Service manual

G420F(FE) Service Manual Chapter 5. Engine Management System (EMS) 164
pressure regulator outlet pressure changes by the
same amount and in the same direction. The end
result is that a change in FTV modulation changes
the outlet pressure of the regulator/fuel inlet
pressure of the mixer, and thus the AFR. A major
benefit of this trim system results from the use of
mixer inlet pressure and AVV as the reference
pressure extremes. The pressure differential across
the mixer fuel valve is related to these same two
pressures, and thus so is fuel flow. Given this
arrangement, the bias pressure delta scales with the
fuel cone delta pressure. The result is that the trim
system control authority and resolution on AFR
stays relatively constant for the entire speed and
load range of the engine.
SECM
The Small Engine Control Module (SECM) controls
the LPG lock-off solenoid valve and the FTVs. The
lock-off solenoid is energized when fueling with LPG
and the engine is turning. FTV modulation frequency
will be varied as a function of rpm by the SECM in
order to avoid resonance phenomena in the fuel
system. FTV commands will be altered by the
SECM in order to maintain a stoichiometric air-fuel
ratio. Commands are based primarily on feedback
from the exhaust gas oxygen sensor, with an offset
for fuel temperature.
MI-07 LP Fuel Filter
After exiting the fuel tank, liquid propane passes
through a serviceable inline fuel filter to the electric
fuel lock off. Figure 3 shows a typical inline type LP
fuel filter manufactured by Century. The primary
function of the fuel filter is to remove particles and
sediments that have found their way into the tank.
The LP fuel filter will not remove heavy end solids
and paraffins that build up in LPG fuel systems as a
result of vaporization.
Figure 3. Inline LP Fuel Filter
MI-07 Fuel Lock-Off (Electric)
The fuel lock-off is a safety shutoff valve, normally
held closed by spring pressure, which is operated by
an electric solenoid and prevents fuel flow to the
regulator/ converter when the engine is not in
operation. This is the first of three safety locks in the
MI-07 system.
Figure 4. Electric Fuel Lock Assembly
In the MI-07 design, power is supplied to the fuel
lock-off via the main power relay with the SECM
controlling the lock-off ground (earth) connection.
The lock-off remains in a normally closed (NC)
position until the key switch is activated.
This supplies power to the lock-off and the SECM,
but will not open the lock-off via the main power
relay until the SECM provides the lock-off ground
connection. This design gives the SECM full control
of the lock-off while providing additional safety by
closing the fuel lock-off in the unlikely event of a
power failure, wiring failure or module failure.
When the liquid service valve in the fuel container is
opened, liquid propane flows through the LP filter
and through the service line to the fuel lock-off.
Liquid propane enters the lock-off through the 1/4”
NPT liquid inlet port and stops with the lock-off in the
normally closed position. When the engine is
cranked over the main power relay applies power to
the lock-off and the SECM provides the lock-off
ground causing current to flow through the windings
of the solenoid creating a magnetic field. The
strength of this magnetic field is sufficient to lift the
lock-off valve off of its seat against spring pressure.
When the valve is open liquid propane, at tank
pressure, flows through the lock-off outlet to the
pressure regulator/converter. A stall safety shutoff
feature is built into the SECM to close the lock-off in
case of a stall condition. The SECM monitors three
engine states: Crank, when the crankshaft position
sensor detects any engine revolutions; Stall, when
the key is in the ON position but the crankshaft
position sensor detects no engine revolutions; and
the Run state, when the engine reaches pre-idle rpm.