Woodward GECO, Technical Manual, Page: 13 / 90

Manual 03554 GECO

Stoichiometric
Woodward 5
Chapter 2.
System Description and Application
GECO™

Stoichiometric Air/Fuel Ratio Control
The GECO™
 Stoichiometric gas engine control is a microprocessor-based
air/fuel ratio control for carbureted, four-stroke, gaseous-fueled engines operating
with a near-stoichiometric air/fuel ratio. It is designed to work in conjunction with
a three-way catalytic converter to efficiently reduce exhaust emissions. By
automatically maintaining an optimum air/fuel mixture, emissions compliance is
achieved and catalyst life is maximized without operator supervision.
Figure 2-1. System Components
In supplementary mode, the carburetor remains in place on the engine, adjusted
slightly lean. During operation, the GECO
 Stoichiometric control adds fuel
through a supplementary fuelling system to bring the air/fuel mixture to the
precise value that will make the catalytic converter most efficient. In full-flow
mode, the carburetor remains in place on the engine, adjusted slightly rich.
During operation, the GECO stoichiometric control restricts flow using a full-flow
trim valve located inline upstream of the carburetor to bring the air/fuel mixture to
the required precise value that will make the catalytic converter most efficient.
A catalyst that simultaneously eliminates hydrocarbons (HC), carbon monoxide
(CO), and oxides of nitrogen (NO
x
) is referred to as a “three-way” catalyst. The
use of a three-way catalyst will reduce the emissions of a stoichiometric engine.
However, for these reductions to be reliable and maximized and to protect the
catalyst element from premature aging or damage, a very precisely controlled
air/fuel ratio is required.
If a three-way catalyst receives exhaust gas containing emissions in the
proportions shown in the “Stoich” window of Figure 2-2, the resulting emissions
exiting the catalyst will be reduced to the levels shown in Figure 2-3.
The exhaust gas oxygen sensor (EGO) will generate a voltage signal that is
characteristic of this ideal stoichiometric window. The GECO control will use this
voltage value to keep the engine at this correct air/fuel ratio. For natural gas this
voltage is usually around 0.780 V.