If the octane is the same, does it matter if the fuel is unleaded?

Phil Smith from Iowa wrote to ask about leaded vs. unleaded octane. He heard at a seminar that “”?when the octane is maintained, lead is NOT necessary in an aircraft engine.””


Phil Smith from Iowa wrote to ask about leaded vs. unleaded octane. He heard at a seminar that “”?when the octane is maintained, lead is NOT necessary in an aircraft engine.””

The use of lead in gasoline is one of the great mysteries of science. We know that as the fuel/air mixture in the combustion chamber burns, it is not an instantaneous explosion, but rather a flame front that starts at the spark plug and goes outward. As the flame front travels, there is a pre-flame reaction occurring at the leading edge. We also know that tetra-ethyl-lead (TEL) tends to greatly reduce or retard this pre-flame reaction.

When the advantage of TEL in gasoline was first discovered about 80 years ago, scientists were evaluating a long list of heavy metals that seemed to reduce knocking in an engine. Of all of the tested compounds that worked, TEL had the least number of negative side effects. By design, engines were developed to, more or less, live with these problems. For example, shorter spark plug and exhaust system life were just accepted.

The entire gasoline business, both automotive and aircraft, was built around designing engines to run on leaded fuels. In addition, all of the test procedures were developed to evaluate only leaded fuels. This changed in the automotive world in the mid-1970s with the advent of catalytic converters on most, and then all, cars.

However, if you go back to reports from the late 1960s, you can read about a phenomenon called the “”lead bonus.”” It was well documented that, even in an auto engine, leaded fuel would give greater antiknock protection than an unleaded fuel of the same octane number.

The other facet is the octane rating systems. The number you see on the auto fuel pump is an average of the research and motor methods for measuring octane. These methods have been used in the automotive industry ? almost exclusively on unleaded fuels ? for the last 30 years. They correlate well with actual in-use service for automotive engines.

In aviation, we use the lean and rich octane methods. The lean rating is derived from the automotive motor method number, so it works well on unleaded fuels. The rich rating is a totally different test procedure and measures the onset of knock as opposed to full power knock in the other two methods. The results I have seen to date indicate that the rich rating method correlates to actual aircraft service better than the lean rating. The rich rating works fairly well with leaded fuels, but not with unleaded fuels with varying composition. For example, in one run in the mid-1990s, an unleaded fuel with a rich rating ran with less knocking in an aircraft engine than an unleaded fuel with a rich rating more than 10 numbers higher.

Back to the original question: a 100-octane unleaded fuel will probably not perform as well as a 100-octane leaded fuel in an aircraft engine. Now, unleaded fuels can be made to work in aircraft engines. However, most turbo/supercharged engines will need to be modified to run on them. Most naturally aspirated engines will probably operate quite satisfactory with no modifications, but there will be some differences.

Unleaded vs. leaded fuels is like comparing apples to chipped beef on toast: They are both foods, but there are some differences.

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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