A Comparison Study on the Performance of the Multi-Stroke Cycle SI Engine under Low Load

Pumping Mean Effective Pressure (PMEP) is the main factor limiting the improvement of thermal efficiency in a spark-ignition (SI) engine under low load. One of the ways to reduce the pumping loss under low load is to use Cylinder DeActivation (CDA). The CDA aims at reducing the firing density (FD) of the SI engine under low load operation and increasing the mass of air-fuel mixture within one cycle in one cylinder to reduce the throttling effect and further reducing the PMEP. The multi-stroke cycles can also reduce the firing density of the SI engine after some certain reasonable design, which is feasible to improve the thermal efficiency of the engine under low load in theory. The research was carried out on a calibrated four-cylinder SI engine simulation platform. The thermal efficiency improvements of the 6-stroke cycle and 8-stroke cycle to the engine performance were studied compared with the traditional 4-stroke cycle under low load conditions. The simulation results showed that multi-strokes consumed less energy in pumping loss and heat transfer loss compared with the four-stroke cycle, but more in frictional loss and exhaust energy loss. The relationship between the two groups influences the improvement in thermal efficiency. When the average torque was lower than 70Nm, the eight-stroke cycle had the most obvious improvement on Brake Specific Fuel Consumption (BSFC), and the maximum increase was 25.6%, which was higher than the low-pressure six-stroke cycle (LSC) and the high-pressure 6-stroke cycle (HSC). However, the optimization range of thermal efficiency and the power range of different stroke cycles influenced each other. The stronger the optimization effect was, the narrower the corresponding power range was.