Multec 3.5 Fuel Injector Application Manual Software
Delphi Energy and Chassis Systems
Revision: 11/05-1 5-7
5.2.2.2 Injector Offset
Since the injector flow curve does not go through the origin, an “offset” is
needed (See Figure 5-2). “Offset” refers to the amount of time that is
added to (or subtracted from) the engine controller-calculated pulse-width.
It is a function of voltage and pressure.
Offset can be determined by doing the following:
Method 1
• Flow the fuel system at the same voltages (and manifold vacuums
for non vacuum biased fuel systems) as in the offset table in the
vehicle software. Calculate a linear regression at each test
condition.
• Note the points where the regression line intersects the time-axis
at each voltage and vacuum (x-intercepts) and enter them into the
table (refer to Figure 5-2).
Method 2
• Use the average regression line slope at 13.5 injector volts.
Intersect this slope with the average per cylinder flow at each
voltage and a specified pulse-width. The pulse-width selected
should represent a typical, highly used injector pulse-width during
the vehicle emissions test procedure, and falls within the linear
portion of the flow curve.
• Extrapolate the regression line until it intersects the time axis at
each voltage and enter them in the table.
Note
It is only necessary to use voltages that cover the operating range of the
engine controller.
Note
For systems using a vacuum biased regulator, the pressure drop across the
injector and resulting slope term is typically constant. A single set of
injector offsets can be used independent of engine vacuum.
Note
For non-vacuum biased systems, the injector slope varies with the
pressure drop across the injector. Injector offsets will be affected by this
change in slope. Fuel rail flow characterization will include measuring the
offset change vs. both injector voltage and manifold vacuum.
5.2.2.3 Low Pulse-Width Correction
The Low Pulse-Width Correction table is used to compensate for the
difference between the linear approximation of the fuel flow and the actual
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