Impacts of VVT Coupled Miller Cycle Strategies (LIVC and EIVC) on Thermal Efficiency in a Boosted DISI Engine

抄録

The downsized boosted direct-injection spark-ignition (DISI) has become one of the most promising technique to improve engine fuel economy. However, the geometric compression ratio of downsized boosted engines is often lower than that of naturally aspirated engines because of the higher knocking tendency resulted from high boosting ratio, which deteriorates the fuel economy of boosted DISI engines. It has been proved that Miller cycle has the potential to decrease the effective compression ratio, which can ease or solve the knocking issue. In this paper, the effects of VVT coupled Miller cycle strategies on thermal efficiency was investigated on a 2.0 L boosted DISI engine over a wide range of engine operating conditions. Miller cycle strategies are realized by two different camshaft structures, which are late intake valve closing (LIVC) and early intake valve closing (EIVC) camshafts. A large map of engine experiments were conducted, in which the engine speed spans 1000 rpm to 2500 rpm with the load ranging from low to high. Comparing different Miller cycle strategies with the reference case (using base camshaft), it was found that an up to 9.56% reduction of fuel consumption was achieved. Also, experimental results show that, at high load, the fuel consumption decreased with the delay of intake valve opening (IVO) under LIVC; whereas that under EIVC first declines and then increases. At part load, brake specific fuel consumption (BSFC) for EIVC monotonically decreases as the retard of IVO; while BSFC for LIVC rises first and then decreases. In order to fully understand the mechanism of fuel consumption, contributing factors to thermal efficiency were quantified under both part load and high load conditions. It was found that the degree of constant volume heat release and mechanical efficiency are the first order of importance to thermal efficiency for both LIVC and EIVC under high and part loads, respectively, despite of the different trend of IVO effects.