Unit Injector (abbrev. UI) is a diesel engine fuel injection system, combining the injection pump and the injector nozzle in a single component, which is (usually) driven by the engine camshaft.
Unit Injector systems were commonly associated as Detroit Diesel Allison fuel systems, however in 1911 a patent was issued in Great Britain for a unit injector resembling those in use today to Frederick Lamplough.
Commercial usage of unit injectors in the U.S. started in 1931 on Winton engines, with a design by C.D. Salisbury, and in 1934 Arthur Fielden was granted U.S. patent No.1,981,913 on the unit injector design adopted for the General Motors two stroke diesel engine.
In 1994, Bosch supplied first electronic unit injector for commercial vehicles and other manufacturers soon followed.
Today, major manufacturers include Robert Bosch GmbH, Delphi Corp., Detroit Diesel Allison, Cummins and now defunct Lucas Automotive.
Design and technology
Design of the unit injector eliminates the need for high pressure fuel pipes, and with that their associated failures, as well as allowing for much higher injection pressure to occur.
The unit injector is fitted into the engine cylinder head, where the fuel is supplied via integral ducts machined directly into the cylinder head.
Each injector has its own pumping element, and in the case of electronic control, a fuel solenoid valve as well. The fuel system is divided into the low pressure (<500 kPa) fuel supply system, and the high pressure injection system (<1500 bar).
The basic operation can be described as a sequence of four separate phases: the filling phase, the spill phase, the injection phase, and the pressure reduction phase.
A low pressure fuel delivery pump supplies filtered diesel fuel into the cylinder head fuel ducts, and into each injector fuel port of constant stroke pump plunger injector, which is camshaft operated.
The constant stroke pump element on the way up draws fuel from the supply duct in to the chamber, and as long as electric solenoid valve remains de-energized fuel line is open.
The pump element is on the way down, and as long as solenoid valve remains de-energized the fuel line is open and fuel flows in through into the return duct.
The pump element is still on the way down, the solenoid is now energized and fuel line is now closed. The fuel can not pass back into return duct, and is now compressed by the plunger until pressure exceeds specific "openning" pressure, and the injector nozzle needle lifts, allowing fuel to be injected into the combustion chamber.
Pressure reduction phase
The plunger is still on its way down, the engine ECU de-energizes the solenoid when required quantity of fuel is delivered, the fuel valve opens, fuel can flow back into return duct, causing pressure drop, which in turn causes the injector nozzle needle to shut, hence no more fuel is injected.
The start of an injection is controlled by the solenoid closing point, and the injected fuel quantity is determined by the closing time, which is the length of time the solenoid remains closed. The solenoid operation is fully controlled by the engine ECU.
The use of electronic control allows for special functions, such as temperature controlled injection timing, cylinder balancing (smooth idle), switching off individual cylinders under part load for further reduction in emissions and fuel consumption.
Further development and applications
In 1993, CAT introduced "Hydraulically-actuated Electronic Unit Injection" (HEUI), where the injectors are no longer camshaft operated. First available on Navistar's 7.3 L, V8 diesel engine. HEUI uses engine oil pressure to power high pressure fuel injection, where usual method of unit injector operation is the engine camshaft.
Unit injector fuel systems are being used on wide variety of vehicles and engines from various manufacturers, e.g. Volvo, Cummins, Detroit, CAT for commercial vehicles, and as well as for passenger vehicles e.g. Land Rover, Volkswagen Group (Volkswagen AG), among others.
The Volkswagen Group use unit pump systems (branded "Pumpe Düse", or "PD") in their PD SDI and TDI diesel engines.In North America, the Volkswagen Jetta, Golf, and New Beetle TDI 2004-2006 are Mk4 Pumpe Düse (newer models use Mk5 BEW engines and older models use ALH engines).
Early Lucas electronic diesel unit injector Delphi E1 electronic diesel unit injector
Delphi Electronic Unit Injector on the Volvo D13A engine Unit Injector engine ECU