What is a sensor?
A sensor is a device that can detect an analogue signal e.g. movement, chemical composition, temperature etc. We personally use sensors for touch, taste, smell, hearing and sight. The second function of a sensor as discussed here is to convert this analogue signal into an electronic signal that can be sent to a processing device, our brain or for a motor vehicle the electronic control unit (ECU). The processing device can then decide if it needs to change something to compensate for the signal it is receiving e.g. move your hand away from something that is too hot or in the case of a lambda sensor instruct a change to the fuelling system on an engine.
Why are Lambda sensors fitted?
With so many vehicles in use on our roads the reduction of pollutants produced by the internal combustion engine is of ever increasing importance. To encourage advances in technology that can bring this about governments have introduced progressively tougher exhaust gas emission legislation. Lambda sensors are a very important part of the technology used to achieve these legislated targets and as a result of their function the engine can also deliver the best economy and performance available. From the end of 1992 most petrol engine vehicles sold in the UK had Lambda sensors fitted.
How does the Lambda sensor work?
Most NTK Lambda sensors can be divided into two non-interchangeable types Zirconia Sensors and Titania Sensors. These are classed as binary or Lambda 1 sensors. They use different types of ceramic element and function in different ways but they have a common goal – to ensure the catalyst works efficiently and harmful gases are kept to a minimum. In order to achieve this an engine needs to attain as close to complete combustion as possible. The ideal ratio of air and fuel to achieve this is around 14.7:1; this means that for 14.7 kg of air 1 kg of fuel would be used. This chemically correct air fuel ratio is known as a stoichiometric ratio or Lambda (λ)1.0. A fuel rich mixture would have a lower value e.g. 0.8 and a fuel lean mixture would have a higher value e.g. 1.2
A relatively small but increasing number of vehicles now use sensor types that can precisely measure the air/fuel ratio over a large range of fuel rich and fuel lean conditions. These sensors are called UEGO (Universal Exhaust Gas Oxygen) sensors, wideband, broadband or linear types.
Zirconia Binary type
Under the metal protective end of the sensor there is a hollow thimble shaped ceramic body made from zirconium dioxide. The protective metal shell has specially designed holes to allow the exhaust gases to come into contact with the outside of the ceramic element. Both sides of this ceramic element are coated with a thin micro porous layer of platinum. These layers are the electrodes that carry the sensors signal to the wire cables. Over the outside electrode a thin additional layer of porous ceramic is added to protect the platinum from erosion by the exhaust gases. The inside of the thimble is hollow and is used to hold ambient air as a reference gas.
At temperatures in excess of 300°C the zirconia element possesses a property that causes a transfer of oxygen ions. This movement creates a voltage. The greater the difference of oxygen concentration between the exhaust gas and the ambient reference air in the centre of sensor thimble the higher the voltage produced. The voltage produced in the fuel lean position should be approximately 0.1 volt and in the fuel rich position approximately 0.9 volt. The very useful part of this function is that at around the stoichiometric point there is a relatively large change in voltage. This allows the sensor to keep the engine emissions within strict limits by constantly bringing the fuelling system back from a fuel lean or fuel rich position to retain the stoichiometric mixture. The time taken to switch from fuel lean to fuel rich is approximately 300 milliseconds.
Titania type
Externally these sensors look similar to the zirconia types however the sensor body may be generally smaller. These sensors do not generate a voltage (as the zirconia type does) but the electrical resistance of the titania changes in relation to the oxygen content of the exhaust gas. If there is a surplus of oxygen in the exhaust gas (fuel lean) the element resistance rises and as the concentration of oxygen decreases (becoming fuel rich) the resistance falls. At the stoichiometric point there is large change in resistance. The output of an applied voltage will therefore change in relation to the fuelling condition. As there is no need for a pocket of air as a reference gas and due to certain other design differences the sensor can be smaller, stronger and have a faster reaction time. The control system for this type of sensor is very different to that used for the zirconia type. All titania type sensors have internal heating elements.
ZFAS-U type (Air/Fuel Sensor)
Also known as a UEGO, wide band or linear sensor, the easiest way to identify this type of NTK sensor (apart from the part number) is by the number of lead wires – they usually require at least five and are always heated types. The sensor is of layered constructed with two ceramic substrate components, a Zirconia detection element and an Alumina heating element. No external reference air is required as the sensor generates its own. The detecting cavity is exposed to exhaust gas through a gas diffusion layer.
Put very simply the sensor tries to maintain a stoichiometric air/fuel ratio in the detection chamber by pumping oxygen in or out of the chamber. The value of the pumping current required to achieve this corresponds to the air fuel ratio of the exhaust gas. Not only does this type of sensor have an extended window of measurement and can be used successfully where lean burn strategy is employed, it also provides exceptional accuracy around the stoichiometric point which is useful in the quest for emission reduction. This type of sensor will also be used in conjunction with diesel engines as they operate with an excess air factor.
