Lambda as a Diagnostic Tool
The lambda calculation determines the ratio between the amount of oxygen actually present in a combustion chamber vs. the amount that should have been present to obtain perfect combustion.
Let’s learn more about this remarkable tool, beginning with lambda’s meaning. Lambda represents the ratio of the amount of oxygen actually present in a combustion chamber compared to the amount that should have been present in order to obtain “perfect” combustion. Thus, when a mixture contains exactly the amount of oxygen required to burn the amount of fuel present, the ratio will be one to one (Ll) and lambda will equal 1.00. If the mixture contains too much oxygen for the amount of fuel (a lean mixture), lambda will be greater than 1.00. If a mixture contains too little oxygen for the amount of fuel (a rich mixture), lambda will be less than 1.00.
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The Wide-Band sensor generates a variable signal as opposed to the simple rich/lean signal of a standard oxygen sensor. Because the signal varies in strength and also in current flow direction (polarity), it’s impossible to directly view the signal with anything except an oscilloscope. However with the right supporting equipment, the Wide-Band sensor can be used for adjusting air/fuel mixture on any engine.
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We all know that perfect combustion requires an air/fuel ratio of approximately 14.7:1 (by weight) under normal conditions. Thus a lean air/fuel ratio of, say, 16:1 would translate to a lambda value of 1.088. (To calculate, divide 16 by 14.7.) A lambda of .97 would indicate an air/fuel ratio of 14.259:1 (derived by multiplying .97 by 14.7).
Here’s the magic: Lambda is completely unchanged by combustion. Even complete combustion or a total lack of combustion has no effect on lambda! This means we can take our exhaust gas samples at any point in the exhaust stream without having to worry about the effects of the catalytic converter.
What’s wrong with this car?
HC: 2882 ppm CO: .81%
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CO2: 13.69% O2: 2.18%
Is it a mechanical problem? An ignition problem? An air/fuel ratio imbalance? What are these emissions readings trying to tell us? At first glance, the high hydrocarbon (HC) reading would seem to indicate an abundance of available fuel, yet the very high oxygen (O2) reading might lead us to wonder if we’re looking at a lean misfire condition. The relatively low carbon monoxide (CO) figure seems to rule out a rich mixture, while the carbon dioxide (CO2) reading might suggest either an inoperative catalytic converter or an engine mechanical efficiency problem.
In this instance, lambda indicates a substantially rich mixture-just the opposite of what we might have thought based on the individual gas readings alone. After all, CO, normally the indicator of a rich condition, is considerably lower than Oz, which is the telltale indication of a lean exhaust. Coupled with the high HC readings, most of us would probably have pegged this as a lean misfire condition.
In fact, these readings were taken on a Ford Escort with one plug wire grounded. The converter had been allowed to cool briefly (in hopes of avoiding a red-hot meltdown), but the heated oxygen sensor rapidly returned to closed-loop. The extra O2 content in the exhaust stream from the dead cylinder caused the PCM to command a rich mixture in response.
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