Woodward GECO, Technical Manual, Page: 32 / 90

GECO

Stoichiometric Manual 03554
24 Woodward
The actual pre-catalyst EGO voltage, which provides the lowest emissions,
varies a little depending on many parameters, including fuel composition,
ambient conditions, catalyst activity, etc. To compensate for these variations, an
oxygen sensor downstream of the catalyst provides a feedback signal that
correlates more consistently with actual emissions. In other words, emissions are
more tightly regulated by maintaining a constant downstream EGO signal.
Unfortunately, the increased feedback delay from a downstream sensor alone
would make the response of the fuel control too slow. The GECO system solves
these limitations with the use of both upstream and downstream oxygen sensor
feedback. The upstream sensor (EGO1 and EGO2), mounted close to the
engine, provides quick response to fuel control changes and allows precise
control at a specific oxygen sensor target. The control then modifies this
upstream EGO target based on information from the downstream EGO3
sensor—the system adaptively learns the optimum control target for the current
engine operating conditions.
Figure 4-2. Adaptive EGO Closed Loop Control Mode Diagram
The most common upstream EGO target value is entered as the “EGO1 Base”
and the “EGO2 Base” values. These values automatically become the EGO
target value until the post-catalyst control modifies the EGO1/2 Base values with
the “Adaptive” signal.
EGO1 Base + Adapt = EGO1 Target
The EGO1/2 Base adjusted by the Adaptive value becomes the EGO target. The
Adaptive signal is the output of the post-catalyst EGO3 control. In the event that
the EGO3 sensor signal is lost, the EGO target reverts back to the EGO1/2 Base
value.
The “EGO3 Base” value is the “EGO3 target” value after it has been modified by
the “EGO3 Offset.”
EGO3 Base + EGO3 Offset = EGO3 Target
Normally, the EGO3 Base is the optimum post-catalyst exhaust oxygen sensor
voltage at engine full power. The EGO3 Offset is an adjustment to this EGO3
Base value for operating conditions at less than full power.
Dynamic adjustment of the pre-catalyst control loop is made with the closed loop
gain called “K_CL” and the EGO1/2 filter tau. Adjustment of the post-catalyst
control loop is with the Adaptive gain called the “K_Adapt” and the EGO3/4 filter
tau.
+
Post-Cat
Exhaust
Oxygen
(EGO3)
EGO1
Base
+
-
+
+
+
-
Fuel
Trim
Valve
Pre-Cat
Exhaust
Oxygen
(EGO1)
Filter
Filter
EGO3
Target
EGO1 Target
+
EGO3
Base
+