Temperature Really Matters
Understanding Critical Temperature Limits
We’ve frequently addressed temperature limits in our discussions, exploring both their significance and, occasionally, their lack of it. It’s crucial to discern which temperatures are vital to monitor and manage actively, as opposed to those that, while still important to track, don’t require the same level of scrutiny. Many pilots fixate on prominently displayed but less critical readings like exhaust gas temperatures (EGTs), possibly overlooking more essential ones. This article won’t rehash the full details from earlier posts but will underscore their key points due to their importance, along with some new insights for consideration.
Managing the Impact of Outside Air Temperature
While we can’t control outside air temperature (OAT), strategic planning allows us to adapt flight schedules to its diurnal fluctuations. OAT influences not just engine performance by affecting density altitude but also plays a critical role in engine cooling. Consequently, we must adapt cylinder head temperature (CHT) limits to match the OAT, especially since CHTs serve as proxies for internal cylinder pressure (ICP) and, by extension, the engine’s internal stress.
On colder days, below the standard temperature, it’s prudent to lower the CHT limits to compensate for increased air density and its impact on engine cooling and performance. Conversely, on warmer days, you might consider slightly higher CHTs but remain vigilant not to surpass the established maximums. This precaution is necessary not only to manage ICP but also due to the significant loss of tensile strength in the aluminium cylinder heads at elevated temperatures.
Note: Exercise caution with any upward adjustments to CHT limits to avoid compromising the integrity of the aluminium cylinder and heads.
The need for pre-heating becomes more pronounced with lower OATs. Generally, any engine core temperature below 0°C warrants pre-heating, and at temperatures colder than -10°C, it becomes imperative. This is especially true for new or recently overhauled engines, where factory tolerances are tightest. While high-time engines with worn bearings may tolerate colder starts better, pre-heating is still advisable when temperatures fall below 10°C, and it’s a necessity below freezing.
Understand that merely warming the oil is insufficient to prevent damage from cold starts. The focus should be on pre-heating the entire engine, particularly the crankcase and the upper cylinder barrels, to ensure that the critical clearances between moving parts do not reach precarious levels.
Understanding Exhaust Gas Temperature
Contrary to common perception, an EGT probe doesn’t directly measure the exhaust gas temperature. Instead, it records the average temperature of the probe itself. Positioned two to four inches downstream of the exhaust manifold, the probe captures intermittent, rather than continuous, exhaust gas flow. This results in the probe reaching an equilibrium based on the brief periods when the exhaust valve is open. Various factors besides the actual exhaust gas temperature can influence the EGT readings, including probe mass and construction (grounded or ungrounded), cam lobe profile, probe location, valve spring condition, and exhaust manifold design. It’s important to recognise that a well-balanced engine is indicated not by a narrow EGT spread, but by a small “GAMI spread”—the difference in fuel flows at which various cylinders reach their peak EGT.
Furthermore, no specific EGT limits are typically provided in an engine’s operator’s manual. Any limits mentioned in the aircraft flight manual (AFM) or pilot operating handbook (POH) are not absolute but rather based on specific installation considerations, like the muffler. High EGTs don’t signal excessive engine stress. They primarily indicate the inefficient use of fuel energy, which gets expelled through the exhaust rather than converted into mechanical energy for the propeller. Thus, trying to restrict EGTs to safeguard the engine is misguided, as the engine isn’t capable of producing dangerously high EGTs under normal operating conditions.
Turbine Inlet Temperature in Turbocharged Engines
A notable exception where exhaust gas temperature limits are critical is the turbine inlet temperature (TIT) in turbocharged engines. Here, TIT probes are positioned just before the turbine inlet, capturing a more consistent flow of exhaust gases. TIT limits, typically ranging between 1650° and 1750°F, are crucial for protecting turbine blades and must be diligently observed.
Understanding Cylinder Head/Coolant Temperature
Cylinder Head Temperature (CHT) is closely linked with internal cylinder pressure (ICP). This means that high CHT indicates high internal cylinder pressure and, consequently, significant stress within the cylinder and engine. Notably, the tensile strength of aluminum cylinder heads is halved at 400°F. Continental sets their cylinder head limit at 460°F, while Lycoming‘s limit is a notably higher 500°F. However, these are emergency operation limits and should be reduced for optimal operation and engine longevity. We recommend setting a personal CHT limit of 400°F (preferably 380°F) for Continental and 420°F (preferably 400°F) for Lycoming engines. It’s crucial to adhere to these limits and take decisive action when approaching them.
