My last several columns have been on the relationship between knock and unleaded fuels in aircraft engines. I have received several questions from people who have experienced knock in their car engines, especially in the 1970s and 1980s. They note that knock didn’t harm their auto engines, so why am I worried about a little knock in an aircraft engine?
There are a number of significant differences between auto and aircraft engines. The biggest is liquid vs. air cooling.Knock occurs when the piston in a spark ignition engine ignites the intake charge. The spark does not cause an instantaneous explosion, but rather a very rapid burn across the top of the piston.
As the flame front goes across the combustion chamber, the pressure and temperature increase rapidly. If the engine is operating on a fuel with inadequate octane, the gases that are furthest from the spark plug(s) will auto ignite before the flame front reaches it, thus the term “knock.”
Since this occurs before top dead center, it causes an increase in the temperature and pressure “spike” in the cylinder. This can increase the heat load that needs to be dissipated by the engine.
One of the most critical parts to cool in an engine is the exhaust valve. Some of the heat load on the valve is transferred up the stem to the guide, but a majority is transferred to the valve seat during the intake stroke. This is a quick transfer, like putting a red hot piece of metal in water, and depends on the temperature of the valve seat.
In a liquid-cooled engine, the valve seat temperature stays very constant, even with increased heat loading from knocking. This is because the liquid temperature stays constant and to increase it much would require enough heat to boil the coolant (latent heat of evaporation).
In an air-cooled engine, the valve seat will just increase because radiation heat transfer is much less efficient and you do not have the latent heat thing going for you. This allows the valve edge to heat up and, if it gets hot enough, it can ignite the incoming charge early in the compression stoke. This is called pre-ignition and it can damage an engine in a few seconds.
The other significant difference is in the sound transfer to the pilot from the knocking process. In an auto engine, the common cylinders and head make a more metallic or audible sound that is transferred to the entire block assembly. That is why they can use only one knock sensor to control knock in an auto engine.
In an aircraft engine, the knocking sound is not transferred to the other cylinders, so the sound is not as noticeable in the driver’s seat. Also, as we all know, the normal aircraft engine noise is at a greater level than in an automobile.
The third significant difference is what to do when knock does occur. In your car, once you hear knock you can just reduce the acceleration rate and the knock will go away.
In an aircraft, if the plane is fully loaded on takeoff and in a marginal situation, reducing the throttle setting to eliminate the knock may not be a good option.
I have heard knock in an aircraft engine in a test cell. If you are near enough you can hear it, and it sounds different than in an auto engine.
I will admit that I have not been in an actual aircraft when knocking occurred and I do not plan on doing so.
But I have talked to people who have done knock test work in real aircraft many years ago, and it was an interesting procedure. The work was done in large multi-engine radial aircraft with the test fuel only in one engine. They had difficulty rating knock because of the noise level in the plane, so they depended more on a visual rating of the exhaust at night to determine knock activity.
Once the observers noted knock either by exhaust appearance or hearing it, they would quickly have the pilot chop the power from that engine in hopes they beat what they called the “death rattle” in the test engine.
Fortunately this was being done in cooperation with the military and they had a ready supply of replacement engines and the personnel available to replace them quickly.