Model Based Evaluation of UEGO Performance and Sensitivity
Closed loop fuel injection have been in use for two decades but it's not until
the recent five years that the wide band lambda sensor have been utilized. The
goal is to explain wide band and discrete lambda sensors in a simple but
powerful way. Both sensors are modeled by simple mathematics and
accounts for Oxygen, Hydrogen and Carbon monoxide influences. The focus is not just
on the output from the sensors, but also on the underlying function. This means
that all explanations are thorough and methodical. The function of a wide band
lambda sensor is more complicated than a discrete type lambda sensor,
therefore it's harder to get correct readings. The model of the wide band
lambda sensor is used to evaluate different problems in preparation for the
development of an observer. Several potential problem sources are tested and
investigated, these include calibration error, pressure error, air leak
error, gas sensitivity and fuel errors. To evaluate the potential problems and
their ability to explain differences between actual lambda and sensor output, two
sensors with differing outputs have been used. The final result is implemented
in an ECU.
The models indicate that the difference between the two sensors is most
likely explained by different sensitivity for CO, O2 and H2. This can in turn have
one or several explanations. It is suggested that different ability to pump
oxygen, different nernst cells or even different controllers can cause
this. The reason is not investigated further as this would require a
very deep research on the two sensors. Because no usable explanation is found an
observer that estimates the offset at stoichiometric conditions, where
lambda equals one, is constructed. The observer uses the fact that the switch point
of a discrete lambda sensor is insensitive to disturbances. The offset
calculation is performed in real time on an ECU. Tools for calibration
of the observer are also developed. With the observer the error for the two sensors
is roughly halved over the whole spectrum and at stoichiometric conditions,
which is the normal operation for an engine, the error was too small to
measure.
Although the wide band lambda sensor is a very complex sensor it is
shown that it can be understood with simple mathematics and basic knowledge in
chemistry. The developed model agrees well with the real sensor for steady
state conditions. For transient conditions, however, the model needs to be
refined further. The question why the two sensors differ is discussed
but the true origin of the cause remains unsolved. The conclusion is that the error
can be drastically reduced with just an offset. It is also shown that when
building a lambda sensing device the controller is of equal importance
as the sensor element itself. This is due to the sensitivity of surrounding factors
that the controller must be able to handle. These effects are specially
important for engines running at lambda not equal to 1, for example diesel
engines.
Keywords: wide band, narrow band, switch type, uego, ego, o2 sensor,
oxygen sensor, lambda, zirconia
Thommy Jakobsson
2006
Page responsible: webmaster
Last updated: 2021-11-10
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