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Unit Injector (abbrev. UI) is a direct fuel injection system for diesel engines; combining the injector nozzle and the injection pump in a single component, the pump of 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 Britainmarker for an unit injector resembling those in use today to Frederick Lamplough.

Commercial usage of unit injectors in the U.S.marker began in early 1930s on Winton engines powering the locomotives,boats even US Navy submarines, , and in 1934, Arthur Fielden was granted U.S. patent No.1,981,913 on the unit injector design later used for the General Motors two stroke diesel engines.

In 1994, Robert Bosch GmbH supplied first electronic Unit Injector for commercial vehicles, and other manufacturers soon followed.

Today, major manufacturers include Robert Bosch GmbH, CAT, Cummins, Delphi Corp., Detroit Diesel Allison and the Delphi Corp. acquired 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 system allows accurate injection timing, and amount control as in the common rail system .

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&NBSP;kPa) fuel supply system, and the high pressure injection system (<2000&NBSP;bar).

Operation principle

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 filter diesel fuel into the cylinder head fuel ducts, and into each injector fuel port of constant stroke pump plunger injector, which is overhead camshaft operated.
Fill phase: 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.
Spill phase: 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.
Injection phase: 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 "opening" 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.

Summary: 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.

Additional functions

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, and multi-pulse injection (more than one injection occurrence during one engine cycle).

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 , among others.

The Volkswagen Group mainstream marques use unit injector systems (branded "Pumpe Düse", commonly appreviated to "PD") in their Suction Diesel Injection (SDI) and Turbocharged Direct Injection (TDI) diesel engines.

Volkswagen Group major-interest truckmaker Scania ABmarker also use the unit injector system, which they call "Pumpe-Düse-Einspritzung", or "PDE".

See also


  • Diesel Fuel Injection published by Robert Bosch GmbH, 1994 ISBN 1-56091-542-0
  • Diesel Engines and Fuel Systems by A. Asmus & B. Wellington, 1988 ISBN 0-7299-0013-4

  1. Volkswagen Mk4 turbodiesel "how to" index, for 2004-2005 Jettas and 2004-2006 New Beetles and Golfs

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