An evaluation of a 2/4-stroke switchable secondary expansion internal combustion engine

To develop an internal combustion (IC) engine with high energy conversion efficiency, simple structure, and high reliability, a 2/4-stroke switchable secondary expansion IC engine concept is proposed in this paper. The secondary expansion means the burned gas, after working in a traditional 4-stroke combustion cylinder, is transferred into another cylinder to expand again. The 2/4-stroke switchable secondary expansion IC engine is a 4-cylinder engine with two combustion cylinders and two expansion cylinders. The expansion cylinder could switch its working mode between 2-stroke and 4-stroke to perform different secondary expansion modes just by varying the valve timing. The 2-stroke secondary expansion mode has great advantages in high load condition on both fuel efficiency and power output, but it is not suitable in low load condition, so the 4-stroke secondary expansion is applied to maintain the high energy conversion efficiency in the low load condition. A traditional 4-cylinder gasoline IC engine was transformed into a secondary expansion IC engine, however, without the valve time switching device, its expansion cylinder could only work on the 4-stroke working mode. A GT-Power simulation model was established as the supplement of the experiment to evaluate the 2/4-stroke switchable secondary expansion IC engine. The experiment results showed that the fuel consumption of the engine working on the 4-stroke secondary expansion mode was lower than that of the original engine. According to the simulation results, compared with the original engine, the fuel saving rate map of the 2/4-stroke switchable secondary expansion IC engine was made by combining the high fuel saving rate conditions of the 2-stroke and 4-stroke working modes. This innovative engine could save fuel up to 10%–18% in all frequently-used working conditions of a general vehicle IC engine.