Exhaust thermocouple sensors are a key element of modern engine management. Measuring the temperature of exhaust gas can provide an important input into system algorithms to improve efficiency, reduce pollutants and extend engine life.
Exhaust Gas Sensors
Exhaust gas sensors are usually thermocouples due to the very high temperatures involved. Often 800°C or more can be achieved. Thermocouple Type K is the most common although Type J is also found in lower performance engines.
Characteristics of Effective Design
The primary requirement of an Exhaust Thermocouple is to be robust enough to survive a high vibration environment. After that it must be small enough to respond quickly and not disrupt gas flows. The exhaust thermocouple needs a sheath to protect it from the corrosive combustion products, and a gas seal to prevent leakage.
Making probes robust enough usually means building them from mineral insulated cable. This is a very tough product. The cable can go directly into the gas stream, or can have a secondary stainless sheath to toughen it up even more. Mineral insulated cable can be easily bent to suit the required route length.
High temperature combustion products can be aggressive to temperature sensor sheaths. High-Chrome steels perform very well in this environment. It is much better than Nickel based alternatives like Inconel. Sensor sheaths should be made of 446SS or 310SS, applications below 800°C may use cheaper grades of sheath.
Vibration is often a design consideration. Making sheaths in thick cross section and minimising insertion lengths all help extend probe life.
Gas seals can be tapered threads welded to protection sheaths, or compression fittings which screw into the manifold.
Past Exhaust Thermocouple Projects
Peak Sensors has supplied custom made exhaust temperature sensors for LGV diesel engines, sports cars, trucks and even buses. Our sensors are an integral part of systems used to reduce large vehicle exhaust pollutants which then allows them to enter low pollution areas such as London’s LEZ – Low Emmission Zone.
The exhaust thermocouple picture below includes some signal conditioning electronics to produce a linear mV output at a much higher voltage than would normally be expected of a thermocouple. This exhaust thermocouple is used with a compression fitting to form a gas seal.