How the 4-stroke engine works

At present, 4-stroke engines are the most common and have been imposed on 2-stroke engines as they are less polluting than these. An internal combustion engine with a 4-stroke cycle is made up of a cylinder, a connecting rod, a crankshaft, at least two valves, a spark plug and many other components that make everything work in a coordinated way. To understand how it is possible for a mixture of gasoline and air to become moving, we will explain, one by one and thanks to this article , each of the 4 strokes of this type of combustion engine, or also called an Otto engine .

Index of contents

4-stroke engine cycle

The times of a combustion engine are 4 and each one performs a different function, namely:

How does a four-stroke engine work?

Time #1: Admission

In the first time a  mixture of gasoline and air will enter the combustion chamber of the cylinder

To do this, the piston goes down from the upper point of the cylinder to the lower point, while the intake valve (or valves) opens and allows the mixture of gasoline and air to enter the cylinder, to be closed later.

Gasoline is combined with air since, by itself,  gasoline alone would not burn and requires oxygen for combustion.

The theoretical relationship is 1 gram of gasoline for 14.8 grams of air, but it depends on many factors, such as the density of that air. For this reason, in modern engines, one examines the excess combustion gases and informs the control unit about how the proportion of the gasoline/air mixture to be supplied by the injectors should be.

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Time #2: Compression

four-stroke engine cycle In the second stroke, with the piston at its lowest position and the combustion chamber filled with gasoline and air, the intake valve closes, leaving the chamber hermetically sealed. The inertia of the crankshaft to which the piston connecting rod is attached will cause the piston to rise again and thus compress the mixture . Gasoline and air are compressed within an airtight chamber and, by reducing the space in such a way, the molecules collide each other by increasing the temperature of the mixture . Gasoline and air are ready for the third stage: combustion.

Time #3: Combustion

In the third stroke, with the piston in its highest position and compressing the mixture of gasoline and air, is when  the spark plug comes into action.

It is at this precise moment, with the mixture compressed and at a high temperature, when  the spark plug generates a spark that makes the mixture explode violently.

Combustion pushes the piston down forcefully and the connecting rod and crankshaft are responsible for converting this linear movement of the piston, from top to bottom, into a rotating movement.

Time #4: Escape

In the fourth stroke, the last of this process and which will mean the fourth stroke of the piston and the second turn of the crankshaft, the piston is in its lowest part again and with the combustion chamber full of gases burned by combustion products. of gasoline and air.

The piston goes back up in this fourth stroke and in doing so  pushes those gases up to leave the exhaust valve  that opens to let them out and leave the cylinder chamber empty again. Not like during compression, which stayed closed.


It is now, with the piston back at the top, that the cycle starts again from the beginning.

The piston will go back down as the intake valve opens and lets in a new mixture of gasoline and air, and so on.

The cycle of a 4-stroke engine seems simple but imagine you drive your motorcycle at 6,000 rpm, that means that this cycle happens about 50 times per second , that is, 50 explosions per second, which translates into 100 crankshaft revolutions per second. . Something that is difficult to imagine, and even more so if we imagine the engine of a sports motorcycle rotating at 14,000 revolutions per minute .

To better understand the operation of a 4-stroke engine, we recommend this explanatory video on the cycles of a 4-stroke engine with which you can see each element and each time perfectly explained.

What do you think? Has it helped you understand how a 4-stroke engine works ?

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