A Study on Charge Motion Requirements for a Class-Leading GTDI Engine 2017-24-0065
An integral part of combustion system development for previous NA gasoline engines was the optimization of charge motion towards the best compromise in terms of full load performance, part load stability, emissions and, last but not least, fuel economy. This optimum balance may potentially be different in GTDI engines. While it is generally accepted that an increased charge motion level improves the mixture preparation in direct injection gasoline engines, the tradeoff in terms of performance seems to become less dominant as the boosting systems of modern engines are typically capable enough to compensate the flow losses generated by the more restrictive ports. Nevertheless, the increased boost level does not come free; increased charge motion generates higher pumping- and wall heat losses. Hence it is questionable and engine dependent, whether more charge motion is always better.
Besides from the above mentioned tradeoff between pumping / wall heat losses and burn rate, emissions etc. another aspect is the optimum charge motion level for best knock performance at high load. A high charge motion level leads to a faster combustion and by that the knock critical end-gas areas are burned faster. On the other hand, the faster burn rate leads to increased pressure in the end-gas area which then reduces the self-ignition time of the unburnt mass. Hence the optimum charge motion level to suppress knock is a parameter which needs to be tuned carefully.
To understand the influence of charge motion level on fuel consumption, stability, emissions, and performance, the very successful Ford 1.0l 3-cylinder gasoline Ecoboost® engine, which is available in many Ford vehicles worldwide, has been modified such that different tumble levels could be realized. Furthermore, the intake valvetrain system has been modified in a way that asynchronous valve opening timings for each of the two intake valves of the cylinders could be selected individually. By this measure, the intake event could be varied and a swirl flow is introduced.
With the above mentioned engine hardware modifications, this study investigates the influence of charge motion on important combustion characteristics. To improve the understanding of the dyno-based results, 1D and 3D simulations have been conducted. The results obtained during this project will be discussed in depth in this paper.
It is shown that an increased charge motion level improves dilution tolerance and burn velocity but the associated increase in pumping and wall heat losses leads to a deterioration in fuel consumption. For full load operation the higher charge motion level leads to increased knocking.
The overlay of swirl and tumble has a significant effect on mixture preparation and homogenization. In this case this combination of charge motion types leads to lean areas around the spark plug.