The Otto cycle engine is an internal combustion engine in which each cylinder needs four strokes of its piston to complete the sequence of events which produces one power stroke. Therefore, it is also called the ‘four-stroke cycle‘. Both spark-ignition and compression-ignition engine use this operating system. It is named after its inventor Nicolaus Otto, who built the first engine operating on these principles in 1876. It includes the following stages:
1- An intake stroke- It starts with the piston at top center (TC) crank position and ends with the piston at bottom center (BC), drawing fresh mixture into the cylinder. Then the inlet valve opens shortly before the stroke starts, closing after it ends.
2- A compression stroke- When both valves are closed, the mixture inside the cylinder is compressed to a small fraction of its initial volume. Combustion begins towards the end of the compression stroke as the cylinder pressure rises faster.
3- Expansion stroke- It starts with the piston at TC and ends up at BC as the hot and high-pressure gases push the piston down, forcing the crank to rotate. As the piston approaches BC, the exhaust valve opens to begin the exhaust process. About five times more work is done on the piston during the expansion stroke than during the compression.
4- Exhaust stroke- It is when the remaining burned gases exit the cylinder. As the piston approaches TC, they are blown out of the cylinder by the piston as the inlet valve opens. Then the exhaust valve closes and the whole cycle starts again.
You must remember that the Otto cycle needs, for each engine cylinder, two crankshaft revolutions for each power stroke.
Below, diagrammatic drawing which shows the Otto cycle.
