Woodward GECO, Technical Manual, Page: 33 / 90

Manual 03554 GECO

Stoichiometric
Woodward 25
Open Loop Strategy
Open loop means that the closed loop feedback signal is not used or is not
available. In the case of the GECO
 Stoichiometric control, if the EGO1 or EGO2
feedback signals are not available, the control operates in open loop mode.
In open loop operation, the control commands the fuel trim valve to take a
predetermined position stored in the control software. This position comes from
what is called the “Valve-Learn” table. The table contains eight load levels, and
each level can have a different valve position command. Valve positions between
the table values are interpolated. The load levels are identified by a normalized
fuel flow called “Mdot Fuel.” These load levels can be adjusted to match the
engine. Mdot Fuel is calculated from intake manifold pressure (MAP) and engine
speed. It is approximately equal to the pounds per hour of fuel flow per liter of
engine displacement. It requires the MAP and MPU sensor values for its
calculation.
Figure 4-3. Open Loop Control Mode Diagram
These predetermined valve positions in the table are normally entered manually
at the time of installation and setup. They can also be learned through an internal
“Valve-Learn” function during normal closed loop operation. While the engine and
control are operating in one of the closed loop modes, the control “reads” the
correct valve position at the current load and enters it into the table for possible
future use.
The control can be switched into open loop operation at any time by switching
the open-loop/closed-loop toggle switch (the CLE switch) located on the bottom
right corner of the circuit board.
Start-up Strategy
During engine start-up, the sensors do not provide useful information for engine
air/fuel ratio control operation. The GECO
 Stoichiometric control ignores most of
these sensor readings and positions the fuel trim valve as a function of engine
speed and air flow. When the engine reaches “run” speed, it enables open loop
operation. When sufficient load has been added to increase air flow (called
Mdot_air) to the engine to the minimum value (called Mdot_air Breakpoint) and
the appropriate sensors are healthy (>40% health), it enables closed loop
operation.
Intake
Manifold
Pressure
(MAP)
Fuel
Trim
Valve
Valve Position
Table
Fuel Flow Valve 1
0.0 40.0
2.0 45.0
4.0 50.0
6.0 60.0
8.0 65.0
10.0 70.0
12.0 72.0
14.0 73.5
Open Loop/
Closed Loop
Fuel Flow
Estimation
Speed
Closed Loop
Control