Variable Valve Timing (VVT) technology is more and more adopted in modern spark-ignition engines for the optimization of torque delivery. Furthermore, a proper choice of valve timing could reduce the typical pumping losses of these engines thus improving fuel economy at part load.
VVT mainly influences gas exchange processes, then the engine volumetric efficiency; in some circumstances, variations of valve timing could modify the charge composition and therefore the flame development and propagation.
In this paper, the combustion process of a small displacement, 2 valve, spark-ignition engine, with variable valve timing, has been numerically and experimentally analyzed.
The use of VVT allows obtaining combined internal EGR and Reverse Miller Cycle effects so to achieve a significant dethrottling at part load operation.
A 3-D computer code has been utilized in order to calculate the details of the flow field within the cylinder and the combustion rate at different valve points. Calculated in-cylinder pressure values have been compared to measured data. Retarded positions of the cam phaser allow controlling engine load with a moderate increase in pumping work, however the consequent increase in residual gas fractions could worsen the combustion quality. This point has been analyzed in details by means of both numerical simulations and experimental tests.
The VVT technology has been resulted an effective way in reducing pumping losses at part load; associating this technique to charge motion optimization allows obtaining good combustion quality. In a few words, the engine efficiency could be increased.