Detonation Insights

Animated .gif (169k)

The cylinder's fuel-air charge expands as it's ignited by the sparkplugs. Properly done, this isn't an explosion so much as it is a smooth, progressive burning of the combustible gases within the cylinder. As these gases expand, they increase pressure, pushing the piston downward during its power stroke.

Detonation occurs when the fuel-air charge explodes, burning in a short, fierce burst of energy rather than a controlled, even burn. Such an explosion can be very damaging and sometimes fatal to the airplane engine. Detonation occurs after the sparkplug fires, when the unburned portion of the fuel-air charge, ahead of the burning charge, suddenly explodes. High engine and cylinder head temperatures increase the likelihood of detonation. They may aid in the abnormal exploding vs. the normal burning of the ignitable gases within the cylinder. As an interesting note, the EGT reading may actually decreases during detonation, while the cylinder head temperatures increase. Of course, your ability to detect this on your EGT gauge depends on several factors: number of EGT probes, location of probes, etc. Even so, why would detonation result in a decrease in EGT?

With the exploding of the cylinder's fuel-air charge, combustion takes place earlier. This suddenly and dramatically releases the heat from the combustion process to the cylinder, piston and valves (too much heat at one time - that's the problem). The exhaust gases that are released have already expended their heat as shown in the animated diagram above. They're cooler, which is reflected in a lower EGT reading. If the throttle and mixture aren't moved and you see the EGT decrease while cylinder head temperatures increase, suspect detonation. Do whatever you need to do to cool the engine.

If you suspect detonation, there are several ways you can cool the engine's cylinders. First, enrich the mixture. Second, lower the climb angle, improving airflow over the cylinders. Third, if the airplane has cowl flaps, open them. Finally, reducing power is always a good practice if detonation is suspected. Do what you need to do to reduce those cylinder head temperatures.

The following are several known causes of detonation:

1. Using a lower grade of fuel than what is recommended by the manufacturer. Higher octane fuels have additives which help prevent detonation.
2. Using a time-expired fuel.
3. Over-leaning. Excessive leaning can raise cylinder head temperatures, increasing the likelihood of fuel exploding instead of burning within the cylinders.
4. Overheating an engine by climbing too steeply on hot days, as well as using excessive engine power (some engines have restrictions on the time maximum power can be used).
5. An abrupt opening of the throttle.

Idea excerpted from Rod Machado's Private Pilot Handbook