In order to enable the inner surface of the cylinder to work normally at high temperature, the cylinder and cylinder head must be properly cooled. There are two cooling methods, one is water cooling and the other is air cooling. The water-cooled engine has cooling water jackets around the cylinder and in the cylinder head, and the cylinder block and cylinder head cooling water jackets are connected. The cooling water circulates continuously in the water jacket, taking away some heat and cooling the cylinder and cylinder head.