An analysis of limits for part load efficiency improvement with VVA devices

Highlights: • Variable valve actuation aims at reducing pumping losses for spark-ignition engines. • Fully unthrottled operation is never reached because of combustion degradation. • Present paper quantifies the combustion degradation origins for various strategies. • Fully unthrottled CAI combustion mode is a non-combustion-limited alternative. • Combustion limitation is, however, replaced by a heat loss limitation. - Abstract: The implementation of Variable Valve Actuation (VVA) in Spark-Ignition (SI) engines generally aims at increasing part-load efficiency by reducing pumping losses. However, any innovative valve strategy has effects on the combustion process itself, introducing new limitations and mitigating the fuel consumption benefits. The experimental analysis of such valve strategies identifies the optimum settings but does not explain the origin of benefits and the sources of unexpected drawbacks. In the present study, the experimentally-optimised operating conditions for different valve strategies were numerically compared with 3D CFD to gain knowledge about causes for efficiency benefits and consequences of valve strategy on combustion progress. We compared standard SI operation in a single-cylinder port-fuel injection gasoline engine to mixture leaning, early intake valve closure (Miller cycle), late intake valve closure (Atkinson cycle), as well as Controlled Auto-Ignition (CAI). All alternative methods reduced pumping work and improved fuel consumption. However, all alternative methods also altered combustion progress and thermodynamic state within the combustion chamber, so that the observed fuel consumption benefits never reached the expected values. An energy balance provided the additional losses induced by each strategy while in-cylinder turbulence and temperature quantification helped explain the trends in combustion speed.