Unit injector (UI) is an integrated direct fuel injection system for diesel engines, combining the injector nozzle and the injection pump in a single component. The plunger pump used is usually driven by a shared camshaft. In a unit injector, the device is usually lubricated and cooled by the fuel itself.
Commercial usage of unit injectors in the United States began in early 1930s on Winton engines powering 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. Most mid-sized diesel engines used a single pump and separate injectors, but some makers, such as Detroit Diesel and Electro-Motive Diesel became well known for favoring unit injectors, in which the high-pressure pump is contained within the injector itself. E.W. Kettering's 1951 ASME presentation goes into detail about the development of the modern Unit injector. Also Cummins PT (pressure-time) is a form of unit injection where the fuel injectors are on a common rail fed by a low pressure pump and the injectors are actuated by a third lobe on the camshaft. The pressure determines how much fuel the injectors get and the time is determined by the cam.
In 1985 the Detroit Diesel division of General Motors Corporation supplied the first electronic unit injector for commercial vehicles in their Series 92 2-stroke diesel engines followed by introduction in the Series 60 4 stroke engines in 1987. Other manufacturers soon followed. In 1995, Electro-Motive Diesel converted its 710 diesel engines to electronic fuel injection, using an EUI which replaces the MUI.
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 kPa) fuel supply system, and the high-pressure injection system (<2000 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 overhead camshaft operated.
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).
In 1993, CAT introduced "hydraulically actuated electronic unit injection" (HEUI), where the injectors are no longer camshaft-operated. First available on Navistar's 7.3L /444 cubic-inch, V8 diesel engine, and later in Navistar DT466E, HEUI used engine oil pressure to power high-pressure fuel injection, in contrast to the traditional method of unit-injector operation via the engine camshaft.
Today, the various types of unit-injector fuel systems are found on a wide variety of vehicles and engines; commercial vehicle from manufacturers such as Volvo, Cummins, Detroit Diesel, CAT, Navistar passenger vehicles from manufacturers such as Land Rover and Volkswagen Group, among others, and locomotives from Electro-Motive Diesel.
The Volkswagen Group mainstream marques used unit injector systems (branded "Pumpe Düse", commonly appreviated to "PD") in their Suction Diesel Injection (SDI) and Turbocharged Direct Injection (TDI) diesel engines, however this fuel injection method has been superseded by a common-rail design, such as the new 1.6 TDI.
In North America, the Volkswagen Jetta, Golf, and New Beetle TDI 2004-2006 are Pumpe Düse (available in both the MK4 and MK5 generations, with BEW and BRM engine codes respectively; older models use a timing belt driven injection pump).
Volkswagen Group major-interest truck and diesel engine maker Scania AB also uses the unit injector system, which they call "Pumpe-Düse-Einspritzung", or "PDE".