The four stroke engine

The four strokes of a four-stroke engine refer to intake, compression, combustion, and exhaust. It was first proposed by Nikolaus Otto in 1876 and is also known as the Otto cycle.

The 4-stroke technical terminology is a four-stroke cycle. Four-stroke engines are widely used in current internal combustion engines due to their high ventilation efficiency. Most car and truck engines use 4-stroke.

The four stroke engine

The four cycles correspond to a complete cycle of the internal combustion engine. It is worth noting that the internal combustion engine only outputs energy outwards in the third stroke (the stroke of the piston moving to the bottom dead center during combustion), and the energy in the other strokes is provided by the rotational energy of the flywheel.

The four strokes of a 4-stroke internal combustion engine are as follows:

In the four strokes, the direction of piston movement changes in the two adjacent strokes. The internal combustion engine completes a complete cycle (4 strokes) and the crankshaft rotates 720°.

Intake stroke

When the intake valve is opened and the exhaust valve is closed, the piston moves from the top dead center to the bottom dead center.

Intake stroke

The cylinder volume above the piston increases, and the vacuum is generated. The pressure in the cylinder drops below the intake pressure, and the vacuum suction is passed.

The gasoline vaporized by the carburetor or the gasoline injection device mixes with the air to form a combustible mixture, which is sucked into the cylinder by the intake port and the intake valve.

The intake process continues until the piston passes the bottom dead center and the intake valve closes. The ascending piston then begins to compress the gas.

Compression stroke

4-stroke gasoline engines compression stroke

Gasoline engine compression stroke

For 4-stroke gasoline engines, the intake and exhaust valves are all closed. The piston moves upward to the top dead center and the combustible mixture in the cylinder is compressed.

The temperature of the mixture rises and the pressure rises.

Before the piston approaches the top dead center, the pressure of the combustible mixture rises to about 0.6 to 1.2 MPa.

At the end of compression stroke, the temperature can reach 330 ° C to 430 ° C.

4 stroke diesel engine compression stroke

Diesel engine compression stroke

The working principle of the four-stroke diesel engine is the same as that of the four-stroke gasoline engine.

It is also composed of four components: intake, compression, work and exhaust.

The difference is that the intake stroke of the diesel engine is pure air. When the compression stroke approaches the top dead center, the diesel fuel is injected into the combustion chamber by the injector.

Since the temperature in the cylinder has far exceeded the auto-ignition temperature of the diesel at this time, the injected diesel fuel is burned by itself after a short ignition delay, and the work is performed externally.

Combustion stroke

Combustion stroke

When the compression stroke approaches the top dead center, the spark plug mounted above the cylinder head emits an electric spark to ignite the compressed combustible mixture.

The combustible mixture emits a large amount of heat after combustion, and the gas pressure and temperature in the cylinder rise rapidly. The maximum combustion pressure can reach 3-6 MPa, and the maximum combustion temperature can reach 2 200 °C to 2 500 °C.

The high temperature and high pressure gas pushes the piston to move to the bottom dead center quickly, and works externally through the crank connecting rod mechanism.

At the beginning of the power stroke, the intake and exhaust valves are closed.

Exhaust stroke

When the power stroke is nearing the end, the exhaust valve is opened. Since the pressure in the cylinder is higher than the atmospheric pressure, the high-temperature exhaust gas is quickly discharged out of the cylinder.

Exhaust stroke

This stage belongs to the free exhaust stage, and the high-temperature exhaust gas is discharged through the exhaust valve at the local sound speed.

As the exhaust process progresses to the forced exhaust phase, the piston moves past the bottom dead center to the upper dead center, forcibly exhausting the exhaust gas in the cylinder, and the exhaust process ends when the piston reaches the vicinity of the top dead center.

At the end of the exhaust, the gas pressure in the cylinder is slightly higher than the atmospheric pressure, about 0.105 to 0.115 MPa, and the exhaust gas temperature is about 600 ° C to 900 ° C.

Since the combustion chamber occupies a certain volume, it is impossible to completely remove the exhaust gas at the end of the exhaust, and the remaining part of the exhaust gas is called residual exhaust gas.


The intake and exhaust of the four-stroke engine together account for 360 ° of the crank angle.

When the piston is exhausted, the piston is lifted up to the top dead center, and the exhaust gas is forcibly discharged.

When the intake air goes down to the bottom dead center, its advantage is “the fresh mix that enters.

Almost all of the gas participates in the combustion, and there is basically no fresh gas that is wasted, so the fuel consumption is low and it is suitable for long-distance use.”

A significant disadvantage of 4-stroke internal combustion engines over two-stroke engines is their lower power output.