In a past column I gave some background on what happens during break-in of a new or overhauled aircraft engine. Since then I have received numerous questions about why leaded fuel is needed during break-in.
Unfortunately, I do not know anyone who has a perfect answer for this. My opinion is based on lab tests and reports from field overhauls and manufacturer’s reports.
An example of a lab test that we ran back in the late 1960s was one run in a 430 CID Buick engine. The engine was installed on a test stand and run for over 20,000 simulated miles at relatively high RPM and load. We monitored the valve stem height and, at the end of the test, there was no significant exhaust valve recession.
The test was repeated with a new tank and fuel lines, plus new heads that had never been run on a leaded fuel. (We found out that the original new engine had been run at the factory on a leaded fuel.)
Now after about 15,000 simulated miles, most of the exhaust valves had receded enough to take up the lash allowed by the hydraulic valve lifters.
As far as field data, about 20 years ago, a west coast oil company started to market an 80/87 unleaded avgas. This was perfectly legal since the ASTM D-910 spec for avgas specifies that the fuel only meet a maximum of 0.5 grams per gallon lead level. There is no minimum level.
A few months after the introduction of this fuel, I started to receive numerous valve recession complaints, but only from the west coast. Further investigation revealed that every failure occurred on aircraft that had operated on the unleaded fuel.
Subsequent to this I have heard numerous reports of valve recession on aircraft that have operated exclusively on unleaded mogas. Now it does not happen to every aircraft every time, but it does happen a significant part of the time.
So what is the mechanism here and what does the lead do to prevent exhaust valve recession?
This is one of those things that if you talk to 10 experts, you will probably get 20 to 25 different answers.
The most common ones are: Lead acts as a solid type lubricant to protect and cushion the valve and seat; another is that the lead alloys with the seat material; and still another is that it improves the heat transfer from the valve and heat tempers the seat.
In the Buick engine test, the low level of lead that was needed to protect the valves was due to the liquid cooling of these engines and the subsequent lower seat temperature compared to an air cooled aircraft engine.
When an engine is running, especially under high load, the exhaust valve is exposed to direct flame temperatures when it first opens. To keep the exhaust valve from getting too hot, engine manufacturers design a direct heat path up the valve stem to the guide, valve tip, and the oil flow.
But the most critical point is the valve tulip edge, and that is designed so that most of the heat is transferred to the valve seat during the time the valve is closed. There is not much time for the heat to be transferred, and the transfer depends on the difference in temperatures between the valve edge and the seat.
Since a liquid-cooled engine exhaust valve seat runs several hundred degrees cooler than that seen in an air-cooled engine, it is easy to see why aircraft engines are much more critical.
So what is the answer? I feel it is a combination of things.
First, in a new engine, the valves and seats are both ground. The surface of the valve and seat are not perfectly smooth. The micro ridges and valleys from grinding allow some leakage after the first startup, and provide a poor heat transfer path from the valve to the seat.
With a leaded fuel, the byproducts of combustion tend to coat the valve and seat, which yields improved sealing and a better heat transfer path. Over time, the seat does harden somewhat and the engine goes on to live a long productive life.
With an aircraft engine, it becomes much more critical, especially during break-in. After that a small amount of lead is required during the life of the engine to protect the valve and seat.
It is not a precise amount, but during break-in I would recommend running at least 25% 100LL for the first 50 hours. After that, a small amount every so often should keep all running well.
Take a look at the Adept Airmotive water cooled aircraft engines that are manufactured in South Africa. Live and learn..
I rode an air-cooled engine called a motorcycle for years. It ran on unleaded fuel. It ran well and probably under harder conditions than the airplane engines do. Frequent starting and stopping, sitting at traffic lights for long durations in hot summer weather and cruising at higher RPM’s than my aircraft. I believe that it most of these problems with aircraft engines are because the engines are way behind the modern curve of better metal technology. Implement the new technologies into the older aircraft engines and lead will not be necessary.
So your saying not even 100 LL is good foe engine break in?
When they got the lead out of the car engines & either harden the valves or chromed coated them. So I wish I could recall but as I recall that was the solation.
I believe hardened valve seats were part of the solution.
Leaded fuel has not been available for automotive applications for many years. I do not see valve problems in car engines, in fact it’s not unusual to see cars with over 200k miles that have not had any major engine work done. These motors have never seen leaded fuel. What is being done on the automotive side that cannot be done in aircraft engines? With the advent of unleaded aviation fuel on the horizon, will we be seeing a lot of valve problems in the fleet?
Liquid cooled engines have a much more efficient heat transfer, especially in the heads where it is most critical. Visser discusses this in his article. Also automotive engines have a much less demanding duty cycle, seldom producing maximum power and normally running at a considerably lower portion of rated power, perhaps 20%, compared to an aircraft engine at 65-75 %.
Most automotive engines are liquid cooled. I don’t know of many, if any, aircraft engines that are liquid cooled.
Rotax 912,914
Edit: There are some liquid cooled. However the big issue is with the air cooled engines.
Interestingly, the Peterson MOGAS STC suggests running a tank of 100LL every 75 hours. No mention as to why, though.
Would your advice also apply to liquid cooled aircraft engines such as the Rotax 100 ULS during break-in or the first 50 hours? I seem to recall the Rotax people saying that while the engine runs satisfactorily on 100LL it prefers premium Mogas to lessen the fouling of spark plugs.
Sorry I butchered that one. I meant to say the 100hp Rotax 912 ULS.