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Fuel Mixture Display For Cars, Pt 1

Has digital readout and bargraph displays. A PIC microcontroller makes it a snack to build.

By John Clarke

Your car engine’s air-fuel ratio has a considerable bearing on fuel consumption and air pollution. For this reason, modern cars use an exhaust gas oxygen (EGO) sensor mounted in the ex­haust system to continuously monitor air-fuel ratios and generate corresponding output voltages. This information is then fed to the engine management computer (EMC) which continuously adjusts the mixture to provide optimum power and economy, consistent with low exhaust emissions.

Click for larger image

As can be imagined, if the EGO sensor is not functioning correctly, engine performance suffers and this can lead to very high fuel bills. Conversely, a properly functioning sensor en­sures good engine performance and helps keep fuel costs down – something that’s even more important than ever given the recent petrol price hikes.

A system that’s in good nick also minimises air pollution. In modern cars, the combustion products from the engine are made safe by a catalytic converter which is mounted in the exhaust system. Combustion byprod­ucts such as carbon monoxide (CO) are converted to carbon dioxide (CO2), unburnt hydrocarbons to CO2 and water (H2O) and nitrous oxide (NO) to nitrogen (N2) under the action of the catalysts within the converter.

Main features

  • Suits vehicles with Zirconia EGO probes
  • Compact size
  • 3-digit LED readout plus 7-LED bargraph display
  • Dot or bar mode option for bargraph
  • Air-fuel ratio matched for Bosch LSM11 (0258104002) EGO probe
  • Indicates air-fuel ratios from 11.8 to 20.6 for petrol
  • Indicates air-fuel ratios from 12.7 to 21.5 for propane/LPG
  • Fully lean and fully rich air-fuel indication
  • Bargraph display follows the non-linear response of the EGO sensor
  • 13 indication levels in dot mode;, 7 levels in bar mode
  • Fast 220ms update time for bargraph; 440ms for 3-digit display
  • Automatic display dimming
  • 0-1V display for setting up adjustments

Note: this device is not suitable for use on cars that run on leaded petrol. If your car doesn’t already have an EGO sensor, you can fit one yourself but the engine must run on unleaded fuel, propane or LPG in order to use the Fuel Mixture Display continuously (leaded petrol poisons the sensor).

However, this only works properly if the air-fuel ratio is held within certain limits. And that in turn is dependent on the EGO sensor. An EGO sensor, by the way, does not last indefinite­ly. Depending on the car, it’s usually a good idea to replace it every 50,000 to 80,000km.

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