To solve this problem,a 4-stroke secondary expansion mode is proposed in the paper,which has an advantage on fuel consumption in the low load condition.The 2/4-stroke switchable expansion IC engine is an engine whose expansion cylinder could switch its operation between 2-stroke and 4-stroke to achieve high fuel efficiency in both high and low load condition.With the development of camlessvalve-train system,such as electro-mechanical valve and electro-hydraulic valve,the valve time can be controlled flexibly,so the 2/4-stroke switchable engine has already been proposed in previous work.Ricardo proposed a 2/4 sight engine concept,and Y.Zhang also developed a 2/4-stroke switchable GDI engine