Use of CHT and EGT Gauges and Monitors
Just so we are on the same page, CHT stands for Cylinder Head Temperature and EGT stands for Exhaust Gas Temperature and in the case of this article, these refer to the aircraft engine cylinder and aircraft engine exhaust gases (same terminology is also used in high-performance cars).
Before we get on to the use of CHT and EGT gauges and monitors, we need to first understand CHT and EGT data. CHT and EGT provides information of two different things that is happening inside the aircraft engine. The former i.e. CHT data specifically points to the Otto cycle power stroke that happens before the exhaust valve opens, while EGT data pertains to the exhaust stroke after the exhaust valve opens.
CHT gauge or monitor measures heat energy wasted during the power stroke. At this stage, the cylinder is under maximum stress from high internal pressures and temperatures. The EGT gauge or monitor measures heat energy wasted during the exhaust stroke, when the cylinder is under relatively low stress.
A higher than normal CHT reading could indicate that the engine is under excessive stress. By contrast, a higher than normal EGT reading indicates that a lot of energy from the aviation fuel is being wasted out the exhaust pipe rather than being extracted in the form of mechanical energy. This is why the CHT and EGT gauges are important because they are both prime indicators of the health of the aircraft engine and fuel setting and economy. For this reason CHT and Airplane Gauges and monitors are used for aircraft power plant management.
For Bonanzas and Baron aircraft as an example, the general rule-of-thumb is that optimum engine longevity is achieved by keeping CHT’s to roughly 380°F. Anything over 400°F would indicate stress.
The aircraft maintenance booklet will usually contain the maximum stress figures for your aircraft engine. The Fuel Gauges in combination with power settings, mixture etc. is used to help you keep your engine below those figures.
The EGT in combination with specific air-fuel ratio is used to maintain fuel economy. Again, for Bonanzas and Baron aircraft, the best power mixture is generally about 100°F RP, while best economy mixture is generally 20°F to 50°F LOP. Generally, pilots do not use EGT for leaning unless they are curious about the fuel-air ratio being used. As long as CHT is does not exceed the stress levels, whatever mixture the pilots set is usually fine.
Importance of CHT and EGT gauges in diagnosing engine problems
An EGT reading of a cylinder that is higher by 50°F could indicate a fouled or malfunctioning spark plug or even ignition wire. A 50°F higher reading on all cylinders on the other hand could indicate a bad magneto.
Similarly, if the pilot experiences engine roughness and elevated EGT and CHT on the affected cylinder (for ROP mixture), or reduced EGT and CHT on the affected cylinder (for LOP mixture), it could be indicative of a partially clogged fuel injector and so forth.
Pilots also use EGT and CHT to detect abnormal combustion events, including detonation and pre-ignition events. A digital EGT and CHT especially of the type manufactured by J.P. Instruments provides invaluable assistance to flight safety and economy.
