How a diesel engine works - in layman's terms

I have always enjoyed explaining how a diesel engine works to people who have no idea, so for all the boffins out there, this will be boring (please feel free to skip this section). I have tried to use simple layman's terms and still maintain accuracy as far as possible.

Unlike petrol, diesel is not flammable but rather combustible (self ignites under extreme pressure). Diesel engines therefore don't have spark plugs to ignite the fuel, but rely on heat to bring about combustion. When you put your finger over the end of a bicycle pump and pump, you will find that your finger becomes hot. This is due to compression of the air trapped inside the pump (the air molecules have less space to move about and collide with each other more frequently causing energy to be given off in the form of heat).

The same thing happens in a diesel engine: as the piston moves upwards, the air trapped inside is compressed, causing it to heat up (the temperature reached is about 400 degrees C). Just before the piston reaches the top of its' stroke, (this point is known as the Top Dead Centre or TDC) the pump and injectors spray a very fine mist of diesel into the piston chamber. The intense heat of the trapped air in the piston causes the diesel to ignite, forcing the piston down and producing the power stroke which goes through the engine via the gearbox / diff / wheels etc and off you go!

As you can see, diesel engines are really very simple, all you need is a good amount of hot compressed air, a fine mist of diesel injected at the right time and hey presto, you're on your way.


This sounds impressive, but in reality a turbocharger is little more than a double headed fan. The fan turns as a result of the exhaust gases passing over the fins at one end. This in turn causes the fins of the other fan to force air into the combustion chamber from outside More air in the piston space/combustion chamber produces a more powerful explosion/ignition. The result is a more powerful engine.


Many turbocharged vehicles are also intercooled. In this instance, a radiator is fitted between the turbo and the inlet manifold. The warm, compressed air from the turbocharger is cooled by passing through the radiator / intercooler. This in turn results in even more air/oxygen being forced in to the piston area(the colder the air, the more molecules present in a given amount).


Once the engine has a nice dollop of good clean, cool air trapped in the combustion chamber, the piston begins to move up compressing the air. The temperature of the air rises rapidly and as the piston nears the top the injector sprays in a very fine mist of diesel which self ignites forcing the piston down in to the power stroke. If the injector is faulty and larger droplets of fuel are sprayed into the piston, the fuel will be unable to burn completely and the performance will be poor with black smoke exhausted (unburned fuel.)


When I first started working with diesel engines fitted to cars, the breaking pressure (squirting pressure) of the injector was 115 Bar(1690PSI). Due to further development it was found that by increasing the injector pressure, and by altering various engine components and design, the engines would be more powerful and produce less emissions . The breaking pressure of the injector went up to 280 Bar(4116PSI). These 'conventional' injectors basically just squirted a measured amount of fuel into the cylinder and a further reduction of emissions became difficult so the 'Two Stage Injector' was developed.

The idea of the Two Stage Injector is to release a measured amount of fuel into the cylinder over a longer period and at a higher pressure, from 190 Bar (2 800 PSI ) to about 400 Bar (5 880 PSI ). By doing this more of the fuel is atomized causing better combustion.

This still not being good enough, the 'high pressure common rail diesel injector' was developed. With a rail pressure of 1800 Bar ( 26460PSI ) to 2000 Bar (29400 PSI ) and each injector able to inject on demand from the vehicles brain box we now have the injectors being able to inject fuel up to 3 times on one compression / exhaust stroke. More fuel, less fuel, high pressure, low pressure every thing is now possible to insure that the fuel is used to its best ability allowing for less emission, more power and more fuel efficient.

All of this at a price, the conventional injector was repaired for about '20.00, the Two Stage Injector for about '60.00 and now Common Rail Injectors for about £250.00.