Four stroke cycle

The four piston strokes.

A four-stroke engine is an alternative internal combustion engine of both the Otto cycle and the diesel cycle, which requires four strokes of the piston or plunger (two complete turns, but of the crankshaft) to complete the thermodynamic cycle of combustion. These four times are: admission, compression, combustion or explosion or expansion and exhaust.

In 1861, German Nicolaus Otto experimented with his first four-stroke gas engine, which he had to abandon due to technical difficulties. In 1867 the Germans Otto and Langen devised a four-stroke gasoline engine, with a mixture ignition (gasoline, compressed air), which they presented the following year in Paris. Otto's great success was sparking the compressed mixture instead of just aspirated, which increased both the efficiency and performance of the engine.

Cycle times

Here the different times (activities carried out during the cycle) and their characteristics are detailed.

1. First time or admission: In this phase the decrease of the piston aspires to the mixture of fuel air in the provoked ignition engines (MEP) or air in compression ignition engines (MEC). The exhaust valve remains closed, while the intake valve is open. In the first time the stork rotates 180° and the cam tree gives 90°. The admission valve is open and the piston stroke is descending.
2. Second time or compression: Upon reaching the end of the lower race, the admission valve closes, compressing the gas contained in the chamber for the ascent of the piston. In the second time the stork gives 180° and the cam tree gives 90°, and also both valves are closed and their career is ascending.
3. Third time or explosion/expansion: Upon reaching the end of the higher race the gas has reached the maximum pressure. In the triggered ignition engines or Otto cycle the spark jumps in the spark plug, causing the inflammation of the mixture, while in the diesel engines, the very pulverized fuel is injected through the injector, which is self-inflamed by the pressure and temperature existing inside the cylinder. In both cases, once combustion is started, it progresses rapidly increasing temperature and pressure inside the cylinder and expanding the gases that push the piston. This is the only phase in which work is obtained. At this time the stork rotates 180° while the cam tree rotates 90° respectively, both valves are closed and their career is descending.
4. Fourth time or escape: In this phase the piston pushes, in its upward movement, the combustion gases that come out through the exhaust valve that remains open. Upon reaching the top race point, the exhaust valve is closed and the admission valve is opened, the cycle restarted. At this time the stork turns 180° and the cam tree turns 90°.