Introduction To Cr Injectors; Cr Injector Operation; Injector Valve Types; Solenoid Valves - Hartridge HK900 Operating Manual

HARTRIDGE LIMITED Operating Manual

3. Introduction to CR Injectors

3.1 CR Injector Operation

One of the main differences between Common Rail injectors and traditional mechanical injectors is
the introduction of electrical control; with Common Rail injectors the timing and duration of injection
are controlled using an electrically-operated valve. There are currently two types of valve, described
in section 3.2 below.
In terms of the hydraulic operation, high pressure fuel is supplied to the nozzle body and to a chamber
on top of the control needle. The electrical valve opens/closes a leak off path from this chamber into
the return circuit. When the valve is closed, the high pressure acting on top of the control needle
keeps the injector firmly closed. Opening the valve relieves the high pressure from the top of the
needle, opening the nozzle and starting the injection. Closing the valve re-establishes the high
pressure on top of the control needle, quickly shutting off the injection.
The volume of fuel delivered is proportional to both the duration of the pulse (pulse width), and the
applied pressure, as shown by the graph in Figure 3.1. Typical pulse widths are in the range of 200 to
2000s. There is a minimum pressure below which the injector will not open; this is typically around
200 bars.
b a r

s
Figure 3.1: Effect of Pressure and Pulse Width on Delivery

3.2 Injector Valve Types

3.2.1 Solenoid Valves

These are electromagnetic coil valves; Figure 3.2 gives a schematic of the drive signal (showing
current against time). There is a higher "pull-in current" (I
) to initially open the valve, followed by a
p
lower "hold current" (I
) to keep the valve open. The overall pulse width (T) is the combined width of
h
the pull-in and hold phases. There is a maximum limit on the pull-in current to protect the solenoid.
Injectors with solenoid valves are currently manufactured by Bosch, Delphi, and Denso. Each of these
manufacturers uses a different specification for operating voltage and current levels.
HL030(EN), Issue 1, June 2005 15
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