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 Obtaining
the correct amount of fuel flow from a group of injectors in an EFI
system is similar to determining the right size jets for a carbureted
engine. The fuel flow of both systems must be matched to the airflow
requirements of the engine over a broad rpm operating range.
A
carburetor may have three or more separate control circuits such as
idle, mid-range, and power, to deliver a fuel curve over the engine’s
operating range. An injector also must be able to supply a small amount
of fuel to support engine idle, a large amount at wide-open throttle
to prevent high-rpm lean-out, and transitional amounts to cover all
of the operating conditions in between. These wide operating boundaries
demand that an injector have a wide ”dynamic range” of operation.
The
amount of fuel delivered by an electronic injector is controlled by
how long it is held open by the Engine Control Unit (ECU). It is “pulsed
or energized” open for short periods of time at idle, and held
open longer as rpm and airflow increase.
Just
as the wrong-sized jets in a carb can cause driveability problems such
as rough idle, surging, poor throttle response or even high-rpm lean-out,
so can incorrect injectors. Following is a guideline equation for approximating
fuel flow per injector based on estimates on engine Horsepower (HP)
and Brake Specific Fuel Consumption (BSFC). Take note of these conditions
for equation accuracy:
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Engine HP must
be realistic estimate of engine output.
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BSFC is determined from engine dynamometer mea- surements. It typically
ranges from 0.4 - 0.6 for gasoline powered engines. A BSFC of 0.5
is a reasonable initial estimate for most engines.
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The 0.8 multiplier of the "Number of Injectors" helps us derive
a practical, maximum "Injector Flow Rate" for each injector based
on an effective real world injector operat-ing pulse time and fuel
flow. It's unrealistic to establish the fuel flow to the engine
based on an injector operating pulse time of 100% (wide open all
the time). This formula uses an injector operating cycle based on
80%. Some full race engine management systems may operate at 85
- 95% duty cycle, but doing so for some time increases the likelihood
of overheating the injectors which may cause irregular fuel rates
or a decrease in low rpm operation.
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For example,
to calculate the individual injector size for a 650 HP V8 using eight
injectors and assuming a BSFC of 0.5:
Use MSD Competition Fuel Injector PN 2013, rated
at 50 lb/hr static flow at 43.5 psi (3 barometric) fuel system pressure.
If
you have a known injector fuel flow rate you can solve the above equation
for a rough estimate of fuel system capacity like this:
NOTE: Keep in mind your application and other mechanical modifications
that have been made to your engine. The number of cylinders or extremely
high rpm engines (such as rotary engines) may require larger injectors
due to on/off times.
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