Evaluation of variable compression ratio (VCR) and variable valve timing (VVT) strategies in a heavy-duty diesel engine with reactivity controlled compression ignition (RCCI) combustion under a wide load range

Abstract

Variable compression ratio (VCR) and variable valve timing (VVT) are two effective strategies to adjust the effective compression ratio, which is beneficial for controlling the combustion process of advanced combustion modes. In this study, systematic evaluation of the two strategies was conducted based on reactivity controlled compression ignition (RCCI) engine in terms of combustion process control, fuel efficiency, and emission characteristics. By coupling an updated KIVA-3V code with the genetic algorithm, the combustion of a heavy-duty RCCI engine with VCR and VVT strategies was respectively optimized, aiming to simultaneously realize high fuel efficiency and low emissions. The optimal VCR and VVT strategies were compared under a wide load range. The results indicate that, at low and mid loads, high effective compression ratio, large premix ratio, and early fuel injection can be utilized to realize Euro 6 nitrogen oxides (NOx) limit with ultra-low soot emissions and low fuel consumption for both VCR and VVT strategies. The increase of load from low to mid narrows the optimal range of exhaust gas recirculation (EGR) rate for VVT strategy whereas the range for VCR strategy is still wide. At high load, compared to VVT strategy, a further decreased effective compression ratio can be utilized for VCR strategy, which allows early fuel injection, leading to the improvements of fuel efficiency and soot emissions. This suggests that the VCR strategy is more practical for high-load operation of RCCI combustion and the commercialization the RCCI engine in the future compared to VVT strategy.