G420E/G424E
Tier
LP Engine
System Operational Overview
24
Heated Exhaust Gas Oxygen
Sensor (HEGO)
The HEGO sensor (Figure 24) installed in the exhaust
manifold before the catalytic muffler is a basic
zirconium type oxygen sensor comprised of a hollow
cone-shaped internal element made of zirconium
dioxide (ZrO2, a ceramic material), which is coated
with a thin layer of micro-porous platinum. The outer
layer is exposed to the exhaust stream, while the
inner layer is vented to the atmosphere and attached
to a wire that runs to the SECM. It operates like a
galvanic cell with the zirconium dioxide acting as the
electrolyte and the platinum layers serving as
electrodes. Once the ZrO2 reaches approximately
600 deg. F., it becomes electrically conductive and
attracts negatively charged ions of oxygen. These
ions collect on the inner and outer platinum surfaces.
Naturally, there's more oxygen in plain air than in
exhaust, so the inner electrode will always collect
more ions than the outer electrode, and this causes a
voltage potential for electrons to flow. The
concentration of oxygen in the exhaust stream
determines the number of ions on the outer electrode,
hence the amount of voltage produced. If the engine
is running rich, little oxygen will be present in the
exhaust, few ions will attach to the outer electrode,
and voltage output will be relatively high. In a lean
situation, more oxygen will be present, and that
translates into more ions on the outer electrode, a
smaller electrical potential, and less voltage. In order
for the sensor to conduct and create an electrical
signal below 600 deg. F., a heated element is added
to the sensor housing. Two wires provide the
necessary 12VDC and ground signal for the heater
element. A fourth wire provides an independent
ground for the sensor.
The HEGO stoichiometric air/fuel ratio voltage target
is approximately 500mV and changes slightly as a
function of speed and load. When the HEGO sensor
sends a voltage signal less than 500mV the SECM
interprets the air/fuel mixture to be lean. The SECM
then decreases the duty cycle of the FTV lowering the
amount of air valve vacuum (AVV) acting on the
atmospheric side of the N2001 secondary diaphragm,
increasing the regulator vapor propane output to
richen the air/fuel mixture. The opposite is true if the
SECM receives a voltage signal above 500mV from
the HEGO. The air/fuel mixture would then be
interpreted as being too rich and the SECM would
increase the duty cycle of the FTV.
CAUTION
The HEGO sensor used is calibrated to
work with the MI-04 system. Using
alternate sensors may impact drivability
and the ability of the system to diagnose
rich and lean conditions.
Figure 24
