Granted that Exhaust Gas Temperature (EGT) and Cylinder Head Temperature (CHT) have applications in auto industry, the sensor probes and instruments designed to capture and display these temperatures were primarily designed for use in aircraft.
The Exhaust Gas Temperature (EGT) sensor or probe is placed in each exhaust pipe of the aircraft, roughly 4 to 6 inches from the cylinder head. These probes read the temperature of the gas as it leaves the cylinder. The exhaust gas temperature is not uniform and varies depending on the power setting, ambient air temperature, altitude and cylinder compression.
Other factors that come into play and have a bearing on the exhaust gas temperature include spark plug ignition timing and cylinder leakage i.e. compression loss. To an expert, the EGT sensor offers a peephole into the combustion process taking place within the cylinder. Usually, the health of the aircraft engine is not based on one particular EGT reading but an average. So if the overall average is abnormally high or low then you have a problem.
In fact for a pilot, it is the bigger overall picture that is drawn by the relative value of EGT i.e. relative to other cylinders and also relative to performances over normal days as well as the way that EGT responds to changes in the fuel-air mixture that are of primary interest.
The single most important reason to pay attention to the EGT reading is to know whether or not your engine is running and this is especially true in a multi-engine aircraft. Contrary to belief, in a multi-engine aircraft the pilot may not immediately realize that one of the engines is not working. This is because other than the EGT, all other displays will read normal.
For example, the aircraft RPM indicator, oil pressure, manifold pressure and even oil pressure and oil temperature will all read normal. A look at the EGT on the other hand will instantly tell the pilot which engine has stopped functioning (because of the fast reducing temperature reading).
Apart from knowing whether or not the aircraft engine is functioning and overall health of the engine, the other important use of the Aircraft Gauges is to help you perfect your aircraft lean mixture. When you pull the fuel-air mixture control back, the F/A ratio will reduce while the EGT increases. As the F/A ratio continues to decrease, it will reach a point when the EGT begins to back down. This is the point at which the ratio of air-to-fuel is at its optimum best.
Cylinder Head Temperature or EGT Gauge for short, measures temperature at one spot on the cylinder head – usually the threaded boss in the cylinder. The difference between the EGT and CHT reading is usually between 30 and 60 degrees and therefore the CHT helps confirm EGT readings. Of course the difference between the EGT and CHT will also depend on the type of (carbureted or fuel injected) engine with the fuel injected engines displaying a bigger temperature difference.