stated challenges, an advanced research single cylinder engine with electro hydraulic valve actuation has been developed and operated under different combustion modes. In this paper, the performance, combustion and emission were measured and compared between throttled SI, EIVC and CAI combustion with negative valve overlap (NVO). At part-load condition, it is found that the CAI combustion produced the lowest fuel consumption and NOx emissions. The EIVC operation led to a moderate improvement in the fuel conversion efficiency over the throttled SI operation but it was characterised with the slowest combustion and worst HC emissions. The particulate emission results showed that soot is the dominant particles in the exhaust, which could be reduced by leaner mixture combustion.