A lambda probe (or oxygen sensor) can be found on the exhaust system of
most cars running on unleaded fuel. Having reached its normal operating
temperature (of about 600 degrees Celsius!) the lambda probe supplies an
output voltage proportional to the amount of residual oxygen measured
in the exhaust gas. This information is indicative of, among others, the
air/fuel ratio supplied by the carburetor(s) and hence the combustion
efficiency. In modern car (and motorcycle) engines, this information is
used to (electronically) adjust engine parameters like ignition timing
and fuel injection. The indicator described here is intended for
permanent installation on a motorcycle of which the air/fuel ratio
needed to be watched, with the obvious aim engine power tuning after
fitting a different set of carburetors.
Apart from this obvious technical use the unit’s bright LEDs will no
doubt attract the attention of curious motorcyclists. At the local
junkyard a single-wire lambda probe may be salvaged from a wrecked car.
Once a suitable nut has been found, the probe can screwed into the
exhaust pipe of the motorcycle, at about 30 cm from the cylinders. Since
we’re talking of welding and drilling in an expensive (chrome-plated)
exhaust pipe, you may find that actually fitting the probe is best left to
specialists! The starting point for the design of a suitable electronic
indicator is that in the noble art of carburetor tuning an air/fuel
ratio of 14.7 to 1 is generally considered ‘perfect’, the range covering
16.2 to 1 (‘lean’) to 11.7 to 1 (‘rich’).
The perfect ratio typically corresponds to a probe output voltage of 0.45 V. Referring to the circuit diagram, that is the input level at which 5 of the 10 LEDs will light, including the green one, D5. If one of the red LEDs lights, the mixture is definitely too rich. Note that in general it is better to have a mixture that is a little to rich than one that’s on the lean side, hence a yellow LED lights between the green LED and the first red one. Also note that the engine needs to be at its normal operating temperature before a meaningful indication is obtained.
The perfect ratio typically corresponds to a probe output voltage of 0.45 V. Referring to the circuit diagram, that is the input level at which 5 of the 10 LEDs will light, including the green one, D5. If one of the red LEDs lights, the mixture is definitely too rich. Note that in general it is better to have a mixture that is a little to rich than one that’s on the lean side, hence a yellow LED lights between the green LED and the first red one. Also note that the engine needs to be at its normal operating temperature before a meaningful indication is obtained.
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