Did you know that piston aircraft engines are only able to convert approximately one-third of the fuel’s energy into useful power?
You can think of heat as being the wasted energy of an inefficient engine. If an engine were using 100 percent of the energy contained in the fuel that it was consuming, it would be the same temperature as the ambient air around it. Aircraft and engine manufacturers are aware of the areas where the majority of this inefficiency happens and have installed sensors and gauges to help the pilot manage them. Two of these gauges that seem to be commonly misunderstood are the cylinder head temperature (CHT) and exhaust gas temperature (EGT).
Cylinder Head Temperature (CHT)
CHT is measured by a temperature-sensing probe located at the cylinder head, and it measures heat energy wasted during the power stroke, when the cylinder is under maximum stress from high internal pressures and temperatures.
High CHTs generally indicate that the engine is under excessive stress. Limiting CHT to the temperature range outlined by the manufacturer is crucial for safe operation and cylinder longevity. As an example, Lycoming recommends regulating CHTs to under 400°F. CHTs above 400°F should be considered damaging to the engine and grounds for “doing something right now” to bring them down.
There are many factors that will affect CHT, such as power setting, magneto timing, and spark plug condition, but the best way for a pilot to control the CHT is by using the mixture control to determine the burn rate of the fuel/air mixture in the cylinder and through the natural cooling effect of fuel.
Important facts about CHT:
Changing the mixture can affect CHT by changing the rate at which the air-fuel charge inside the cylinder burns, therefore causing combustion to occur earlier or later.
Increased power increases CHT.
Reduced cooling airflow over the engine increases CHT.
Timing, spark plug failures, and magneto problems can all affect CHT by changing when combustion occurs in the cylinders.
Exhaust Gas Temperature (EGT)
While CHT mainly shows what’s going on in the cylinder during the power stroke before the exhaust valve opens, EGT mainly shows what’s going on during the exhaust stroke after the exhaust valve opens.
EGT is measured by temperature-sensing probes located downstream of the exhaust valve that indicate heat energy that is being wasted when the exhaust valve is open. Given that the exhaust valve is closed during the majority of intake, compression, and power strokes, the exhaust gas is only flowing past the sensing probe for a small portion of the engine operation and during the time of the lowest stress on the cylinder. The EGT gauge is therefore displaying the average of a relatively cool temperature when the exhaust valve is closed and the spike in high temperature when the valve is open.
Early EGT gauges only showed tick marks representing twenty-five degree increments instead of a numerical temperature because knowing the actual temperature really doesn’t matter with EGT. The tick marks were designed to help the pilot determine how many degrees lean of peak (LOP) or rich of peak (ROP) their mixture setting was. High EGTs do not represent a threat to engine life because the engine is simply not capable of producing EGTs high enough to harm anything. As a general rule, the best power mixture is approximately 100°F ROP, while the best economy mixture is generally 20°F to 50°F LOP.
Over the years, engine instruments have evolved to show the pilot more information than ever before, and it is now standard for EGTs to indicate numerical temperatures (absolute values) instead of relative temperatures like gauges of the past. While these modern gauges can still be used to fine-tune mixture settings in the same manner, they can cause confusion for pilots concerned about damaging the engine due to perceived high or inconsistent EGTs.
Important facts about EGT:
Peak EGT can be found by leaning the mixture control and observing the maximum EGT that you are able to achieve on the gauge.
Mixture settings can be used to establish mixtures richer than peak (ROP) or leaner than peak (LOP).
High EGTs do not represent a threat to engine life.
When adjusting power settings and mixture, it is important to pay close attention to CHT because it is the best representation of stress on the engine. You should establish a target CHT that represents the maximum stress you want to place on your engine based on the operational limits set by the manufacturer, and then adjust your power settings, mixture, and (if applicable) cowl flaps to ensure that all cylinders are operating at or below acceptable levels. From there you can use EGT to set the mixture to a specific fuel/air ratio that is ROP or LOP. It’s not essential to use EGT for leaning unless you’re particularly curious about what fuel/air ratio you are using.