Exhaust Gas Temperature (EGT) Sensors & Pyros
Measure Exhast Gas Temperature (EGT)
Exhaust gas temperature is a measure of heat in the cylinder during combustion, and is most commonly measured close to the head. Since all metals melt, deform, or undergo transformation under excessive temperatures for that particular metal, it is easy to have too high of EGT and cause damage to engine/turbo components. Thus a tuner must measure the EGTs and keep them in check or suffer the consequences. High EGTs also contribute to knock. You cannot tune your car by EGTs alone, and you should not tune your car without measuring the EGTs.
Benefits of EGT sensors for engine tuning
Operation at peak EGT, particularly on long runs like in road racing or circle tracks, will give a fuel savings meaning less pitting for fuel (increased range) as well as less likelihood of spark plug fouling. Temperature variations will occur between in between individual cylinders when using a probe for every cylinder. Typical variations of about 100 degree F between each individual cylinder can be observed with fuel injection systems, and about 200 degree F with conventional carburetion. Carburetion variation is greater because the atomization of the fuel is not as effective when compared with fuel injectors. In colder air temperatures, the mixture atomization is poorer for both fuel injected and carbureted engines. However, with the carburetor, the fuel/air ration and atomization is less effective resulting in larger temperature variations when comparing individual cylinders.
Types Of EGT Pyro Sensors
Thermocouples can be relatively low cost sensors (if using type K) as compared with thermistors or RTDs. However, the electrical system is significantly more expensive since there must be compensation for voltages produced whenever there is a change in wire material (often called cold-junction compensation).
Thermocouples can be made with very little mass which allows for a fast response with changing temperature. In order for the sensor to minimize drift and be durable in a vehicle exhaust environment however, the thermocouple must be protected by a sheath, and made thicker. Thus much of the fast response advantage is eliminated.
Resistance temperature detectors or RTDs are based on the natural change in a metal's resistance with temperature. In vehicle exhaust applications the material of choice is platinum due to its capability to span the entire range of temperatures while having a near linear output. Industry standard for a RTD is 100Ωat 0°C, however the standard resistance for vehicle exhaust applications is 200Ωat 0 °C. The higher resistance doubles the sensitivity and makes vehicle wiring even less significant.
The types of thermistors found in a vehicle exhaust environment will typically produce a negative temperature coefficient (NTC), meaning the resistance will decrease with increasing temperature. Thermistors offer a high sensitivity over a smaller range in temperature than either thermocouples or RTDs. At 0qC the resistance can be over 100,000 Ω,at 200°C 200 to 500 :, and at 800°C 50:. Thus,thermistors can achieve very high sensitivities over a particular range of temperatures. However, achieving nearly the same accuracy over a large range in temperatures is not possible (unless several pull up resistors are used) due to the highly nonlinear characteristic response.
•Thermocouples have the lowest sensitivity, followed by RTDs or thermistors depending on the temperature range. Below a range of 500 to 600°C, thermistors have very high sensitivity. Above 600°C the RTD has the highest sensitivity. Signal levels are again lowest with thermocouples followed by RTD and then thermistors up to 200°C to 600°C (depending on the type of thermistor). Linearity of the sensor is also a desirable trait. The platinum RTD is the most linear followed by thermocouples. Being linear provides the ability for only one pull up resistor to be used over the entire measurement range without losing accuracy.