Types of Diesel Fuel Injection Systems

Throttle body injection (TBI) is very similar to carburetion; so similar, in fact, that many throttle body injection units were actually adapted from gas carburetors. TBI differs from carburetion in that it uses one or a set of downward-facing injectors to shoot fuel into the engine under pressure rather than allowing it to be drawn in by engine vacuum. TBI is fairly uncommon on modern diesel engines, primarily since it doesn't work well with turbocharging and because there were already better options on the scene when it was developed.

Some industrial diesel engines use multi-point fuel injection (MPI) similar to that used in most cars. MPI systems utilize a single or dual fuel pumps to feed fuel injectors mounted in the intake port of the engine. The injectors spray fuel into the engine's intake valves through the intake port. This design is fairly rare on diesels, primarily since it was developed after the mechanical direct injection used on most diesels and offers no real advantage for compression-ignition engines.

Direct injection has been used on diesel engines since the 1950s. These systems use a powerful, positive displacement fuel pump to pump diesel through injectors mounted directly in the cylinder. These injectors sit about where the spark plug would be on a gas engine and work well with diesel engines because they can introduce huge amounts of fuel into the combustion chamber just as the piston reaches its peak compression. This makes the combustion event much faster, quieter and more powerful than it would otherwise be.

One interesting component that sets direct injection apart from other types is the fuel pump. A direct injection pump is almost identical in form and function to a tiny four-stroke engine, but passes liquid diesel fuel instead of air and fuel. These "positive displacement" pumps develop the very high and stable fuel pressures needed to inject fuel against the compressive force of a rising piston. Without a positive displacement pump, the air on top of the rising piston would overcome the fuel pressure and force air backward into the fuel line.

Common rail injection (CRI) is a combination of both direct injection and multi-port injection. Like multi-port injection, CRI uses a common fuel rail (reservoir) to feed multiple fuel injectors, but those injectors are mounted inside of the cylinder-like direct injection. This design gives engineers the flexibility to control the amount of fuel injected and fuel injection timing with a computer, an impossible feat for traditional mechanical direct injection that relies only on injector pump timing to regulate fuel flow. The secret to CDI's success is its single, powerful fuel pump, which can pressurize the (very thick) fuel rail to more than 23,000 pounds per square inch.