Unit injectors have been available for many years in heavy commercial diesel engines. These include the 'Cummins Pressure Time System' and the Detroit Diesel System.
Diesel fuel injectors are all very similar in construction. For both systems actuation is a result of an extra lobe on the engine camshaft for each cylinder. This extra lobe actuates the injector at the appropriate point in the engine cycle. Each cylinder has an injector which is fed by a common fuel and return line.
In the Cummins Pressure Time system, the fuel pressure in the fuel lines is very high, and the cam lobe has a low point which allows the injector needle to be lifted by the fuel pressure and enter the cylinder at a given time which is determined by the cam lobe on the cam shaft.
The amount of fuel that is injected at any particular time is controlled by adjusting the fuel pressure in the system.
In the Detroit Diesel System, each injector is like an individual jerk pump and fuel injector in one assembly. The injection cam on the camshaft has a “high point” on it and rapidly increases the fuel pressure in the unit injector, forcing the fuel down into the nozzle which the lifts the needle and allows the fuel into the cylinder. The amount of fuel that is injected is controlled by a small control rack in the unit injector, which is connected to the throttle system via a common rod that links all the unit injectors together.
