Decided to pop this out from the "fun video" thread. This stuff is fascinating...
Do the older radial engines have the same issues with needing a lead "cushion" in the valve seats that auto engines needed when we still had leaded gas?
A lot of car builders these days rebuild motors with stainless steel valve seats so that they can run modern fuels without damaging the valves or putting in lead additives. Wondering if the same holds true on radials?
Do you have a link to the video that you referenced?
As for valve seats, there is nothing special about them regarding radial engines. After all, a radial is still just a push rod engine like a horizontally opposed engine. As for types of metals used in the seats, it is typically a bronze alloy or an "austenitic steel". The bronze alloy is utilized in the intake valve seat, but in some applications may also be used in the exhaust valve seat.
Now, I am not a metallurgist. It is my understanding that austenitic steels are stainless. Your question lead me to do a lot of reading on the internet, and I can't find anything to refute that an "austenitic steel" is anything other than a type of stainless. My A&P instructor used to just always classify "austenitic steel" as being non-magnetic.
Alright, so the exhaust valve and seat of any engine is subjected to higher temperatures than the intake valve. This is due to the exposure to the hot exhaust gases scavenged during the exhaust stroke. In addition, the intake valve runs cooler as it is cooled by the incoming vaporized fuel/air mixture. So the exhaust valve and seat need to be able to withstand higher temperatures. Thus, often the seats are made of "austenitic steel". Also, the exhaust valves themselves may be sodium filled to aid in heat dissipation.
Lead as a valve cushioning property is a little overstated. Yes, that is a property, but it is not the primary purpose of lead in avgas. In fact, 100LL is much higher in lead content than the older 80/87 that many older engines were certified to run. This contributes to excessive lead build up which can lead to valve sticking! This is especially an issue at prolonged running at low power,
think taxiing, where the scavenging effect is low. Most people are not taught to lean for taxi, primarily I suspect to prevent them from trying to take off in a overly lean condition.
No, the PRIMARY purpose of lead in aviation fuels is to prevent detonation! All fuels have a critical temperature & pressure. Tetra-Ethyl Lead raises the threshold for gasoline, allowing for higher effective pressures within the cylinder without risking detonation. Of course there are other variables too, including mixture setting & power setting, that effect the propensity of the fuel to detonate within the combustion chamber. However, the fact remains that the addition of TEL to avgas is primarily to prevent detonation, regardless of the manner of the arrangement of the cylinders around the crankshaft.
Hope this helped!
