How To Control Your Aircraft’s Engine Temperature
Controlling Your Engine’s Temperature: Why It Matters.
Managing engine temperature is critical, especially during demanding operations like aerotowing or high-power climbs. Following our discussion about general engine and oil temperature management, a key question arises:
What is the best way to reduce your engine’s temperature? Should you keep the throttle fully open and use other methods, or reduce throttle to lower power and accept the resulting decrease in power output?
There are both obvious and less obvious techniques to control your engine’s temperature. Let’s explore them.
Cowl Flaps
Cowl flaps are often the first method pilots think of for engine temperature control. Designed specifically for cooling, they effectively manage airflow through the engine cowling. Cowl flaps are widely used, from touring motor gliders to high-performance aircraft.
By adjusting the cowl flaps, you control the engine cowling’s air outlet. This changes the outlet’s cross-section and modifies airflow, amending the pressure difference between the upper and lower plenums. The result? Changed airflow and cooling.
How the Cooling System Works
Air-cooled engines, like those form Continental and Lycoming, rely on the pressure difference between the top and bottom plenums. Air enters the top plenum and slows down, converting kinetic energy into pressure (potential energy). This pressure difference forces air over the cylinders and cylinder heads, cooling them. Cowl flaps lower the bottom plenum pressure by enlarging the outlet and creating a low-pressure area around it, enhancing airflow and cooling efficiency.
Airspeed
Adjusting airspeed is another way to influence engine temperature. Faster airspeeds increase the pressure in the top plenum by providing more kinetic energy to convert into potential energy (pressure). This leads to improved airflow and cooling.
While airspeed changes also affect bottom plenum pressure by altering the airflow around the outlet, the primary effect occurs in the top plenum. Increased airflow through the engine at higher speeds provides better heat dissipation, making airspeed adjustments a simple yet effective cooling technique.
Mixture, Propeller, and Throttle Control
Beyond external factors like airflow, pilots can directly control engine temperature through the mixture, propeller, and throttle settings.
Mixture Control
The mixture lever adjusts the air-to-fuel ratio. Operating lean-of-peak (LOP) or rich-of-peak (ROP) can significantly impact engine temperatures. Leaning the mixture rich-of peak reduces fuel flow and raises combustion temperatures, while enriching it provides more fuel to absorb heat and lower temperatures.
Propeller Control
Engine RPM affects power output and heat generation. Lower RPM generally reduces power output and heat production, particularly in ROP operation, where fuel flow exceeds air availability. Reducing RPM limits air supply, effectively lowering combustion temperature and waste heat.
Throttle Control
Throttle settings directly control the air supply and, consequently, engine power output during ROP operation. In most systems, the fuel flow adjusts proportionally to the air supply, ensuring the correct mixture at mean sea level and standard conditions.
Fuel-Injected vs. Carburetted Engines
Fuel-injected engines adjust fuel flow proportionally to airflow, maintaining consistent mixture control across power settings. This predictability allows for precise temperature management.
Carburetted engines, however, often include a Power Enrichment Valve (Economizer). At wide-open throttle (WOT), this device enriches the mixture to prevent detonation and provide additional cooling. Reducing the throttle disengages the enrichment, returning the mixture to its normal metering, which may result in a leaner mixture and higher temperatures during ROP operation.
Key Takeaway
For fuel-injected engines, reducing throttle generally lowers temperature. However, for carburetted engines, keeping the throttle fully open at high power may provide better cooling due to the economizer’s enrichment function.
Water Injection
Water injection is a specialized cooling technique used in high-power applications, such as air racing or military aircraft. A fine mist of water (sometimes mixed with methanol) is injected into the intake air, reducing combustion temperatures and increasing detonation resistance.
This method allows engines to produce more power without exceeding thermal limits. Although rare in general aviation, water injection remains an effective solution in extreme environments, such as Reno Air Races, where maximum engine performance is critical.
Conclusion
There is no single method for reducing engine temperature. Instead, a combination of techniques—like adjusting cowl flaps, airspeed, mixture, propeller RPM, and throttle—can keep your engine within safe operating limits.
While external measures like cowl flaps and airspeed adjustments are straightforward, internal controls demand a deeper understanding of your engine and its systems. Carburetted engines, in particular, require knowledge of their enrichment mechanisms to prevent overheating during high-power operations, such as aerotowing.
If high engine temperatures persist under normal conditions, investigate the underlying cause. Often, the issue can be resolved with proper maintenance or operational adjustments, ensuring reliable and efficient engine performance.
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.