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What is an Oxygen Sensor?

  1. What do Oxygen Sensors do?
  2. How do they work?
  3. Types of Oxygen Sensors

 

What do Oxygen Sensors do?

Almost all petrol powered vehicles made since 1986, when new laws requiring all new passenger vehicles to operate on unleaded fuel in Australia were introduced; we have seen the constant evolution of engine management systems. This major change saw the introduction of Oxygen Sensors and Catalytic Converters.

Most vehicle since 1986 have at least one oxygen sensor, originally called Lambda Sensors, and newer vehicles often have two or more oxygen sensors. An oxygen sensors function has changed little over the last 30 years from when they were first used in European fuel injected cars, but their importance in engine operation has increased significantly. Oxygen sensors are now critical in modern vehicles engine management and emission control systems.

An oxygen sensor monitors the amount of oxygen (O2) in the exhaust gas. The amount of oxygen in the exhaust gas is a good indicator of engine combustion efficiency and is also the best place to monitor the air to fuel ratio. It turns out that there is a particular ratio of air and fuel that is perfect, and that ratio is 14.7:1, also known as the stoichiometric ratio (different fuels have different perfect ratios, the ratio depends on the amount of hydrogen and carbon found in a given amount of fuel).

Too much oxygen in the exhaust gases indicates a lean mixture. A lean mixture tends to produce more nitrogen oxide pollutants NOx, and in some cases it can cause poor performance including misfire and even engine damage with the engine running too hot.

Too little oxygen indicates a rich mixture, which wastes fuel and reduces economy, and the unburned fuel results in excess emissions.

Either condition can shorten the life of the expensive catalytic converter.

The oxygen sensor is located in the exhaust manifold downpipes before the catalytic converter or between the exhaust manifolds and the catalytic converters.

Engine management systems also now control and monitor the operation of the vehicles catalytic converter, so in recent years there has been a rapid increase in vehicles on our roads using pre and post catalytic converter oxygen sensors. With the fitment of sensors after the catalyst, the efficiency of the catalytic converter can now be measured.

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How do they work?

Oxygen sensors works as a result of the varying amounts of oxygen in the vehicle exhaust verses the amount  of oxygen in the atmosphere. An oxygen sensor generates a voltage signal, or changes its resistance, proportional to the amount of oxygen in the exhaust gas and sends this signal to the onboard computer or ECU (Engine Control Unit), which in turn regulates the air fuel mixture to the desired optimal level.

The sensing element on an oxygen sensor is generally a ceramic cylinder plated inside and out with porous platinum electrodes and protected in a housing which protects it against mechanical effects and facilitates mounting. The ceramic body is made of stabilised zirconium dioxide (Zirconia). Its surfaces are coated with electrodes made of a gas permeable platinum layer.

The outside of the cylinder is exposed to the hot exhaust gases, while the inside is vented internally through the sensor body or wiring to the outside atmosphere. A voltage is produced by the difference in the two amounts. If the amount of oxygen in the exhaust is closer to the amount in the air, the engine is lean and the voltage is low (normally 0.1 to 0.3 volts). If the engine is rich the voltage is high (generally 0.8 to 0.9 volts).

       

 (Click to enlarge)

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Types of Oxygen Sensors

To be sure of fitting the correct replacement oxygen sensor it is good to have an understanding of the different types available:

Narrow Band Oxygen Sensors (Zirconia dioxide or Zirconia)

Narrow band sensors are usually just called oxygen sensors because for many years there was only this one basic type of sensor. As of now most cars still use this type of sensor and it is the most common sensor, and come in heated and unheated forms.

It's called a narrow band sensor because it can only detect a very narrow band of air fuel ratio. It's basically an on -off switch in that it will either read lean or rich, but it doesn't tell the ECU how lean or how rich the mix is. It communicates with the ECU via the voltage it produces.

These sensors can have 1, 2, 3 or 4 wires. Narrow band sensors communicate their information by providing a voltage that is read by the ECU

There are two different types of Zirconia sensor:

Zirconia Thimble type

  • Element- Zirconium dioxide, thimble type
  • Types - one, two, three and four wire, 18mm thread
  • Output signal – usually oscillating 0.1 and 0.9 volts

Zirconia Thick film type

  • Element - Zirconium dioxide, planar type
  • Types - four wire, 18mm thread
  • Output signal – usually oscillating between 0.1 and 0.9 volts

Schematic picture -A planar Zirconia sensor (Click to enlarge)

 

Narrow Band Sensor (Titanium Dioxide)

There's another type of narrow band sensor called a Titanium Dioxide sensor. We only see this rarely. It uses a voltage of 0 to 5 volts, and the logic of rich and lean are backwards from other sensors (low voltage means rich and higher voltage means lean).

