D
Volvo Trucks North America
Date
Group
No.
Page
Service Bulletin
11.2006
230
247
7(11)
Unit injectors for diesel engines have both injector
pump element and the nozzle assembly in the same
body. The pump stroke is activated by a camshaft lobe
which rotates at one half crankshaft speed in 4 stroke
engines. The pump stroke is always constant but the
delivery through the injection nozzle is controlled by an
electrically operated solenoid valve, usually named a
Spill Valve (SV). The SV is located between the high
pressure fuel Line and the low pressure fuel Line. When
the SV is closed, all the fuel from the pump chamber is
forced through the nozzle and into the engine combustion
chamber. When the SV is open, the fuel is routed into the
low pressure fuel feed line instead and so no fuel passes
through the nozzle. The SV is operated by the Engine
Electronic Control Unit (EECU) by software and certain
power stages capable of driving the solenoids.
The injected fuel quantity per pump cycle is determined
by how long the SV is closed during the pump stroke. The
start of injection, i.e. injection timing, is determined by
when the SV is closed.
The injection pressure achieved is balanced out by a
number of factors:
1 The pumping capacity (pump element diameter, cam
rate) verses the restriction in the nozzle (the nozzle
hole area).
2 The engine speed, i.e. fuel/sec from the pump
chamber verses the nozzle hole area.
3 The nozzle opening pressure.
For a given unit injector/cam/nozzle specification, the
injection pressure is well defined over the speed/load
range for a specific engine. Engine exhaust emissions
are very dependent on the fuel spray characteristics and
are, to a large extent, influenced by the nozzle spray hold
design and the injection pressure. Therefore, it is of great
benefit to be able to change these parameters and today
it is possible to vary at least one of them, namely the
injection pressure. The method is to let the pressure build
up to a desired level before opening the nozzle needle.