A Lambda lesson

Tim Howes from NGK Spark Plugs (UK) Ltd, explains the role of the Lambda sensor

Published:  12 January, 2015

By Tim Howes, Deputy General Manager, Supply Chain & Technical at NGK Spark Plugs (UK) Ltd

NGK has recently expanded its range of NTK Lambda sensors by introducing 49 new sensor types. It now comprises more than 770 types and offers a perfect oxygen sensor for around 70% of the UK car fleet.

The NTK range of oxygen sensors has evolved significantly since their introduction several decades ago and we have subsequently seen the introduction of NOx sensors which work in conjunction with associated exhaust after-treatment devices. To support the ever increasing complexity of exhaust gas management technology, NGK has also introduced a new range of NTK exhaust gas temperature sensors (EGTS) into the UK aftermarket.

EGTS are thermistor devices that monitor the exhaust gas temperatures to enable the efficient operation and protection of vital components, including turbo chargers, located directly in the flow of those hot gases. EGTS from NTK are original equipment (OE) in approximately 35% of global passenger car sensor applications and provide extreme resilience against heat and vibrations, have high measuring accuracy, fast light-off times and an exceptionally wide measuring range.

The most common type is the binary sensor. This utilises a solid state electrolyte made from the ceramic material zirconium dioxide (ZrO2). A useful property of this material is that when it reaches a temperature of about 350°C, it becomes permeable for oxygen ions (not the gas). The outer surface of the sensor element is in direct contact with the exhaust gas whilst ambient or 'reference' air reaches the inner surface.

As soon as a sufficient temperature is reached, oxygen ions can migrate from the reference air side towards the exhaust gas side where, as a result of the combustion process, a lower oxygen concentration (partial pressure) exists. During this migration through the element, electrons are absorbed by the electrodes. The result is a potential difference on both sides of the element. If the engine burns a lean mixture, the sensor produces a voltage between 0 and 150 mV. A rich mixture creates a voltage of 800 to 1,000 mV. As the stoichiometric point is passed, the voltage jumps abruptly, giving a nominal output of either zero or one Volt. This gives rise to the term 'binary' sensor. The sensor signal oscillates around the ideal value of ? =1 with a frequency from 1 to 2 hertz and 'informs' the engine management about deviation from the ideal, stoichiometric mixture.

Originally only heated by the hot exhaust gas, as environmental regulation became stricter, the sensor was equipped with an internal heater to reduce the resulting delay in operation from cold and now has at least three wires - one for the sensor signal and two for the heater supply and ground. Most of the heated sensors now have four wires as they require a separate signal ground connection (ISO-HEGO).

Sensors that can allow an engine to operate away from stoichiometric are useful for cars using a lean burn technology and wideband sensors were developed for this use. NTK wideband sensors, which have five wires, were first used in significant volume production from the mid-1990s when the first generation of lean burn engines appeared.

The wideband sensor generates a signal, directly proportional to the residual oxygen of the exhaust gas. In order for the engine management system to understand this output, an NTK ASIC (Application Specific Integrated Circuit) or 'chip' must be incorporated into the vehicle's ECU.

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