Software Multec 3.5 Fuel Injector Application Manual
5-10
Delphi Energy and Chassis Systems
Revision: 11/05-1
open-loop characterization determines the accuracy of the actual delivered
air/fuel ratio.
5.2.3.1 Volumetric Efficiency (VE)
Volumetric Efficiency corrects the base pulse-width for pumping
inefficiencies and various errors inherent in the system. The speed density
equation assumes perfect induction, combustion, and exhaust. Since this
is impossible, the VE correction is used to model system flaws based on
engine speed and load.
The VE calibration is the basic building block for fuel delivery.
The VE table is calibrated to achieve a stoichiometric air fuel ratio at all
conditions.
5.2.3.2 Vacuum Biasing
For ease of fuel calibration, it is desirable to maintain a constant pressure
differential across the injector. This is achieved through referencing the
fuel pressure regulator to manifold vacuum. This maintains a constant
pressure differential across the injector, keeping slope constant. This
makes injector “on” time (and flow) become a simple prediction based
upon the injector slope.
Newer demand fuel systems (returnless) locate the pressure regulator at
the fuel tank where vacuum biasing is not practical. For demand fuel
systems the pressure drop across the injector pressure changes with engine
vacuum. Correction tables for slope and injector offset vs. engine vacuum
are required. See section 5.2.2 for notes on Demand Fuel Systems.
Note
Useable injector dynamic range is reduced without vacuum biasing. (See
section 3.6.4 for pressure compensation.)
5.2.3.3 Variable Fuel Pressure Compensation
Fuel systems capable of variable fuel pressure control will require injector
flow data over a wider range of injector pressure differentials (pressure
differential = fuel pressure + engine vacuum). The control system will
typically use the differential pressure to look up the injector slope and
intercept for the current operating point.
Variable fuel pressure can be used to enhance injector flow range by
reducing pressure at low fuel consumption conditions to increase
minimum commanded pulse widths. Likewise, during maximum power
conditions the fuel pressure can be temporarily boosted to increase
injector flow if the pump has sufficient capacity.
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