To provide a method for taking a process for expanding longer than a process for actually compressing a four cycle diesel engine in terms of strokes, and to obtain measures when the process is taken excessively long.

In the compression process, an exhaust valve is opened and closed at a bottom dead center and before a top dead center, respectively. The exhaust valve is opened as measures when the exhaust valve is opened excessively in the compression process, and before excessive expansion results by explosion for resisting rotation in the expansion process, respectively, and is closed at the bottom dead center.