In vehicle application, most of time gasoline engines are part load operated, especially in city traffic, part load operation covers most common operation situations, however part load performances deteriorate due to pumping losses and low thermal efficiency. Many different technologies have been applied to improve part load performances. One of them is to adopt over-expanded (Atkinson/Miller) cycle, which uses late/early intake valve closing (LIVC/EIVC) to reduce pumping losses in part load operation. But over-expanded cycle has an intrinsic drawback in that combustion performance deteriorates due to the decline in the effective compression ratio (CR). Combining with high geometry CR may be an ideal solution, however there is a trade-off between maintaining a high CR for good part load fuel consumption and maintaining optimal combustion phasing at higher load. As a result, variable compression ratio systems, which include variable engine mechanisms and variable timing, need to be implemented to resolve these problems.
This paper analyzes potential benefits of meaningful concept of high CR over-expanded cycle gasoline engine combining over-expanded cycle with high geometry CR, while two stage variable valve lift (VVL) system is employed to achieve functional synergies. Compared with conventional over-expanded cycle engines, Atkinson cycle and Miller cycle work in concert to overcome their own drawbacks. LIVC (Atkinson cycle) with high valve lift is implemented at high load to reduce effective CR, which can optimize combustion phasing and reduce knock tendency. But thermal efficiency is still relatively high due to high geometry CR and un-affected expansion ratio. At part load, EIVC(Miller cycle) with low valve lift is adopted, effective CR is maintained at a high level to improve thermal efficiency, at the same time, pumping losses decrease as volumetric efficiency is reduced. The paper describes the fuel economy benefit obtained with this concept. In addition, power performance is also discussed.