Contact Information

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G.W. Dale Ltd
139 Newcastle Street
Stoke on Trent

Phone: +44 (0) 1782 837 665
Fax: +44 (0) 1782 839 550

Direct (unit) injection

Unit injector injector

The unit injector combines a high-pressure pump and nozzle with a solenoid valve to form compact assembly. As a result, high-pressure lines are no longer necessary and injection can be controlled by the integrated and extremely precise solenoid valve at pressures of up to 2000 bar.

In this animation, you can see the fuel being released by the pump injector. A cross section of the pump injector also shows the pump piston (in green) push downward at high velocity, ensuring a high injection pressure that is attained quickly.

The injection lobe, which is attached to the camshaft serves two purposes:

  • For the first phase, a small amount of fuel is injected into the cylinder through the first nozzle at lower pressure. This begins the burning process and also reduces the shock of the main injection cycle, thus resulting in quieter engine operation, especially when the engine is cold.

  • For the second, main injection phase the rest of the fuel quantity is injected at extremely high pressure, which is regulated by an electromechanical solenoid (or valve) which governs the exact timing and duration of the injection pulse. This time period is governed by many factors through the main engine control unit (or ECU), which include engine speed and load, exhaust gas levels (obtained through the O2 sensor), EGR operation as well as both engine and ambient air temperatures.

This precise metering of fuel and the timing of the injection pulse gives the TDI-PD engine its tremendous efficiency and fuel economy as well as an almost imperceptible amount of visible exhaust emissions.

Each cylinder has a unit injector fitted between the valves in the cylinder head where it is powered by the engine's overhead camshaft.


The unit injector is used in both passenger cars and commercial vehicles.

unit injector

The Bosch Unit Injector system was first used in the VW Passat TDI in 1998, after which it rapidly found favour within the VW range. With the V10 TDI, VW recently presented what is currently the most powerful diesel engine for use in a car.