Software Multec 3.5 Fuel Injector Application Manual
5-6
Delphi Energy and Chassis Systems
Revision: 11/05-1
3. The BPW is then compared to the minimum PW. If the BPW is less
than the minimum allowable PW, the BPW is set to the minimum PW,
or the injector firing scheme is altered, to increase the PW above the
minimum.
4. The X intercept (injector offset) is then added to the final BPW,
forming the total PW that is delivered by the engine controller.
Note: Demand (or returnless) fuel systems where fuel pressure is not
biased for manifold vacuum require the injector slope and intercept to be
measured as a function of injector tip vacuum at a constant fuel gauge
pressure. (Vacuum levels of 0 to 80 kPa in 20 kPa increments are
typically used.)
5.2.2.1 Injector Slope
The injector slope characterizes the maximum injector flow. The slope
term correlates very closely to static flow and is constant over a wide
range of battery voltages, although it does vary with pressure — the
higher the pressure, the steeper the slope. The slope term is obtained
using the following method:
1. A flow curve for the fuel rail assembly that measures each port’s flow
is taken at 13.5v at a constant repetition rate. Flow points are
determined by the injector firing scheme.
2. A linear regression is done for the flow based on the points determined
in step 1.
3. The linear regression is then used to determine the slope and offset
(which is used in section 5.2.2.2).
Note
Do not use the actual static flow point from the flow curve because most
injectors deviate rich near static flow (see Figure 3-14). Also, do not use
the “offset” from the flow curve. The engine controller initially assumes a
zero offset and then makes corrections later.
4. If the flow data is a measurement of total rail flow rate rather than
individual cylinder flow rate, divide the slope by the number of
cylinders.
Note
It is a good practice to use an average of several fuel systems’ flows
(minimum 3) to calculate the fuel system slope and offset.
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