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  #101  
Old 08-12-2021, 05:03 PM
AndrewR AndrewR is offline
 
Join Date: Jun 2017
Location: Ballarat, VIC
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Quote:
Originally Posted by jcarne View Post
If that is true then why does the "data we are yet to see" indicate more sticking valves on the LOP side? Even if an engine is run fairly deeply LOP the EGT's are still notably higher than a typical ROP cruise.
EGT is also an imperfect measure of combustion temperature. It measures the temperature after the gases have expanded and done their work. The temperature early in the power stroke when the cylinder volume is much smaller will be much higher.

If we slow the burning and delay the peak pressure, we will also decrease the temperature. This is why the CHT goes down. It also means that temperature dependent reactions like this will be slower.

So this would seem to be the link to CHT - all else being equal, a higher CHT is an indication of higher combustion temperatures earlier, which will help the lead scavenging reactions.
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  #102  
Old 09-02-2021, 10:52 PM
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jcarne jcarne is offline
 
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Well after emailing a couple of concise questions I haven't heard back again. I resent the email after a week. I'll keep you guys updated if I hear anything but right now Ed isn't answering...
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  #103  
Old 09-30-2021, 05:21 PM
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jcarne jcarne is offline
 
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Alright everyone. Sorry I'm just getting around to posting this. Life has been crazy with a new born and learning the ins and outs of pediatric life flight billing...



Anyways, Ed sent me a response. Here is my email to him with his responses in red.


1. You mention in your article that exhaust valve temp doesn't follow EGT. This seems to be a sticking point with many guys. I understand exhaust valves are quite a bit hotter in places than the cylinders and they use the seat/guides to cool; however, isn't the reason they are hotter than the cylinder due to exhaust gas passing by them? So wouldn't it stand to reason that exhaust valve temperature follows EGTs to some degree? Would really love some insight/data on why valve temp does or doesn't follow EGTs.

The exhaust temperature in a sense follows both the EGT AND the CHT. It is a complex combination of heat flow in and heat flow out, that makes for the confusion. The combustion temperature is around 4000 degrees F and the heat going out the exhaust port heats the valve and the head. The cylinder is cooler and acts as a heat sink drawing heat out of the valve. IF the head gets hotter it will reduce the heat flow out of the valve and into the head. George Braly had some exhaust valve data from Pratt and Whitney (circa 1936) that showed as you sweep the mixture from lean to rich (LOP to ROP) the exhaust valve temperature peaks and then goes down. The CHT continues to go up after the EGT peaks as does the power AND the exhaust valve temperature. Here is Lycoming data that shows all but the exhaust valve temperature. I have seen George’s data and it is perfectly reasonable to believe the valve temperature goes up with increasing power and CHT. If George can find the P&W paper I will share it.

2. The TRW picture you reference in your articles shows a comparison between a solid and sodium filled valve. Do you know if this is just a generic picture with generic temperatures and not referenced to a Continental and Lycoming at comparable power setting (say peak egt 65% power)? This leads to the final question.

This is TRW data not Continental or Lycoming. It is an example of how sodium filled valve stem conduct heat more effectively.

3. The reported sticking exhaust valve claims seem to point mostly to Lycoming engines. However, if we look at the TRW picture a sodium filled valve carries more heat to the stem for guide contact dissipation. The picture even reflects a sodium filled valve being hotter than a solid valve in the area where these deposits are forming. So at first glance it seems like a solid valve (Continental) would be more prone to condensation of the lead oxy-bromide than a Lycoming. What am I missing here?

Continental’s only produce lead oxy-bromide when the pilot alters the peak combustion temperature by running deeply LOP at reduced power.

Lycoming’s can experience valve sticking through lead oxy-bromide being carried up the valve stems (hot side in high power engines) OR from carbonaceous deposits caused by blow-by fuel contamination (cool side). BOTH seem to be exacerbated by the use of sodium cooled exhaust valves leading to higher stem temperatures but for very different reasons.


Thanks for your help in understanding this better Ed. I look forward to your response.

-Jereme
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Last edited by jcarne : 09-30-2021 at 05:30 PM.
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