What causes exhaust valve recession in an engine? — General Aviation News

Jan 30, 2024

By Ben Visser · November 23, 2022 · 8 Comments

That's a question I receive often and it's a very good question.

But as with so many things in aviation — and life — there's not a very definitive answer.

There have been several theories about the cause, including one that states the lead in fuel acts like a lubricant to protect the valve seat from wear.

This may be sort of true, but it does not answer why when you break in an engine on leaded fuel and then switch to unleaded fuel, the seats are usually OK.

Another theory is that the lead additive in avgas is somehow alloyed into the seat or somehow hardens the seat material.

Again that may be possible, but my limited metallurgical knowledge does not completely buy that.

The iron exhaust seats are usually as hard as they can make them when manufactured. I have been told that if they were made any harder, they would be brittle and prone to cracking. A little soft lead probably would not help.

So what is the answer?

The theory that makes the most sense to me is that the lead helps sealing and improves thermal heat transfer.

When the seat and valve are installed, there are small minute imperfections in the mating surfaces from the grinding process. This allows a small amount of gases to leak past the valve, which increases the temperature of the valve and seat.

This is a very small amount, but like the ring to cylinder wall seal, it needs some wearing in to "break-in" the seat.

The lead byproducts of combustion fill in those imperfections and provide a better seal for the valve.

The second part of this is thermal transfer of heat to the valve seat.

When the exhaust valve opens at the end of the combustion cycle, it is hit with the super-hot flame front from the combustion process. This elevates the temperature of the exhaust valve — especially the valve edges and seat area.

To cool the valve, much of the heat is transferred up the stem to the valve guide area and even to the oil in the valve spring area.

This is why some engines have sodium-filled valves to improve the heat transfer rate of the valve.

But the edge of the valve or the seat area gets most of the heat from the exhaust gases. To cool that area, much of the heat is transferred to the valve seat during the intake, compression, and power cycles.

When a leaded fuel is used during break-in, the lead by-products of combustion fill the tiny valleys left by the grinding process. This improves the heat transfer rate which, in turn, helps keep the temperature of the edges of the exhaust valve under control.

In the absence of lead, a much hotter valve face strikes the seat which, over time, can cause erosion.

There are many factors that affect the recession process. One is load.

That is why many automobiles were not affected when forced to use unleaded fuels in the 1970s. They operate at a much lower load factor than an aircraft engine at cruise.

In addition, most of them started life on leaded fuels and leaded fuels were still produced into the 1980s.

Another factor is RPMs. The lower the RPM operating range, the better the heat transfer rate to the seat.

That is why vehicles like farm tractors that work at 1,000 to 1,200 rpm did not have a significant problem switching to unleaded fuels. They are also all liquid cooled, so the temperature in the valve seat area could be better controlled.

The most significant problem was in gasoline-powered trucks. Even though they were liquid cooled, they operated at higher RPMs and under higher loads.

Will valve recession be a problem in aircraft operated on unleaded fuel?

I think there is a possibility, especially in older aircraft after being overhauled using new old stock parts.