Fundamentals Multec 3.5 Fuel Injector Application Manual
2-20 Delphi Energy and Chassis Systems
Revision: 11/05-1
Likewise, it is sometimes necessary to protect the catalyst from high
temperature degradation by enrichment of the intake fuel charge. This
condition is typically referred to as catalyst protection or catalyst over-
temperature protection (COT). Such a condition might be seen under high
load conditions, such a towing a trailer or climbing a steep grade. Typical
A/F ratios might be similar to those seen with PE.
Such enrichment modes are determined by the engine controller and must
be considered when sizing a fuel injector for an application.
2.2.9 Injector Flow Tolerances
Consistent fuel delivery is
critical
Cylinder-to-cylinder air/fuel variation is a function of both air and fuel
flow fluctuations. To meet air/fuel ratio specifications, injector-to-injector
flow differences in the rail must be minimized as any variation adversely
affects the air/fuel ratios for all the cylinders. The engine controller,
working in concert with the oxygen sensor (if present), usually cannot
control the air/fuel ratio in each cylinder. Instead, it averages the air/fuel
ratios for each cylinder together. So if one injector runs rich, the engine
controller compensates by commanding all injectors lean. (Some V
engine systems control each bank of cylinders independently through
separate oxygen sensor feedback.)
Good system design requires consideration of optimum fuel spray
targeting and preparation for consistent fuel delivery, and having a rail
design that eliminates injector-to-injector pressure variations.
Injector flow tolerance increases for fuel supply rates that are outside of
the “working flow range” (see section 3.10.2). This increase in tolerance
results from operating the injector at pulse widths close to its opening and
closing response times. The voltage supply available, injector operating
temperatures, manifold absolute pressure and the pressure drop across the
injector will also affect its flow performance. Typically compensation
tables are used in the vehicle calibration software to correct for changes in
voltage and manifold absolute pressure. Corrections for injector
temperature are currently being developed.
2.3 Fuel Flow Considerations
Correct sizing of both the fuel injector (including rail assembly) and the
fuel pump is essential for proper operation on the vehicle (refer to Section
3). For the fuel injector to operate properly, the fuel pump must meet the
following requirements.
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