Quote:
Originally Posted by hharney
When operating LOP are the exhaust valves in jeopardy because of the decreased amount of cooling and lubrication from using less fuel? It seems that I have heard that cylinders are changed out more often on Cont. 360's when operated at LOP. Is this a management issue? It is not true?
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Herb:
Exhaust valve temperature is most affected by CHT. The lower the CHT, the lower the exhaust valve temperature. EGT has no effect on exhaust valve temperature. As a matter of fact, if you are at 40dF ROP and lean the mixture to peak, the EGT will be going up but the exhaust valve temperature will be going down. 40-50dF ROP is the mixture where the exhaust valve runs the hottest.
There is no difference in valve lubrication across the mixture sweep. Valve guides are lubricated by oil. The notion that fuel acts as a lubricant is misplaced. Fuel is a solvent, not a lubricant. Some claim that the lead in the fuel acts as a lubricant, but this is also misplaced. Lead exists in during the combustion event as a salt of bromide--lead-oxy-bromide. Salts are abrasive. Abrasives make crummy lubricants!
So, if running the exhaust valves cooler is desirable (it is) and running the combustion chamber cleaner to keep the exhaust valve cleaner and a better valve-seat interface is desirable (it is), then LOP mixture management is less stressful or damaging for the exhaust valves than ROP mixtures.
There is no hard data to support the claim that more cylinders are changed when operated LOP. There is hard data to support that the converse is true. There are over 400 million flight hours of data supporting the reality that LOP is easier on cylinders than ROP operation. There is data to support the contention that not running the mixture rich enough when ROP does put increased stress in the form of heat and pressure on the exhaust valve. That's probably where the notion came from that if you run too lean (not rich enough on the rich side) you'll burn up the exhaust valves.
Does that help?