Exhaust Gas Temperature (EGT) is a direct reading of how hot the exiting gases are at the exhaust manifold, revealing both the intensity of combustion inside the cylinders and the degree of "afterburning" that continues after the exhaust valve opens. A slender temperature-sensing probe, inserted within three inches of the exhaust port, feeds a thermocouple-type pyrometer whose dial the pilot or driver watches in real time. Because the measurement is taken while the cylinder is under comparatively low stress, the figure chiefly reports heat energy that is being wasted during the exhaust stroke rather than converted into useful work.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is exhaust gas temperature (EGT) in aviation?
EGT is a measurement of the temperature of the exhaust gases at the exhaust manifold, and it measures heat energy wasted during the exhaust stroke when the cylinder is under relatively low stress. The EGT gauge displays an average of relatively cool temperature when the exhaust valve is closed and a spike in high temperature when the valve is open.
What does exhaust gas temperature (EGT) indicate?
Exhaust gas temperature indicates how hot the combustion process is in the cylinders and how much afterburning occurs in the exhaust manifold. It is a measurement of the temperature of the exhaust gases at the exhaust manifold, and it mainly reflects what is going on during the exhaust stroke after the exhaust valve opens. The gauge tells the temperature of the exhaust gas, and it is an indicator of the engine's combustion process and air-fuel ratio. By monitoring EGT, the driver can get an idea of the vehicle's air-fuel ratio and can be warned of dangerous conditions before damage occurs. High EGTs indicate that a lot of energy from the fuel is being wasted out the exhaust pipe rather than being extracted as mechanical energy, and they also indicate higher fuel consumption. Lower or normal EGT readings indicate better fuel efficiency, while deviations from the known good baseline indicate a need for further investigation. In a diesel, the richer the air-fuel ratio, the higher the EGT will be.
In a turbine engine, EGT indicates temperature after the turbine and signals compressor or turbine damage; it does not alone indicate thrust. Inter-turbine temperature is another measurement that indicates whether the engine is running within parameters. The hot accelerated gases do work on the turbine, thus losing energy, and the transfer of power to the turbine reduces temperature. EGT margin is the difference between the engine's EGT during full-rated takeoff and the EGT limit. Neither EGT nor TIT alone indicates how much thrust the engine is producing. A single parameter shows thrust level as percentage, and the flight-deck gauge displays degrees Celsius.
In a piston engine, EGT is an indication of the fuel mixture coming out of the cylinder after the combustion process occurs, and it verifies that all cylinders are firing. Mixture control determines the fuel-air mixture, and varying the mixture changes the rate at which the air-fuel charge burns. Lean of peak (LOP) and rich of peak (ROP) are mixture settings. Leaner mixtures are referred to as LOP, while richer mixtures cause higher EGT. Prior experience of what was previously deemed normal for the particular combination is needed to interpret actual temperatures.
How is turbine engine exhaust gas temperature measured?
Turbine engine Exhaust Gas Temperature is measured by temperature sensors placed at the Low-Pressure Turbine (LPT) of the engine. Multiple thermocouple probes are installed at the exhaust of the turbine, usually past the high-pressure turbine, and the resulting signal is routed to an EGT indicator.
Thermocouples, technical heat detectors, are set in a circle arrangement to take the temperature cross-sectional across the whole gas passage. I observed how the probes were set to prevent straightforward impact from the burning, so the quantification is not taken straight from the gas flow itself. Each thermocouple creates an electrical potential relative to the temperature it meets, and this input is sent to the motor's electronic engine command part.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
What is peak EGT in aviation?
Peak EGT is the mixture at which EGT stops rising and starts falling. Peak EGT occurs when EGT reaches maximum. The mixture setting allows positioning of the control to operate the engine on either side of peak EGT. Mixture settings rich of peak EGT provide more power, while lean of peak EGT settings run cooler, although many engines run unacceptably rough on lean side of peak.
Peak EGT is the hottest exhaust gas temperature a pilot can observe while leaning the mixture. It occurs at the stoichiometric ratio of 14.7 pounds of air (6.67 kg) for each pound of fuel (0.45 kg), a condition in which no excess air or fuel remains to cool the exhaust stream. Finding that hottest reading is the key to the EGT method because it gives a repeatable reference from which every other mixture setting is judged.
