TBI vs PFI
There are two basic types of Electronic Fuel Injections sytems:
-
Throttle Body Injection (TBI) - also known
as
Central Fuel Injection (CFI)
-
Port Fuel Injection (PFI) - also know as
Multi
Point Injection (MPI) - or Multi-point Fuel
Injection (MFI)
Throttle Body Injection
- TBI
The TBI system uses one (or two on a V-block engine) fuel
injectors mounted above the throttle plate on the intake
mainfold. As air is drawn into the intake, fuel is sprayed
to mix with the incoming air. The atomized air/fuel mixture
is then drawn into the cylinders by the downward movement
of the pistons on the intake stroke. This produces a much
better air/fuel mixture than a carbureted system, but can
still allow fuel to condense in the intake during cold weather.
Port Fuel Injection - PFI
The PFI system uses one injector for each cylinder located in the
intake manifold, just at the base of the intake valve. This
results
in increased fuel efficiency, as the atomized fuel is not mixed
with
the air until it is needed - as the air is entering the cylinder
on the
intake stroke. Injectors may be controlled in banks, in pairs,
or
individually depending on the system.
Idle Air Control
When the accelerator pedal
is released, the throttle plate in the air intake closes completely.
Because there are three requirements for combustion to take place
- air, fuel and ignition - the
engine would stall. To prevent this from happening, a passageway
is built into the throttle body
that allows air to bypass the throttle plate when it is closed.
To control this opening - and maintain
a correct idle speed - the passageway must be controlled by a valve.
This is done in two ways:
-
Idle Air Control Solenoid
A solenoid controlled valve is activated by the computer. Because
a
solenoid only has two positions - open or closed - the computer must
open and close the valve at a predetermined rate to control air flow.
The rate at which the valve is opened and closed is know as the
duty cycle. The duty cycle will increase as more air
flow is needed.
A small electric motor is used to position a valve in the bypass
passageway. The motor will vary the size of the opening by moving
a pintle back and forth to increase or decrease the size of the
opening,
and control idle speed.
Next - Computer Inputs and
Outputs
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