  • Element- Titanium Dioxide, planar type
  • Types- three and four wire, 12mm and 18mm thread
  • Heater resistance - 4 to 7 ohms
  • Output signal - oscillating between 0 and 1 volt or between 0 and 5 volts (depending on make and number of wires)

Wide Band Oxygen Sensors - Air Fuel Ratio (AFR) Sensor

Air Fuel Ratio Sensors are used as a substitute for conventional oxygen sensors in many late model vehicles. These sensors are generally more technologically advanced and incorporate extra internal circuitry for more precise air/fuel ratio management of your engine.

This is a variation on the Zirconia sensor; called the "wideband" sensor also incorporates an electrochemical gas pump. An electronic circuit containing a feedback loop controls the gas pump current to keep the output of the electrochemical cell constant, so that the pump current directly indicates the oxygen content of the exhaust gas.

Schematic picture - A planar wideband Zirconia sensor (Click to enlarge)

Wide band (Air / Fuel Ratio) sensors are a newer type of oxygen sensor. They started being used on a few Toyota models in 1997, but have been used more and more in recent years. Many Japanese and German cars are the early adopters of this type of sensor, and they are just now starting to show up in American cars over the last few years.

The task of the air fuel ratio sensor is just that same as that of the oxygen sensor, the difference is that the output signal from the air fuel sensor is not changed right when the air/fuel mixture goes rich or lean. The wide band sensor not only tells the ECU if the mix is rich or lean, but how rich or how lean it is. Wide band sensors communicate their information by providing a current flow, the strength of which is directly proportional to the amount of oxygen present in the exhaust stream, and is very accurate. The amount of current is relatively small (0 - 30 mA).

In this way, it's easier for the ECU to adjust the mix without a lot of overshoot and guess work. This sensor eliminates the lean-rich cycling inherent in narrow-band sensors, allowing the control unit to adjust the fuel delivery and ignition timing of the engine much more rapidly.

Commonly used in aftermarket high performance and driver air fuel display equipment. The wideband Zirconia sensor is used in stratified fuel injection systems, and can now also be used in diesel engines to satisfy the upcoming EURO and ULEV emission limits.

Wide band sensors need to maintain a correct tip temperature, and therefore the ECU will control the amount of heat supplied by the heater circuit. Usually its heater ground runs to the ECU, and then the ECU can make or break the ground, as needed, to maintain the tip temperature in an acceptable range. This differs from the narrow band sensors which just supply a constant 12 volts to the heater.

The advancement of the air fuel ratio sensors is very ideal for the low emission engines as well for tuning performance engines. Some vehicles may have air fuel ratio sensor Pre-Catalytic Converter and a Narrow Band Post-Catalytic Converter. e.g. Late Toyota Aurion & many others.

A Planar Air Fuel Sensor is a combination of a standard Zirconium Oxide Oxygen sensor and a Pump Cell to maintain a constant sensing of a stoichiometric air fuel ratio through the extreme rich and lean conditions. The pump cell is a diffusion gap in the Zirconium Oxide of the sensor that is connected to a control circuit

NOx Sensor

A high temperature device built to detect NOx in the exhaust that in appearance is similar to an Air / Fuel Ratio sensor. Direct injection petrol engines use stratified fuel injection to reduce fuel consumption by allowing a very lean burn process. As lean mixtures reduce petrol consumption they generate harmful NOx compounds. NOx is a generic term for mono-nitrogen oxides NO and NO2 (nitric oxide and nitrogen dioxide).

During engine lean operating cycles, the Nitrogen Oxides can no longer be satisfactorily converted to a harmless state by the conventional catalytic converter.  An additional NOx storage catalytic converter in the exhaust is used to temporarily store the Nitrogen Oxides.

The NOx Sensor monitors the converter storage capacity. The engine will try and operate in low consumption mode for as long as possible, until the NOx sensor in the NOx catalytic converter determines that the NOx catalytic converter is full, and together with the Engine ECU increases the fuel mixture causing an increase in the NOx converter temperature to transform the Nitrogen Oxides to harmless Nitrogen.

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