Doosan G643, Руководство по обслуживанию

Страница: 154 / 260

G643(E) Service Manual Chapter 5. Engine Management System (EMS) 152
Dual Dither Valve
The key to meeting emissions requirements when
operating in LPG is the dual dither valve hardware in
the fuel system. Similar to the MI-04 system, the
dual dither system modulates the fuel pressure
regulator outlet pressure by providing an offset to
the regulator secondary stage reference pressure.
By adding a second dither valve, or fuel trim valve
(FTV), to the MI-07 system, smoother, more
accurate control of supply pressure is achieved,
resulting in better control of air fuel ratio and
emissions. This smoother control also minimizes
wear on fuel system components such as the
regulator diaphragm and lever by significantly
reducing the pressure pulsations observed with a
single FTV.
Regulator Pressure Offset
Regulator pressure offset is achieved through the
use of a fixed orifice and a variable orifice in series.
The inlet to the fixed orifice is connected to the
mixer inlet pressure (roughly equal to ambient
pressure). The outlet of the fixed orifice is connected
to both the pressure regulator reference port and the
inlet to the two FTVs (the variable orifice) that act in
parallel. The outlets of the FTVs are connected to
the mixer outlet, referred to as Air Valve Vacuum
(AVV). Thus, by modulating the FTVs, the pressure
regulator reference pressure can be varied between
mixer inlet pressure and AVV. For a given change in
the pressure regulator reference pressure, the
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