The two stroke engine is an engine which only produces power on every second stroke of the piston. They are not in common use as automobile engines, but are used mainly for powering motorcycles and lawnmowers. However, many large marine diesel engines are of the two stroke variety with some having twelve cylinders and producing up to 20,000hp. Two stroke diesel engines may also be found in heavy commercial vehicles and locomotives.
In the two stroke engine as the piston moves up on the compression stroke it uncovers the inlet port and allows fuel mixture to enter the crankcase. As it reaches the top of the cylinder the fuel mixture is ignited and the piston is forced back down the cylinder. Towards the bottom of its stroke the piston uncovers the exhaust port and the used gases start to escape, at the same time as the exhaust port is opened a transfer port opens allowing the fresh fuel mixture to enter the cylinder from the crankcase. The top of the piston is shaped to direct the fuel mixture towards the top of the cylinder. As the fuel mixture enters the top of the cylinder the increased pressure helps to force the remaining exhaust gases through the exhaust port. The main disadvantage with the two stroke engine is that when the exhaust gases are pushed through the exhaust port some of the fresh fuel mixture also gets expelled. The same can be said with the exhaust gases as some are mixed with the fresh fuel mixture to be reburnt on the power stroke. Though two stroke engines are simpler in operation than four stroke engines, they are not as fuel efficient as a percentage of fuel is lost without ever being used.
