Why lead reduces knock

In the Sept. 23, issue of General Aviation News, there was a letter to the editor from Jack Thompson, who wrote: “Re: Visser’s Voice: Knock free, Aug 26 issue: It is a sad commentary given that the IC engine is such an integral part of aviation that the user community is so ignorant of its operation and engineering issues. What percentage of the users understand the impact of fuel detonation resistance and the if-then relationships of fuel suitability for a given service?”

The actual truth is that no one really completely understands the interaction, especially how lead additives work in a fuel to reduce knock.

In the original research work for Tetraethyl Lead (TEL), they tried a long list of metallic additives and chemistries in test engines. Many of them worked to reduce knocking, but TEL was selected because it caused the least amount of negative side effects on engine life.

I recently received a nice letter from Gerald Whitcomb, who quoted a booklet put out by GM in 1940, titled “The Power Primer.” The booklet described TEL as being able to “run interference” between fuel and oxygen and thus reduce the speed of the advancing flame front.

This is basically true. The best theory that I have heard is that the TEL retards the pre-flame reaction at the flame front, which reduces the temperature and pressure build up in the end gas zone, which in turn reduces the tendency of the end gas to auto ignite.

Let me expand on that. When spark plugs ignite the mixture in a cylinder, it is not an instantaneous explosion. What happens is that the flame propagates out from the plug until the entire mixture is burned. Now as that flame front travels across the piston there is a pre-flame reaction that occurs at the flame front that raises the temperature and pressure. Then when the flame occurs, the temperature and pressure raises even more.

When knock occurs, the temperature and pressure of the mixture furthest from the plug raises above the auto ignition temperature of the end gases and they auto ignite. This is the “knocking” that you hear.

When the temperatures of the pre-flame reaction are reduced by the TEL, the tendency of knocking is reduced. TEL works better in some fuels than others and knocking is dependent on a very long list of factors, such as engine design, air conditions, load, RPM, etc., etc.

The reason for this explanation is not to impress people with what I know, because I am just repeating what other researchers have written. The reason is to give people an idea as to the complexity of the problem.

Many people feel that there is a simple correlation between knocking and fuels. But the problem is that the relationship is different for each engine and condition. For example, to measure octane in a knock test engine, one must run the engine at just one RPM, with constant load and all engine parameters carefully controlled to very narrow limits. In addition, the temperature and humidity of the intake air is carefully controlled, and one must then correct the octane reading for barometric pressure. When you throw in the fact that there are many different engine combustion chamber designs, that aircraft operate over a wide range of atmospheric conditions, loads and RPM ranges, one can see the complexity of the problem.

The auto gas STCs for 80/87 engines work because of the extra margin of “knock” safety between auto fuels and the requirements of these engines.

When we start talking about many of the 100/130LL engines, there is a margin of “knock” safety with leaded avgas. This margin is because all leaded avgas is based on the same refinery process. No matter where it is produced, with TEL, all avgas is almost identical to that produced by other companies.

But when you switch over to unleaded fuels, which may have very different compositions — and do not have the benefit of the TEL — then that margin of “knock safety” may be gone.

Knock knock, who’s there? Unleaded avgas.

Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at Visser@GeneralAviationNews.com.

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