A four-stroke engine is a type internal combustion engine, an alternative engine. It can work both in diesel cycle and Otto cycle ( gasoline engine). It is therefore a thermodynamic and combustion motorcycle.
The most important feature is that you need to perform four strokes of the piston or plunger to complete the cycle. During these 4 races the crankshaft makes two full turns. These four times are: admission, compression, combustion or explosion and escape.
In 1861, the German Otto experimented with his first 4-stroke gas engine. Otto had to abandon the project due to technical difficulties.
Later, in 1867 the Germans Otto and Langen devised a 4-stroke gasoline engine, with the ignition of the compressed mixture, which they presented the following year in Paris.
Otto's great success was to spark the compressed mixture instead of just suctioning, which increased both the efficiency and the performance of the engine. The gasoline engine (Otto cycle engine) carries a spark plug to generate this spark inside it.
The 4-stroke thermal engines carry out the cycle in 4 phases or times. The vast majority of endothermic engines are 4-stroke.
The 4-cycle cycle each phase corresponds approximately to a piston stroke. The phases included are the following phases:
First Time - Admission of the Load in the Cylinder
First time or admission: in this phase the descent of the piston sucks the mixture of combustible air in spark ignition 4-stroke engines (Otto) or air in compression ignition engines.
The exhaust valve remains closed, while the intake valve is open. In the first time the crankshaft gives 180º and the camshaft gives 90º and the intake valve is open and its stroke is descending.
During the first time, the piston in the run to the bOttom creates an aspiration of the fluid in the cylinder. In the combustion chamber, the suction valve is opened in due time to allow the entry of air or the combustible gas mixture.
The valve starts to open before the start of the race and closes after the race.
Second Time - Compression of the Load on the Cylinder
The piston reaches the lower end of stroke. In this second time, the intake valve closes, the piston starts to rise compressing the gas contained in the chamber. In the second time the crankshaft gives 360º and the camshaft gives 180º, and also both valves are closed and its race is ascending.
Closed suction valve during the return stroke of the piston, the load is compressed in the combustion chamber to a maximum value, which is reached at the end of said stroke.
At this moment, the volume of the load is reduced to a fraction of the volume that was at the beginning of the race; this fraction esl a of the volumetric compression ratio.
Third Time - Combustion and Expansion
As it can not reach the upper end of the stroke, the gas has reached the maximum pressure. In gasoline engines, the spark in the spark plug jumps causing the mixture to ignite, while in diesel engines, the fuel that self-ignites is injected with the pressure and temperature inside the cylinder is injected with a syringe.
In both cases, both in the diesel engine and in the Otto engine once the combustion starts, this progresses rapidly increasing the temperature inside the cylinder and expanding the gases that push the piston.
This is the only phase of the 4-stroke engine in which work is obtained. At this time the crankshaft is 180º while the camshaft is 240º, both valves are closed and its stroke is descending.
Shortly after the end of the compression stroke, the ignition of the mixture takes place by means of an electric spark, or also the spontaneous ignition of the fuel injected into the combustion chamber, with the consequent increase in temperature and pressure caused by the heat of combustion.
The value reached by the pressure after the ignition is approximately 2 to 4 times higher than it was initially, so that the piston is pushed down. Before the work stroke is completed, the exhaust valve begins to open and the combustion gases, which are still under some pressure, start to come out.
Fourth Time - Expulsion or Escape of Combustion Products
In this phase of the 4-stroke engine the piston carefully pushes, in its upward movement, the combustion gases that come out through the exhaust valve that remains open. At the end of the upper stroke, the exhaust valve closes and the intake valve opens, restart the cycle. At this time the crankshaft is 360º and the camshaft is 180º and its stroke is ascending.
During the next stroke the return to the top dead center (P.M.S), the piston expels the combustion gases through the exhaust valve.
At the end of the race, or shortly after, the exhaust valve closes. Meanwhile, the suction valve remains open and a new cycle begins, which is repeated regularly.