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Mechanics who are familiar with how gasoline engines work are often amazed when they found out how completely different a diesel engine is, even though the two use roughly the same combustion technology to turn a crankshaft and through that the wheels.

The diesel injector pump is one of those things that makes a diesel engine much different than a gasoline engine.

Diesels operate by direct injection of fuel into a pressurized cylinder. The properties of diesel fuel are such that a spark, which is what normally fires the fuel in a gasoline engine, will not ignite the fuel. Instead, diesel is ignited through pressure (achieved through compression).

As the cylinder nears top-dead-center (maximum compression), diesel fuel is injected at pressure. The combination of sprayed fuel and the pressure of the chamber causes the fuel to ignite, forcing the return stroke.

Some gasoline engines work in a similar manner, but it's uncommon.

Because of this pressure, many diesel injector pumps operate at 100MPa or higher (some as high as 200MPa). This means that those servicing the fuel lines or pump need to be aware of the high pressures involved once the fuel has been pushed through the pump.

This pressure is such that, if not properly and safely relieved, it can literally inject fuel directly into the skin (or right through it) of the person who breaches the lines. More than one mechanic has had his arm amputated after getting injected with diesel fuel.

Two types of diesel injector pumps are common in diesel engines today: the in-line pump which works like a tiny version of the engine to push and pull diesel fuel through itself and give it pressure. The other type is the rotary pump (or distributor pump) which uses an axial cam plate to inject fuel into individual lines via a rotary distribution valve.

The in-line pump is common on larger, heavier diesel engines such as those on trucks, fixed engines, and agricultural vehicles. The rotary pump is common in smaller applications such as cars, light trucks, and small generator engines.

These two mechanical type diesel pumps are beginning to lose favor now as new, electronically-controlled pumps take their place to provide higher efficiency in today's diesel engines. The two most common new type of diesel injector pump are the common rail diesel system and the unit direct injection system.