Exhaust Gas and Cylinder Head Temperature
Sometimes Temperature Limits Matter - Understanding the Importance of EGT and CHT.
Exhaust Gas Temperature (EGT) and Cylinder Head Temperature (CHT) are familiar terms every pilot has encountered. Unfortunately, the vast majority of pilots do not fully grasp their meaning and significance. Often, pilots associate EGT solely with leaning procedures and limits they have learned from flight instructors and fellow aviators, a misconception that can lead to problems. On the other hand, CHT is often overlooked, with little thought devoted to its implications. Both practices are misguided and might be detrimental to your engine. Here’s why you should reconsider your understanding of EGT and CHT.
Exhaust Gas Temperature
The EGT probe does not measure anything meaningful. It does not actually gauge the exhaust gas temperature but rather an average temperature of the probe itself. Positioned downstream of the exhaust manifold, the EGT probe does not “see” a continuous but intermittent exhaust gas flow. So the probe levels on an equilibrium based on the short time when the exhaust gas valve is open and the majority of the time when it is closed and no exhaust gas is present.
Peak EGT occurs at the “stoichiometric” (chemically correct) air-to-fuel mass ratio of 14.7:1, where there is exactly the right amount of air (oxygen) to oxidise all the fuel (hydrocarbon chains). Leaner mixtures cause EGT to decrease simply because less fuel produces less energy. Richer mixtures also lead to a decrease in EGT as excess (unburnt) fuel absorbs heat energy when it vaporises. Consequently, peak EGT can be used to identify a stoichiometric mixture, with both richer or leaner mixtures (ROP and LOP) exhibiting lower EGT values.
In the past, EGT gauges only indicated relative values, which was advantageous. Various engine monitor units (EMUs) and avionics manufacturers, in an attempt to outperform each other, added different features to their products, leading to the advanced engine indications available today. Unfortunately, the addition of absolute values for EGT is detrimental as pilots started to worry about high EGT values and uneven distribution of EGT values easily seen with the bar indication features. The assumptions that “even (as identical) EGT values are good” and “high EGT values are bad” are incorrect.
Even EGT Values
Numerous factors can affect indicated EGT besides the actual exhaust gas temperature. These include probe mass and construction (grounded or ungrounded), cam lobe profile, probe location, valve spring condition, and exhaust manifold geometry, among others. The indicator of a well-balanced engine is not a small EGT spread, but rather a small “GAMI spread” – defined as the difference in fuel flows at which the various cylinders reach peak EGT. Ideally, this spread should be no more than about 0.5 gph. If the GAMI spread is much more than that, the engine is unlikely to run smoothly in LOP operation, and the installation of GAMIjector® or TurboGAMIjector® fuel injector shall be considered.
High EGT Values
There are no EGT limits for the engine shown in the engine’s operator’s manual. Any stated limits or range in the Aircraft Flight Manual (AFM) or Pilot Operating Handbook (POH) are either not actual limits or are based on the actual installation, taking into account the aircraft’s exhaust system, etc. High EGTs do not indicate that the engine is under excessive stress. They simply signify that a lot of energy from the fuel is being wasted out the exhaust pipe rather than being converted into mechanical energy sent to the propeller. High EGTs do not pose a threat to engine longevity as the engine is not capable of producing EGTs high enough to cause harm. Therefore, attempting to limit EGTs in an attempt to be kind to the engine is misguided.
Exemption: Turbine Inlet Temperature (TIT)
The only exemption and reasonable limits of exhaust gas temperature is Turbine Inlet Temperature (TIT) for turbocharged engines. TIT probes are positioned further downstream of the exhaust manifolds just before the turbine inlet and are exposed to a much more continuous flow of exhaust gases. The limits of TIT, aimed at protecting the turbine blades, are usually between 1650° and 1750°F and must be observed.
EGTs are Generally Meaningless Except for Troubleshooting and Engine Condition Monitoring
Magneto Problems
If you notice elevated EGTs on all cylinders with normal or slightly reduced CHTs, perform an in-flight ignition stress test. Exercise caution because the engine might quit on you when switching off magnetos purposely for the in-flight ignition stress test. Anticipate this behaviour and be prepared. In the event of an in-air engine shutdown, throttle back to idle before turning the magneto back on to prevent afterfire and potential exhaust system damage.
Defective (or Fouled) Spark Plug(s) or Ignition Wire(s)
Observe elevated EGTs with normal or slightly reduced CHT on only affected cylinders. Conduct an in-flight ignition stress test and note the cylinder(s) that flame out by observing EGTs.
Partially Clogged Fuel Injector Nozzles
Notice 1) Elevated CHT and EGT on the affected cylinder when ROP & 2) Reduced CHT and EGT on the affected cylinder when LOP. Turn on the fuel boost pump and set to full rich. This action might clear the fuel injector nozzle from the clog. If unsuccessful, land and analyse EMU data.
