How It Works: The Diesel Engine: The 4-Stroke Cycle

Understanding how your diesel engine works is key to knowing how to care for it and repair it if something goes wrong. In the first of this series, we are interested in the 4-stroke cycle

How It Works: The Diesel Engine: The 4-Stroke Cycle

A diesel engine has no ignition system or spark plugs.

Diesel fuel ignites at a temperature of approximately 320° Celsius. So what ignites the fuel and keeps the engine running?

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

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

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

Continued below…

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Let’s imagine an elephant jumping from a height on a bag of fresh air. And imagine that at the same time, an archer shoots an arrow full of diesel aiming to arrive at the air bag exactly at the same time as the elephant.

As the air bag is very quickly compressed by the arrival of the elephant, the arrow with exactly the right amount of fuel arrives and enters the now very hot air bag.

There is only one inevitable outcome: the elephant gets a free spin!

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

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

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

For a real engine mounted on a boat, there is no elephant in the room and you have to get air into the engine, get the exhaust gases out and get the fuel in at the right time.

Below is the four-stroke cycle. Almost all small boat engines work on this principle.

A diagram showing the operation of a 4-stroke cycle diesel engine

1 credit

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

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

This up and down motion is converted by the connecting rod and crankshaft, with the connecting rod under the piston and the crankshaft below.

This arrangement works much like the windshield wipers on a car; they go from side to side, but are driven by a motor.

In an engine, pistons move up and down driving a shaft.

Diesels Afloat New Edition (Fernhurst, £18.99) is available from www. fernhurstbooks.com. It follows the RYA Diesel Engine and MCA Approved Engine Course 1 course syllabus.

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