To manage CHT, consider adjusting airspeed, opening cowl flaps if equipped, reducing power, and appropriately leaning your aircraft’s engine. You can enrichen the mixture significantly on the rich side of peak or lean it slightly on the lean side of peak to lower the CHT and, thus, reduce engine stress. This difference is due to the shallower slope of internal cylinder pressure over the air-to-fuel ratio on the rich side of peak compared to the lean side. Further details on leaning techniques and optimal CHT temperature ranges for performance and engine longevity are discussed in our leaning blog posts. Additionally, CHT can be influenced by ignition timing; advancing it increases CHT, while retarding it decreases CHT.
Additional factors affecting CHT include:
- Outside air temperature (OAT)
- Airspeed (lower cooling at reduced speeds)
- Altitude (air density) affecting cooling at higher altitudes
- Cooling system efficiency (depending on design and conditions)
Exhaust Gas vs. Cylinder Head Temperature
EGT limits, unlike CHT, offer limited practical value:
- EGT is not a direct measure of engine stress. In fact, EGT often decreases when stress increases.
- EGT is primarily useful for troubleshooting and monitoring engine conditions.
On the other hand, CHT limits are critical for engine longevity:
- CHT closely approximates internal cylinder pressure, with high ICP indicating increased stress.
- Elevated CHT heightens the risk of cylinder head cracks.
Understanding Oil Temperature
Oil is crucial for aircraft engine performance, serving multiple functions beyond just lubrication. It cools the engine and its components, keeps the engine internally clean from wear and combustion byproducts, acts as a hydraulic fluid in hydraulic tappets/lifters and propeller governors, and helps protect and preserve internal engine parts.
Many pilot-owners focus on oil temperature primarily for its impact on lubrication, i.e., viscosity. However, this concern is largely addressed by using multi-grade oils and pre-heating the engine. A proper oil temperature is equally, if not more, important for ensuring that water – a byproduct of the combustion process and present in the oil – is evaporated. This is essential to maintain oil and engine cleanliness and to protect and preserve internal engine components. Thus, the oil temperature should be within the limits stated in the Operator’s Manual, but ideally maintained above 100°C (212°F) for several minutes during each flight. Oil temperature can be managed through power settings, airspeed, and, if equipped, thermostats.
Intake (Induction Air) Temperature
The temperature of the induction air is another critical, yet often overlooked, parameter. It significantly influences engine power and, crucially, the margins for detonation/pre-ignition. Unintentional or erroneous activation, or improper installation, of the carburettor heat system can lead to rapid engine destruction. Always use the carburettor heat system correctly and appropriately, and verify the installation if necessary. Monitoring and controlling intake air temperature is vital for maintaining engine health and performance.
Prioritising the Right Temperatures for Optimal Engine Health
In the realm of aircraft engine maintenance and operation, understanding and managing temperatures is paramount. This blog post has delved into various crucial temperature parameters, highlighting their significance and impact on engine performance and longevity.
- Outside Air Temperature (OAT): Though not directly controllable, OAT significantly influences engine cooling and performance. Adjustments to CHT limits based on OAT variations are essential for maintaining optimal internal cylinder pressure and preventing undue stress on engine components.
- Exhaust Gas Temperature (EGT) and Turbine Inlet Temperature (TIT): We’ve debunked the myth around EGT as a crucial metric for engine stress, emphasising its role in troubleshooting and monitoring instead. For turbocharged engines, TIT remains a critical parameter to protect turbine blades.
- Cylinder Head/Coolant Temperature (CHT): As a reliable indicator of internal cylinder pressure and engine stress, CHT is a vital temperature to monitor and control. We recommend establishing personal CHT limits, taking proactive measures to maintain these levels.
- Oil Temperature: Often underrated, oil temperature plays a multifaceted role in engine health, from lubrication to internal cleanliness. Maintaining the oil temperature above 100°C during flights is critical for evaporating water content and preserving engine components.
- Intake (Induction Air) Temperature: Lastly, the induction air temperature, especially in carbureted engines, is crucial for engine power and preventing detonation/pre-ignition. Correct use and installation of carburettor heat systems are vital for engine safety.
By understanding these temperature parameters and their impact, pilot-owners can make informed decisions, leading to enhanced aircraft operation and maintenance. Remember, it’s not just about monitoring temperatures, but actively managing and responding to them for the health and longevity of your aircraft engine.
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.