Diesel Engine Principles For Beginners

Diesel Engine Principles For Beginners

Diesel engine principles are pretty straight forward for the experienced mechanic but what about regular everyday people who have never taken the time to find out more about a diesel engine and how it works?
I am going to explain in layman’s terms the basic principles of a diesel engine.


The first thing you should know is a gasoline engine and a diesel engine are totally different. A gasoline engine is constructed much lighter than the heavier built diesel engine and runs on an air/fuel mixture combined with a high energy spark that is timed to ignite the air/fuel mixture inside each engine cylinder at the precise time creating power and torque [turning force] which drives your vehicle.
A diesel engine uses high compression [intake air that is compressed / squished] into a very small space inside each cylinder causing extreme heat! This is called ‘Heat Of Compression’ which ignites a very fine high pressure mist of diesel fuel that is injected into the cylinder at the exact time.
So you now know that a gasoline engine needs a high energy spark to run while a diesel uses ‘Heat Of Compression’


Every engine today runs on FOUR STROKES or FOUR CYCLES- both these terms mean the same. Here is how the four stroke diesel engine operates.
The four strokes are: INTAKE-COMPRESSION-POWER-EXHAUST. The pistons, valves and injectors work together in each cylinder in a set sequence over and over.

Intake valves in the cylinder head open allowing pressurized air to enter each cylinder while the piston is travelling downward.{the pressurized air supply is made possible by the TURBOCHARGER which pushes air into the intake system giving the diesel engine a boost of air to keep up with instantaneous injection of fuel}

When the piston starts to make it’s way back upward the valves close which traps the intake air in the cylinder which allows compression to take place, the HEAT OF COMPRESSION is reached when the piston reaches the top of the cylinder, the diesel fuel is then injected into the cylinder at the precise time.

After injection takes place an explosion occurs in the cylinder because of the combination of heat and atomized diesel fuel. This causes the piston to be forced downward which produces torque and the horsepower required from a typical diesel engine.

After the power stroke the piston moves upward once again while the exhaust valves open allowing the previously ignited gases to escape to the atmosphere out the exhaust system.

As mentioned before each cylinder goes through this exact sequence over and over in a set firing order. For instance, a 6 cylinder diesel engine has a firing order 1- 5- 3- 6- 2- 4 This is the order that each cylinder goes by, following the 4 strokes mentioned above. This sequence has been engineered to allow the diesel engine to run smoothly with no imbalance.

DIESEL ENGINE TRIVIA: Today’s high performance diesel engines.

Fuel passes through the injector at speeds of nearly 1500 miles per hour – as fast as a jet plane at top speed.

Fuel is injected into the combustion chamber in less than 1.5 milliseconds, the same time it takes for a camera flash to go off.

The minimum amount of fuel injected into a diesel engine is one cubic millimetre – about the same volume as the head of a pin.

Volkswagen has developed a 1 litre diesel powered car that got 100 kilometres out of .89 litres of fuel {60 miles on approxiamately 3/4 of a quart of fuel!}

There is much to learn about diesel engines and a ton of information online. If you have ever considered buying a vehicle with a diesel engine you have my blessing! The extra cost will be to your benefit, so I recommend finding out more about diesel engines before you decide which one to choose..

I hope you have found this information on diesel engines helpful!

John Whelan

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