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How it works: The diesel engine: The 4-stroke cycle

Understanding how your diesel engine works is key to knowing how to look after it, and fix it when it goes wrong. In the first of this series, we look at the 4-stroke cycle

How it works: The diesel engine: The 4-stroke cycle

A diesel engine has no ignition system or sparking plugs.

Diesel fuel ignites at a temperature of around 320° Celsius. So what ignites the fuel and allows the engine to run?

When air is compressed, the effect on the air is to increase its internal energy and thus its temperature.

Provided that the air is compressed rapidly enough so that the heat has little time to escape to its surroundings, the air in a diesel engine cylinder can be made to rise to above the ignition temperature of the fuel by compression alone.

If diesel fuel is then injected into the hot air, the mixture will ignite, releasing energy. This is known as compression ignition.

Let’s imagine an elephant jumping from a height onto a bag of cool air. And let’s imagine that, at the same time, an archer shoots an arrow full of diesel fuel aimed to arrive at the bag of air at exactly the same time as the elephant.

As the bag of air is very rapidly compressed by the arrival of the elephant, the arrow with exactly the correct amount of fuel arrives and penetrates the bag of now very hot air.

There’s only one inevitable outcome: the elephant gets a free ride!

Very simplistic, I know, but the basic diesel engine is as simple as that.

If the air is heated to above the combustion temperature of the fuel very rapidly AND if the correct amount of fuel is injected into this hot air at the correct time, the engine will run.

No electricity is required, except to turn the engine over fast enough to start, which can be done by hand on a small engine.

For a real engine fitted to a boat, there is no elephant and we need to get air into the engine, exhaust gases out and fuel in at the right time.

Shown below is the four stroke cycle. Just about every small boat engine works on this principle.

In these drawings the piston replaces the elephant, we use valves in the cylinder head (top of the engine) to let air in and exhaust gas out.

Fuel is injected through the green component, called a fuel injector. The piston goes up and down, but we want a shaft to rotate to drive our propeller.

This up and down motion is converted by the connecting rod and crank shaft, the connecting rod being under the piston and the crank shaft below this.

This arrangement works a bit like car’s window wipers; they go from side to side, but are driven by a motor.

In an engine the pistons go up and down driving a shaft.


  • Lerwick, Shetland, UK
  • Callum Smedley