Completely Clogged Fuel Injector Nozzles
The affected cylinder will shut down, recognised by reduced EGT and CHT. You might also notice the engine running roughly. Try to unclog by turning on the fuel boost pump and setting to full rich. If unsuccessful, land and fix the fuel injectors
Burned Exhaust Valve
Observe slightly elevated EGT on the affected cylinder only (usually by about 20°F to 60°F, depending on the severity of the valve damage). EGT usually varies between normal and slightly elevated, often (but not always) in a rhythmic fashion with a frequency of roughly one or two cycles per minute. (If the burned valve fails completely, the EGT of the affected cylinder will go cold, and the engine will run rough.)
Engine Condition Monitoring
Data from the exhaust gas temperature can be used to detect upcoming, yet unnoticed by the pilot, problems like exhaust valve failures or necessary fuel flow adjustments.
Cylinder Head Temperature
Cylinder Head Temperature (CHT) correlates with Internal Cylinder Pressure (ICP), which means when the CHT is high, the ICP is also high, indicating significant stress within that cylinder and engine. Not only is the stress high in a cylinder with elevated CHT, but the tensile strength of the aluminium cylinder heads also loses half of their strength at 400°F.
Manufacturer Limits and Recommendations
Continental states a 460°F limit for their cylinder heads, while Lycoming sets the limit at an incredibly high 500°F. Both are limits for emergency operations and should be set far lower for optimal operation and engine longevity. We recommend establishing a personal CHT limit of 400°F (preferably 380°F) for Continental and 420°F (preferably 400°F) for Lycoming engines. Adhere to these limits religiously and make aggressive adjustments when approaching the upper limits.
Managing CHT
To manage CHT, consider changing airspeed, opening cowl flaps (if equipped), reducing power, and adjust your mixture accordingly. You can either enrich a lot on the rich-side-of-peak or lean slightly on the lean-side-of-peak to reduce the CHT and therefore the engine stress. This is because the slope of the internal cylinder pressure over the air-to-fuel-ratio is shallower on the rich-side-of-peak compared to the lean-side-of-peak. More about leaning techniques and optimal CHT temperature ranges for performance and engine longevity is detailed in another blog post. CHT is also affected by ignition timing. Advancing the ignition timing so that the spark plug fires earlier increases CHT. Retarding the ignition timing, so the spark plug fires later, decreases CHT.
External Factors Affecting CHT
CHT is a valuable but imperfect approximation for engine stress as it is also affected by Outside Air Temperature (OAT), airspeed (less cooling at low airspeed, altitude (less cooling at high altitudes due to reduced air density, and cooling system efficiency (depending on design, conditions, etc.). Take these parameters into account when operating your aircraft engine and setting your personal CHT limits.
Note on Fuel Flow During Take-Off
Check and potentially adjust fuel flow during take-off, particularly when PMA replacement cylinders with improved volumetric efficiency or aftermarket intercoolers (for turbocharged engines) are installed. The full power fuel flow marked on your OEM fuel flow gauge may simply not be high enough to keep the CHT in check!
Conclusion
Understanding and interpreting Exhaust Gas Temperature (EGT) and Cylinder Head Temperature (CHT) is crucial for optimal aircraft operation and longevity. It’s vital to emphasise that EGT limits, contrary to popular belief, do not effectively measure engine stress. In fact, EGT often decreases as engine stress increases. Primarily, EGT is a beneficial tool for troubleshooting and monitoring the condition of the engine rather than a parameter to be maintained within a certain limit for optimal engine performance.
On the other hand, stringent attention to CHT limits is fundamental for ensuring engine longevity. CHT serves as the best approximation for internal cylinder pressure, a crucial indicator of engine stress. Elevated CHT not only indicates high internal cylinder pressure and stress but also significantly increases the likelihood of head cracks, a detrimental issue for engine performance and safety.
While EGT readings are important for diagnostic purposes, consistent and vigilant monitoring of CHT is paramount in safeguarding your engine’s health, ensuring smooth operations, and maximising the lifespan of your aircraft’s powerplant. Fly safe, stay informed, and keep these critical parameters in check for a seamless and reliable flying experience.
About Quest Aeronautics
Quest Aeronautics is a state-certified engineering office for aviation, dedicated to shaping the future of general aviation by providing innovative and cost-effective solutions to enhance aircraft performance and operations. With a focus on CS/FAR-23 and experimental/amateur-built (E/A-B) aircraft, Quest Aeronautics provides a range of services including flight testing, aircraft operations and maintenance consulting, high-quality aviation products, and tailored support for E/A-B projects. Collaborating with industry-leading partners, Quest Aeronautics is committed to delivering unparalleled support and expertise to individuals and organisations in the general aviation market.
About Author
Sebastian, the founder of Quest Aeronautics, is a driven and enthusiastic individual with a passion for aviation. Before delving into aviation, he gained valuable experience as a chemical process engineer and laboratory technician. Sebastian holds a Master of Science in Engineering and a commercial pilot licence, with several fixed-wing aircraft ratings under his belt. He has also completed an introduction course for fixed-wing performance and flying qualities flight testing at the National Test Pilot School in Mojave, CA and is compliance verification engineer for flight.