Before discussing Wideband sensors it should be noted that whilst the Wideband sensor is sometimes termed 'Planar', Bosch make a narrowband sensor that also has a planar construction. Instead of the solid electrolyte of the Zirconia types, this is replaced by ceramic laminate layers, and these laminate layers are silk-screen printed to configure them. The big advantage with this method of construction is that the heater can be buried within the sensor layers, thus making the sensor heat up to operating temperature much more quickly.
The current generation of sensors are termed Wideband, Planar, UEGO or 'A/F ratio sensors' (Air/Fuel ratio sensors). They are much better at gauging exactly how much Oxygen is in the exhaust stream, rather than the simple switching action of the narrowband sensors. The term 'planar' comes from the shape of the sensing element which is a flat strip (plane), rather than the thimble shape of traditional sensors.
Figure 11 - Cutaway view of a planar wideband sensor
Wideband sensors have only become necessary as engine management systems have evolved to the point where a more accurate sensor is required to meet low-emission vehicle targets - the old style of sensors had their characteristic 'switching point' for various historical reasons. The wideband sensor is an absolute requirement for lean-burn and fuel ionising mixture control strategies (eg. Volkswagen FSi) as well as diesel vehicles. The wideband sensor allows the ECU to gauge how well combustion is occurring right down to very lean mixtures.
Figure 12 - Wideband sensor output curve (red)
compared with narrowband sensor output range (green)
The sensor works on broadly the same principle as an ordinary sensor (Nernst cell) but with an added internal system (a device called an oxygen pump), and the output current varies in proportion to the Oxygen present in the exhaust. As can be seen from the graph, it can measure a far wider range than a traditional sensor, but more importantly when it is within the range that we are most interested in (from Lambda=0.9 to Lambda=1.1) the response graph is fairly linear, meaning that we can determine the exact oxygen content of the exhaust gas, rather than a steep switching point around the central area. Under cruising conditions in a modern engine, the AF ratio can extend to about 20:1 and the wideband lets us measure accurately at these lean mixtures.
Another, more complex method of decoding the signal is needed by the ECU using a custom ASIC, and the sensors are incompatible with earlier types. This type of sensor is only operational at temperatures of 600 degrees C or more, requiring a powerful heater.
Wideband sensors can be identified by their multi-wire harness (five, six or more wires), and are generally fitted to:
- Any recent car utilising lean-burn or direct-injection engine technology
- Diesel engined vehicles equipped with lamda sensors
- Some Honda vehicles from about 1990 onwards used this type of sensor (L1H1)
- Volkswagen FSi systems, and other stratified charge systems
- Some non-automotive applications such as rolling roads and specialised laboratory gas-testing equipment
The wideband sensor is especially suited to lean-burn, supercharged, turbocharged and high performance vehicles (eg. Subaru impreza 2002 onwards), as ignition detonation or 'knock', the enemy of the high-performance engine, can be avoided under all engine operating conditions by keeping a much closer check on the air/fuel ratio than would ever be possible with a narrowband sensor. This also applies to lean-burn engines, where the average (mean) mixture strength is very weak.
Wideband sensors for Motorsport applications
Wideband sensors are used by racing teams to accurately gauge high performance tuned engines. Lambda meters can be purchased that use a wideband sensor to allow data logging of mixture strength and other engine paramenters. It should be noted that WB lambdas designed for passenger cars are not the same as the ones designed for motorsport use, and are thus not compatible.
As well as being engineered to be more resilient, motorsport WB sensors are calibrated to have a wide output response, whereas the passenger vehicles are calibrated to have most accuracy around stoichiometry and very lean mixtures. A racing car will spend much of its time at the opposite end of mixture strength, over-fuelling for maximum performance.