On the rich side of peak, the engine ingests more fuel than it can burn. The surplus evaporates, absorbs heat, and drives cylinder-head temperatures higher while delivering extra power and making the airplane fly faster. Maximum power for normally-aspirated engines is found roughly 100-125°F (37.8-51.7°C) rich of peak. Turbocharged engines behave the same way but show peak EGT values in the 1,600-1,700°F (871.1-926.7°C) range instead of the mid-1,500s seen on their naturally-aspirated counterparts.
Leaning past peak causes EGT to drop, power to decrease, and the airplane to fly more slowly, yet the engine runs cooler and burns almost exactly 100 percent of the fuel that enters each cylinder. This lean-of-peak setting increases endurance and range, so ‘best economy’ is generally accepted to be 20-50°F ( 6.7-10°C) lean of peak and some engines also tolerate operating right at peak for the same economic benefit. Operating on either side of peak EGT is not inherently harmful provided the flight stays within the power and temperature limits published in the POH or engine manual, and at 65% power or less the mixture legally places anywhere relative to peak without fear of damage.
What is the relationship between EGT and CHT in aviation?
EGT and CHT have a symbiotic relationship in the sense that they help to confirm each other. One distinction between the two is that the EGT indication is instantaneous, while the CHT is usually a little slower to react. CHT is slower because it reflects material heating, whereas EGT responds faster as it measures direct gas temperature. Because of this difference in response time, EGT can change much more quickly than CHT, and EGT changes are larger in magnitude than CHT changes.
Both CHT and EGT can be used to detect detonation and pre-ignition. During detonation, EGT decreases and CHT increases. During pre-ignition, both EGT and CHT rapidly increase. These opposing or aligned responses reinforce the diagnostic value of monitoring both parameters together.
CHT is the residual heat from combustion and mainly reflects what is going on in the cylinder during the Otto cycle power stroke before the exhaust valve opens. It measures heat energy wasted during the power stroke when the cylinder is under maximum stress. CHT is influenced by outside air temperature, airspeed, altitude, and cooling system efficiency. Reduced cooling airflow, increased power, or advanced timing all increase CHT. CHT sensors are located at the cylinder head and serve as the best approximation for internal cylinder pressure and overall engine stress.
Leaning the mixture past peak EGT reduces CHT. Lean-of-peak (LOP) operation results in slightly less power but a relatively large reduction in CHTs - about 25°F (3.9°C) for each 10°F (12.2°C) lean beyond peak. This provides a natural cooling effect through reduced fuel flow and combustion temperature.
Why is it important for the pilot to know an engine's exhaust gas temperature?
It is important for the pilot to know an engine's exhaust gas temperature because exhaust gas temperature gauge is the pilot's earliest notice of how hard the engine is working. Because the pyrometer reacts more quickly than the water temperature gauge, it warns first if internal temperatures begin to climb toward the red limit printed on the dial. High temperatures indicate potential problems like fuel inefficiency or catastrophic failure and an unusually high reading prompts immediate action to prevent damage. During takeoff at TOGA thrust level in warm weather conditions, pilots monitor EGT readings to guarantee they do not exceed the limit, for even a brief overtemperature softens turbine blades or scores pistons.
Beyond limits, the same pointer lets pilots set an airplane's air-to-fuel mixture for either economy or power. Lean mixture burns very cleanly, reduces combustion pressure and temperature, and is great for engine longevity, while rich-running engines burn too much fuel and too little air, wasting fuel and cooling the charge. Pilots keep track of EGT readings, make informed decisions about engine longevity, balancing speed against cylinder life, flight length against fuel remaining, and power against temperature.
What are the benefits of an exhaust gas temperature gauge in aircraft?
The benefits of an exhaust gas temperature gauge in aircraft include that it can be used to monitor the airplane’s air to fuel mixture. Temperature readings are transmitted to the cockpit for the flight crew to monitor; gas temperature is presented on a flight deck gauge in degrees Fahrenheit or degrees Celsius. Lean mixtures burn very clean, reduce combustion pressure and temperature, and result in fuel savings, yet they sacrifice some power and airspeed. A multi-probe cylinder head temperature/exhaust gas temperature system can serve as an early warning device. It can pinpoint location and nature of engine problems long before trouble appears in other ways. If the engine is not running smoothly after startup, the EGT gauge can help quickly identify the problem cylinder. High temperatures can indicate catastrophic failure, so the gauge can be used to prevent damage, while maintenance teams can determine whether the engine is performing efficiently and remains within safe limits.
