Unit injector
The unit injector
The unit injector is driven by the engine camshaft via a rocker arm. Simply put, a unit injector is a
combination of an injection pump and a conventional injector and it consists of three main sections:
the pump section(1)
the valve section with the solenoid(2)
the injection section(3)
The pump piston pumps a constant quantity of fuel back and forth through the injector and there is no
injection as long as the injector solenoid keeps the valve open.
1. Filling phase
This occurs the entire time that the pump piston is on the way up. The fuel valve is open since there is no
voltage applied to the solenoid valve. Fuel can therefore be sucked from the feeder channel, past the
open valve and into the pump cylinder.
2. Spilling phase
The pump piston is on the way down. As long as there is no voltage to the solenoid valve, the fuel valve is
open and the fuel flows back in return to the feeder channel.
3. Injection phase
The pump piston is still on the way down. Now the solenoid valve has received voltage from the control
unit. The valve cone is lifted and the valve closes. Since fuel cannot pass through the valve, a pressure is
quickly built up which lifts the nozzle needle and injection occurs.
Injection continues as long as the valve is closed and the pump piston is on the way down.
The injection timing and fuel quantity is determined by the length of the electrical pulse. It is the control
unit which determines the pulse size via the information it receives from the control system.
4. Pressure reducing phase
The pump piston is still on the way down. The control unit ends the electrical pulse when the engine has
received the amount of fuel needed at that instant. The fuel valve opens and the fuel can once again flow
past in return to the feed channel. The pressure then drops quickly and the nozzle needle closes.
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