Before evolving into the fuel injection systems we enjoy today, a car’s fuel delivery system featured carburetors. Diesel engines, however, have always utilized direct fuel injection.

Unlike gasoline engines that use port injection to spray fuel into the intake manifold just before the intake stroke, diesel engines squirt high-pressure fuel directly into the engine’s cylinders. This occurs at the top of the compression stroke and both initiate and control the combustion, doing the same work as both the ignition system and the throttle in a gasoline engine.

With all the considerations involved–heat, pressure, and distributing the fuel mist evenly–it’s no wonder that the injector is the most complicated component of a diesel engine. The good news is that with no separate ignition system, that’s one less thing that can go wrong compared to a gasoline engine’s fuel system. Plus, with no manifold, it’s a much cleaner setup.

These days, two variants are generally used in diesel vehicles: the unit injector and the common rail.

Unit Injector

This system combines the injector nozzle with the injection pump in one compact component that is cooled and lubricated by the fuel itself. In this process, low-pressure fuel feeds into the fuel ducts of the cylinder head and, if the solenoid valve is open, into the injector’s duct. As the pump plunger descends, the solenoid valve and fuel line close and the trapped fuel is compressed. Once the pressure rises above a particular “opening” pressure, the injector nozzle needle rises so that fuel can be sprayed into the combustion chamber. Finally, the plunger completes its descent, the solenoid and fuel valve are re-opened, and fuel enters the duct. This drops the pressure inside, which shuts the injector nozzle.

Unit injector systems are usually found in large commercial vehicles and in passenger vehicles made by Volkswagen Group and Land Rover.

Common Rail

In this setup, fuel flows from the tank to the common header (also known as the accumulator), then to the injectors to be sprayed into the combustion chamber. The header uses a relief valve to maintain proper pressure and send extra fuel back to the tank. The injection time and quantity are controlled precisely by piezoelectric valves. The name “common rail” comes from the common fuel rail that stores fuel at extremely high pressures (up to 29,000 psi) and feeds the fuel to several fuel injectors.

Common rail systems are found in most other diesel passenger cars worldwide.


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