Comparative study on the pumping losses between continuous variable valve lift (CVVL) engine and variable valve timing (VVT) engine

Highlights: • The gross IMEP is approximately proportional to engine cylinder pressure at BDC. • The mechanism and influence factors of intake loss in CVVL and VVT engines were compared. • The theoretic limitations of engine intake loss were deduced. • The suggestions were proposed for CVVL engine to approach the theoretical minimum pumping loss. • The BSFC of CVVL engine can be further reduced by more than 20%. VVL and VVT are two common approaches to reduce engine pumping loss at part loads, however, the comparative study on their potentials and influence factors of pumping loss reduction is scarce. In this paper, the gas exchange processes of CVVL and VVT engines were investigated by bench testing. The mechanism and influence factors of pumping loss in the two types of engines were comparatively analyzed, the theoretic limitations of pumping loss in intake process were deduced and the suggestions to approach the theoretic minimum were proposed. The results show that the gross indicated mean effective pressure (IMEP) of engine is approximately proportional to cylinder pressure at bottom dead center (BDC) for both traditional throttle control strategy and early intake valve closing (EIVC) strategy. The difference of pumping losses between CVVL and VVT engines results from intake process. Due to the differences of intake modes, the CVVL engine has lower intake loss than VVT engine by nature. The pumping loss in intake process of CVVL engine is much closer to the theoretical minimum, and the BSFC of CVVL engine can be reduced by more than 20% at 2000 r/min and 2.3 bar IMEP by increasing the maximum lift of intake valve. All these have provided guidance for pumping loss reduction of gasoline engine.