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.
Does lowering coolant temperature in cars Helps reducing valves recession??
Great artical on engines and the wear and tear of the inside while we are flying.
I’ve been building custom Harleys many years and working on my 13th build today. 1978 shovelhead. It’s a hobby. I always use Lucas additive in my engins and transmission on all my bikes and cars etc. A while back I ask a good friend who is a certified Harley Davison engine builder to rebuild my engine on one of bikes a 1979 FXEF Shovelhead bike I had been riding 16 plus years. I wanted to know how well the lucas was performing. After teardown he made a comment that the lower end Rods, bearings, cam etc and other parts showed very litte wear. So $500 Set of new rods then trued up the flywheels and other new parts it was put back together. It was very expensive for something that wasn’t broke……
My question do aircraft engine builders use additives like Lucas to help on were and tear plus will add better compression?
Tractors running at 1200 rpm are diesel
In my limited experience it seems like water cooled heads help eliminate this problem as in the Rotax 912/914 series engines that run in the 5000 rpm range and prefer unleaded fuel.
Thanks, you gave an answer that I was going to ask… Thank you…
Did not define what is valve recession
It is where the valve seat material wears away, and the valve recedes into the head.
This reduces the effective valve lift.
Indeed. Cylinder shops know this. Engine overhaul people know this. But it’s impirical data that affects warranty and long term considerations for both. Running clearance between the exhaust valve stem and guide are also considerations, given the relative sloppiness and need for valve-to-face sealing. Unleaded fuel will prove worthy of monitoring, but whether engine OEMs publish acceptance of the fuel is key.
Those are all good points but ultimately heat does the damage, even to the super hard Stellite seats in use today.
Primarily heat is proportionate to power output but there are many other significant factors:
Slow combustion exposes the exhaust seats to more heat.
A slow combustion can have many causes:
weak ignition spark
small plug gap
Not enough timing advance for the operating condition
Lean (also rich) operation slows the combustion and should be accompanied with more timing advance to keep the seats cool.
Large diameter combustion chambers (such as is typical on aircraft engines)
Single ignition operation also generates higher EGTs (if the plug is not centrally located)
Too high an octane rating for the operating condition.
Most of the significant advances in the automotive industry came from improving the combustion to keep the ignition process short and take advantage of the cooling during the expansion stroke (power stroke).