Thermodynamic Analysis of Variable Valve Timing Infuence on SI Engine Efficiency

The large number of mechanical, electro-magnetic and oleo-dynamic systems for variable valve actuation developed by automotive suppliers demonstrates the great interest that is being devoted to their potential application on internal combustion engines.

In the paper, a possible strategy to realize an original engine load control by means of both intake and exhaust variable valve timing (VVT) is briefly presented and the thermodynamic analysis of the performance obtainable with this solution is carried out. The peculiarity of this strategy is that it is possible to directly recirculate the desired mass of exhaust gas with less limitation with respect to the external duct architecture. To highlight its potentiality this solution has been compared with two different engine architectures:

• a traditional Spark Ignition (SI) engine, where the load control is demanded to the throttle body;
• an engine equipped with a VVT system only on the intake valves, where the load control is performed by means of the well-known Early Intake Valve Closing (EIVC) strategy.

The proposed engine load control technique shows a relevant efficiency increase at partial load, with respect to both throttled and EIVC engines. The obtained results are certainly overestimated, due to the theoretical thermodynamic approach used to compare the different engine architectures, but the relevant efficiency increase shown in the paper seems to leave a good possibility to find in real experimentation a still appreciable improvement with respect to a throttled engine.