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Wideband Air-Fuel Mixture Display Unit

This Wideband Oxygen Sensor Display can show your car's air-fuel ratio as you drive. It's designed to monitor a wideband oxygen sensor and its associated wideband controller but could be used to monitor a narrowband oxygen sensor instead. Alternatively, it can be used for monitoring other types of engine sensors.

By John Clarke

Main Features & Specifications

MAIN FEATURES

  • 3-digit LED display plus 7-segment bargraph.
  • Linear display with 0-5V wideband range or 0-1V S-curve range.
  • Alternative display switching (A or B readings for wideband values); petrol or LPG readings for narrowband S-curve.
  • 0V and 5V endpoint value limit adjustments for both A and B displays.
  • Decimal point positioning.
  • Display leading zero suppression.
  • Bargraph can be operated in dot, bar or centred-bar mode for wideband range. S-curve set-up allows for dot or centred bar styles.
  • Display dimming with minimum brightness and dimming threshold adjustments.
  • Quieting period used for input measurement to ensure accuracy.

    SPECIFICATIONS

    Power Supply: 6-15V @ 240mA (full display brightness)

    Input Current Loading: less than ±1mA

    Digit Update Period: 250ms

    Bargraph Update Period: 30ms

    Wideband Display Reading Range: 0-999

    Narrowband Display Reading Range: 11.8 to 20.6 for unleaded petrol with the stoichiometric ratio set for 14.7; 12.6 to 21.4 for LPG with stoichiometric at 15.5. The display shows an "L" for ratios below the lowest value and an "r" for ratios above the highest value.

  • WHY WOULD YOU want to monitor the air/fuel ratio as you drive? Well, for starters, it will allow you to save fuel since the display clearly indicates when the engine is running rich.

    When used in conjunction with a wideband oxygen sensor and controller, the air/fuel ratios shown on this unit are more accurate than can be obtained from the narrowband sensors that are typically used in cars and which are really only accurate close to the "stoichiometric" point (ie, the air/fuel ratio at which there is just enough oxygen in the air to ensure complete combustion).

    Click for larger image
    Fig.1: here's how the display unit is used with a wideband sensor and its associated controller. The narrowband S-curve output from the controller is fed to the engine management computer (see text).

    Under normal driving, most engine management systems operate under "closed loop" control. This is where the air/fuel ratio from an oxygen sensor is monitored and controlled by the car’s engine control unit (ECU) to maintain a predetermined fuel mixture. This is usually stoichiometric but under light cruise conditions the mixture can go
    lean to improve fuel economy.

    Conversely, during acceleration, the air/fuel mixture in many cars is allowed to go rich to improve performance and is not under the control of the ECU. This is called "open loop" and the richness of the mixture depends on other factors such as the throttle setting and the injector opening period.

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