AFR200 Series, Product User Guide
This document is subject to change without notice.
Caution: None of GAC products are flight certified controls including this item
Copyright © 2010, Governors America Corp., All Rights Reserved
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PUG4144 D
3 Operational Description
3.1 Overview
The AFR’s fuel control algorithm maintains a stoichiometric air-fuel ratio for optimized emissions and engine
performance. Engine speed can be governed in either isochronous or droop modes using GAC’s proprietary
Electronic Digital Governor (EDG) algorithm.
The AFR control operates the closed loop fuel system with five major components:
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Digital precise stepper motor fuel control valve to adjust the flow of fuel into the system
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Static venturi mixer to combine fuel and air to the appropriate mixture
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Electrically controlled throttle body valve or universal actuator to control the amount of air mixture that
enters the engines intake manifold based on engine speed input
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Oxygen sensor to monitor exhaust to determine whether the engine is running lean or rich
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Manifold Air Pressure (MAP) sensor to measure intake manifold pressure (or vacuum)
For added control and engine protection, the AFR can also sense the following optional sensors:
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Second (post-catalytic) oxygen sensor
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Engine Oil Pressure (EOP) sensor
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Engine Coolant Temperature (ECT) sensor
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Exhaust Gas Temperature (EGT) sensor; Type K thermocouple
3.2 Air Fuel Ratio (AFR) Controller Feedback Inputs
The following sensors are mandatory and used by the AFR for closed-loop control of the air fuel ratio and engine
speed governing.
3.2.1 Pre-Catalytic Converter Oxygen (O2) Sensor
The AFR uses a narrow-band oxygen sensor, located after the merge point in the exhaust manifold / plumbing
and before any exhaust conditioners. This sensor output is between 0 and 1V based on the oxygen concentration
in the exhaust gas. The closed loop feedback is accomplished by varying fueling based on engine speed and load
in addition to the oxygen sensor output.
When no signal is received from the O2 sensor, the sensor fails, or the sensor is not at operating temperature
(must be greater than 600°F) as is the case when a cold engine is first started, the AFR orders a fixed
(unchanging) rich fuel mixture. This is referred to as "open loop" operation because no input is used from the O2
sensor to regulate the fuel mixture.
Adaptation is further carried out by a feedback / predictive algorithm which uses customizable PIDs. The
predefined table values are then changed real-time to ensure the desired oxygen sensor voltage set point
(lambda value) is constantly maintained. The air fuel mixture is constantly adjusted which results in a switching
from lean to rich and vice versa in order to operate at peak efficiency and minimize emissions.
Voltages near 0.9V indicate that the fuel mixture is rich and there is little unburned oxygen in the exhaust whereas
voltages closer to 0.1V indicate the mixture is lean. At 0.5V the engine is operating at stoichiometry.
3.2.2 Manifold Absolute Pressure (MAP) sensor
A pressure sensitive electronic circuit inside the MAP sensor monitors the movement of the internal diaphragm
and generates a voltage signal that changes in proportion to intake manifold pressure. This produces an analog
voltage signal that typically ranges from 0.5 to 4.5 volts. The output voltage usually increases when the throttle is
