Four stroke engine

Four stroke engine principles explained. On this page you will quickly learn what each individual stroke does and how these four strokes work together to provide power to your automobile.

When you fully understand the theory and operation behind four stroke engines, diagnosis and repair becomes much easier.

The downward movement of the piston increases the volume of the cylinder above it, which in turn reduces the pressure in the cylinder and causes a vacuum. This vacuum physically pulls the air fuel mixture into the cylinder.

The air fuel mixture will continue to enter the cylinder until the intake valve closes. The intake valve closes after the piston has reached bottom dead center.

This delayed closing of the valve increases the volumetric efficiency of the cylinder by packing as much air and fuel into the cylinder as possible.

Four stroke engine operation

After the intake valve closes this will trap the air fuel charge in the cylinder.

The upward movement of the piston compresses the air fuel mixture that is trapped and thus heating the air fuel charge.

The compression stroke is very important to the over all performance of the motor. Weak or leaking compression will cause lack of power and a rough running condition.

If you think you may have a problem in this area you can review my page about testing engine compression.

At the top dead Center (TDC), better known as when the cylinder reaches the end of its upward travel, a combustion chamber is formed.

This small sealed area is where the fuel charge will be ignited and burned.

The total volume of the cylinder with the piston at BDC compared to the volume of the cylinder with the piston at top dead Center determines the compression ratio of the engine. In modern engines, this is usually 8.5 to one.

The final two strokes

The burning fuel rapidly expands, creating a very high pressure against the top of the piston. This process drives the piston down towards the bottom of its travel.

This forced downward movement of the piston is transmitted to the connecting rod and then to the crankshaft. This one stroke is what drives the wheels and continues the engine's rotational movement.

The exhaust stroke is the final step in a four stroke engines operation. The exhaust valve opens just before the piston reaches bottom dead center on the power stroke.

The Pressure in the cylinder causes the exhaust gas to rush past the open valve and into the exhaust manifold.

The movement of the piston from BDC to TDC pushes most of the remaining exhaust gas from the cylinder.

As the piston nears TDC, the exhaust valve begins to close as the intake valve starts to open. The exhaust stroke completes the four-stroke engine cycle.

The opening of the intake valve begins the cycle again. This cycle occurs in each cylinder and is repeated over and over, as long as the engine is turning.



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Now that we understand why they call it a four stroke engine. We can go back to the automotive engine section to learn another piece of the engine operation puzzle. This next link takes you from the four stroke engine to automobile engines



Did you ever want to see what a bad cylinder looks like and how they fix it? I have a video for you that takes you inside for some engine diagnosis



You can search through all of my auto repair content at one time.

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