Diesels were some of the first engines to receive fuel injection. Although many people consider diesels to be little more than agrarian chuggers, the fact that they're often used in industrial settings for hours on end means that cost and public sentiment take a backseat to efficiency, durability and economy.
Throttle Body Injection
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
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.
- Photo Credit diesel special image by longtall_chris from Fotolia.com
What Is a Diesel Fuel System?
Over the past few decades, scientists and engineers have experimented with different types of fuels for use in cars, trucks, construction equipment,...
What Is the Function of a Fuel Injector?
The function of a fuel injector is to spray atomized fuel into the combustion chamber of an internal combustion engine. Fuel injection...
What are the Different Types of Diesel Fuel?
Any fuel used in a diesel engine is called diesel fuel. These fuels are classified as 1D, 2D and 4D. A main...
Explanation of a Diesel Fuel Injection Pump
Patented by Rudolf Diesel in 1892, the diesel engine is an internal-combustion engine that uses pressure to ignite fuel rather than a...
How to Replace a Fuel Injector in a Dodge Ram
The fuel injector in a Dodge Ram mixes fuel with air under pressure and rendered carburetors obsolete during the late 1980s. One...
How to Replace a Fuel Injector in a Jeep Grand Cherokee
The Jeep Grand Cherokee has a fuel injector that mixes fuel with air. This technology almost completely replaced carburetors in the late...
How to Bleed a Diesel Fuel Injection System
Diesel injector fuel lines carry fuel from the lift pump and deliver it to the injector supply lines or the fuel gallery....