This technician’s guide explains the fuel injection system operation and is mainly focused on the electronic unit injector.
The heart of the diesel engine is the fuel injection system. It has the vital function of communicating with the Electronic Control Module (ECM) and pumping a measured quantity of fuel into each cylinder at precisely the right moment. The unit injector does all this with simple design and few parts. The benefits are low unit cost and minimal maintenance cost. The advantage of the 30,000 psi fuel injection pressure capability is high velocity injection which translates into excellent penetration and atomization. As a result of the ability to control injection spray, exhaust emissions and fuel
consumption parameters are optimized. These features help make the most efficient use of each ounce of fuel, minimizing smoke, NOx, and HC emissions.
Diesel Fuel System Overview
The function of the fuel system is to store and supply fuel to the combustion chamber. The major parts of the diesel fuel system are the fuel tank, fuel filters, fuel pump, electronic control module, injection nozzles or injectors, and fuel lines. The fuel pump draws filtered fuel from the tank through low pressure fuel lines leading to the water separator (not all diesel engines have a water separator). In most diesel engines, the fuel passes through a primary fuel filter before reaching the pump. The pump circulates an excess supply of fuel through the injectors, which purges air from the fuel system and also cools and lubricates the injectors. The unused portion of fuel returns to the fuel tank by means of the fuel return line. The primary filter captures large contaminants from the fuel and acts as a water separator. Water is heavier than diesel fuel and falls to the bottom of the primary filter, where in most applications it can be drained. After passing through the pump, the fuel goes through a secondary filter before reaching the fuel injectors to keep them clean and prevent them free from damage. The fuel flows to the fuel injectors where it is injected into the cylinders. The primary job of the entire fuel system is to inject a controlled amount of atomized fuel into each engine cylinder at the precise time. Excess fuel exits at the rear of the cylinder head just above the inlet, through a restrictive return fitting that maintains fuel pressure in the system. It then returns back to the tank. The electronic unit injector injects fuel directly into the combustion chamber. The injector performs these three functions:
- Creates the high fuel pressure required for efficient injection.
- Meters and injects the exact amount of fuel required to handle the power requirement.
- Atomizes the fuel for mixing with the air in the combustion chamber.
Electronic Unit Injector (EUI)
The Electronic Unit Injector (EUI) injects fuel directly into the combustion chamber. The small size of the injector along with the trapezoidal valve placement in the cylinder head allows the EUI to be placed in the center of the combustion chamber for optimal fuel efficiency and low emissions. The EUI is placed in an injector cup insert, and O-rings are used to seal between the injector and the cylinder head as well as the injector cup insert and the cylinder head.