Menu Sign In Contact FAQ
Welcome to our forums

Valve recession with unleaded fuel (UL94)

One large training fleet (UND) has now found valve recession issues with unleaded fuel (UL94 not UL100) and returned to 100LL.

My comment FWIW is that in the same situation when car owners moved to unleaded, circa early 80s, the same valve recession issue was observed with a few types of newly overhauled engines, but not with the same engine designs that had already accumulated some combustion products on the valves/seats. Most car and motorcycle engine designs were unaffected.

Last Edited by Silvaire at 10 Nov 08:18

Silvaire wrote:

One large training fleet (UND) has now found valve recession issues with unleaded fuel (UL94 not UL100) and returned to 100LL.

Interesting. Unleaded fuel has been extensively used by Lycoming powered aircraft in Sweden for many years and no such issues have been reported.

Last Edited by Airborne_Again at 10 Nov 08:23
ESKC (Uppsala/Sundbro), Sweden

My thought is that high utilization UND service included running all their engines from fresh overhaul, and a lot of those freshly overhauled engines were run for lots of hours after overhaul on unleaded only, enough to unearth the issue they’ve apparently now found. Also they were measured periodically after their overhaul cycle, in a structured program looking for valve recession specifically based on previous car experience.

Same thing with valve recession on freshly overhauled engines happened on cars and motorcycles in the late 70s/early 80s after unleaded fuel was first introduced, then used widely and inspected after long use. It wasn’t found until unleaded fuel was used exclusively for lots of hours on engines that had never been run on leaded fuel. Then rebuilders and manufacturers went in all kinds of wacky technical directions for a while before the issue was put to bed by the late-80s or so. It’ll be interesting to see if this is a repeat performance or not.

Last Edited by Silvaire at 10 Nov 08:55

What seems strange of course is that lycoming moved to hardened valve seats back in the 90’s = I doubt any of these engines were running on overhauled cylinders from before that period – so either Lycoming’s quality control for this metallurgy was not great or the Swift formulation has some special sauce that makes it particularly agressive.

Hardened valve seats are not the answer for this issue based on industry experience. For example in about 1980 BMW installed hardened valve seats in their motorcycle engines as a preemptive measure against unleaded fuel then becoming the norm in the US. This ended up being a disaster, because hardened materials have worse thermal conductivity and made the exhaust valves run hot, that worsened the problem and valve recession was extreme. By 1985 they had moved to another alloy for both valves and seats (a matched pair) that solved the problem of their own making. Pre-1980 or so engines never had a major problem then or now regardless. So it’s not so simple but the valve and seat materials that generally work with unleaded fuel are now established.

Last Edited by Silvaire at 10 Nov 13:13
I do not buy this theory that hardened valve seats might be worse for heat conductivity. The wall thickness is just 5-6mm, so not a big factor. And no, the seats are typically not hardened like in hardened. You´d have extreme troubles in machining them after inserting them or later do repairs. They are simply from an alloy with higher chrome content for reducing wear when no lead is the lube. I get tool steel for seats with 12 % chrome and use that unheattreated as the surface will turn into high wearing state from “workhardening” . And definitely get valves with stellited seats to go with the chrome tool steel. So when you don´t have the full picture about materials or any other components in an engine you cannot single out the root of the problem. On all iron engines like with pre-80 engines I put in these tool steel seats as in iron heads there will be wear when no lead fuel is used. Certainly not before mid-30ies was lead added to fuel and not much was reported about seat wear, but then crank speeds were quite low and mileages less than post-war, fuel not so cheap to get. So wear not a big factor then. Interestingly aircraft with DH Gipsy bronce heads like in the Tiger Moth had a lot of wear from exactly lead in fuel, post war when avgas was norm, no more leadfree to be available on airfields. My theory: Same effect like the wear at soldering iron copper tips from lead content in solder, eating into the copper by alloying chemistry. Vic
Last Edited by vic at 10 Nov 17:17

Vic, I’d suggest reading easily found articles like this or this or this to understand that high thermal conductivity of valve seat materials in combination with strength/hardness is a key requirement in cooling the valves of an engine with hot (e.g. air cooled) cylinder heads, and preventing the valves from recessing. Less thermally stressed (lower power density and/or liquid cooled and/or multivalve) engines can more often get away with lower conductivity valve seats as long as they’re hard enough.

Strength/hardness and thermal conductivity are two things that are typically hard to find simultaneously in one material – alloys that one typically don’t have the other. Beryllium copper is an example of an optimum valve seat material in this regard. However it is also costly and toxic – nothing is perfect

Last Edited by Silvaire at 10 Nov 20:14

Thanks to UND for being a live lab for new fuels.
It would be great if they could test 100UL in the same controlled environment.

LFOU, France

Considering the amount of publicity they made about using unleaded fuel, they must have had a compelling reason to roll back that decision. It will be interesting to get more data. I know of many people that have and are currently successfully burning unleaded avgas and mogas in Lycoming engines and “clones”. I have to wonder if there is something else going on.

Fly more.
LSGY, Switzerland
Silvaire, no need to tell me much about valve seat rings. I was just refering to your BMW example above, you saying they had changed for “hardened” rings with seemingly lots of troubles from this. Still I´d like to see if they were anything like hardened of above 50 HRC – which I do not believe. These flat twins got their problems more from sinking rings by excessive heat in aluminium around rings – a different matter. BMWs always had typically sort of cast iron rings, so my disbelieve about noticable poorer heat sink qualities from high chrome tool steel, bronce rings so far not discussed yet. Your links deal mainly about racing engines and mostly copper based seat rings. That is fine as long as no high mileage engines are equipped with that material. And when you look deeply into your links you will find that other than race types you better have higher chrome steel rings for lasting joy, in more modern times iron based sinter material. This can offer some more favourable properties like suitable expansion factors and less shrink/press fit in alu heads. I bet you don´t find any copper type rings in real high powered engines that have to go millions of hours, talk about truck Diesel or methane engines and a few thousand kW generator engines running 24h/d at 1500 resp. 3000 rpm – quite unlike common low stressed aero engines where you lot do leak tests all the time at each bigger service. AMPCO and all copper based rings are OK for short time fun engines like Ducatis, the bevel types not unknown for dropping their alubronce rings. It is not just the material that matters here, the rings need a good support and decent cross sections for not collapsing in use, so not an obvious fact for discussion. I did alu bronce rings for my R 69 S , but that was 40 years ago and these are alu heads. For cast iron car engines not my choice, when several thousands of hours are required from them, so the 12 % chrome tool steel for ex seat was my decision, and in our previous Ford V6 they already did more hours than any of your arero engines will ever hold up. So again same chrome type went recently into the active V 6 car to last out hopefully till we are 90 – good enough for me. Vic
36 Posts
Sign in to add your message

Back to Top