Q: What Is the Difference Between a 2-Cycle and 4-Cycle Engine

The difference between a 2-cycle engine and 4-cycle engine is the 2-cycle only needs one revolution of the crankshaft to get to a power stroke, while a 4-cycle engine needs 2 revolutions. To help explain the two different types of engines we will need to explain how each type of engine works first.

A two-cycle engine piston has only two strokes. The piston starts at top dead center (TDC) in its bore. As the piston moves down, the piston bore there is an exposed exhaust port and any exhaust is expelled from the cylinder, and as the piston continues to go down there is an intake port exposed when the piston reaches close to bottom dead center (BDC). As the exhaust is expelled, the intake port will let in the air fuel mixture till BDC is reached. As the piston starts going up, the piston covers the intake port then the exhaust port. As the piston continues to go up toward TDC, it is compressing the fuel and air mixture brought in from the intake port. Once TDC is reached, the compressed mixture is ignited by a spark plug. This ignites the mixture and produces the power stroke pushing the piston down toward BDC again. As the piston goes down producing power, when the exhaust port is exposed the remaining pressure is expelled as exhaust. The exhaust leaving the cylinder allows the in-rush of fuel and air to come into the cylinder via the intake port. The complete process is in one revolution of the crankshaft. Every time the piston is going down is a power stroke. When the piston is almost to BDC, it expels the exhaust and intakes the fuel and air. Going up is the compression stroke of the mixture.

A four-cycle engine has to make two complete revolutions of the crankshaft. The piston starts at TDC and as the piston starts to go down for its intake stroke, the intake valve opens to let in the fuel and air mixture all the way till BDC of the piston stroke. The intake valve closes and the piston travels to TDC compressing the mixture. This is the compression stroke. Then the mixture is ignited by a spark plug and this explosion pushes the piston down for the power stroke. The piston then travels to BDC. As the piston starts to come back up on its exhaust stroke, the exhaust valve is opened and the exhaust is expelled out the exhaust valve until the engine gets back to TDC. The intake stroke starts the cycle again.

Both of these types of engines can be found on gasoline and diesel engines. The diesel and gasoline engines work using the same principle, except the gasoline engine uses spark ignition and diesel uses compression ignition.