Variable Valve Timing for Fuel Economy Improvement in a Small Spark-Ignition Engine

Nowadays, since fuel economy for the green-house effect reduction is a primary goal of every energy conversion system, the typical fall of partial load efficiency of spark-ignition engines is a no more tolerable inconvenience. Numerous solutions have recently been proposed. In this paper, the potential of a simple Variable Valve Timing (VVT) system has been investigated. This system has been designed to update a small displacement engine pursuing the objective of optimizing both engine performance and, particularly, fuel consumption at part load operation. A Continuously Variable Cam Phaser (CVCP), able to produce a Reverse Miller Cycle effect during the intake phase and a significant internal EGR generation at the end of the exhaust stroke, has been introduced. A numerical approach, based on both 1-D and 3-D computational models, has been adopted in order to evaluate the engine performance when load is controlled by the VVT system and to deeply investigate the influence, on in-cylinder phenomena, of the valve timing variation. The numerical analysis has shown that such unfavourable conditions could rise at strongly retarded valve timings as very high EGR rates. In these situations, an accurate optimization of the flow field within the combustion chamber is necessary in order to balance the effects of an excessive charge dilution. To this aim, a particular solution has been proposed and satisfactory results have been obtained. In this way, the VVT system here analyzed revealed as an effective tool in reducing the pumping losses, hence the specific fuel consumption, at partial load. The numerical approach results have been often compared to the experimental results obtained at the engine manufacturer’s research centre. A good agreement between calculated and measured values has been achieved.