G420E/G424E Tier LP Engine System Operational Overview 23
MI-04 Fuel Trim Valve
The Fuel Trim Valve (FTV) is a two-way electric
solenoid valve and is controlled by a pulse width
modulated (PWM) signal provided by the SECM. The
FTV is used to bias the output fuel pressure on the
LPG regulator/converter (N-2001), by metering air
valve vacuum (AVV) into the atmospheric side of the
N-2001 secondary regulator diaphragm. An orifice
balance line connected to the air inlet side of the
mixer provides atmospheric reference to the N-2001
when the FTV is closed. The SECM uses feedback
voltage from the O2 sensor to determine the amount
of bias needed to the regulator/converter.
In normal operation the N2001 maintains fuel flow at
a constant output pressure, due to the calibrated
secondary spring. The amount of fuel flowing from the
N2001 will vary depending on how far the secondary
diaphragm opens the secondary valve in response to
the negative pressure signal generated by the air/fuel
mixer. One side of the N2001 secondary diaphragm is
referenced to atmospheric pressure while the other
side of the diaphragm reacts to the negative pressure
signal from the mixer. If the pressure on the
atmospheric side of the N2001 secondary diaphragm
is reduced, the diaphragm will close the secondary
valve until a balance condition exists across the
diaphragm, reducing fuel flow and leaning the air/fuel
mixture.
A branch-tee fitting is installed in the atmospheric
vent port of the N2001 with one side of the branch-tee
connected to the intake side of the mixer forming the
balance line and referencing atmospheric pressure.
The other side of the branch-tee fitting connects to
the FTV inlet (small housing side). The FTV outlet
(large housing connector side) connects to the AVV
port. When the FTV is open AVV is sent to the
atmospheric side of the N2001 secondary diaphragm,
which lowers the reference pressure, closing the
N2001 secondary valve and leaning the air/fuel
mixture. The MI-04 system is calibrated to run rich
without the FTV. By modulating (pulsing) the FTV the
SECM can control the amount of AVV applied to the
N2001 secondary diaphragm. Increasing the amount
of times the FTV opens (modulation or duty cycle)
causes the air/fuel mixture to become leaner;
decreasing the modulation (duty cycle) enriches the
mixture.
(Figure 22) shows the Fuel Trim Valve connected in
the MI-04 system.
Fuel Trim Valve
Balance Line
FTV
Figure 22
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