2 ms·

This would now probably get into the big debate around running these engines rich of peak or lean of peak. Generally, speaking engines of this type are manuall

This would now probably get into the big debate around running these engines rich of peak or lean of peak. Generally, speaking engines of this type are manually leaned - full rich for take off, climb and landing, then leaned for fuel conservation in cruise. The debate is around whether it is better for the engine to be run slightly richer or slightly leaner over an extended period of time. Slightly lean of peak is probably better. However, if leaned too much there is the risk of one or more cylinders overheating. Exhaust Gas Temperature (EGT) guages should be used to monitor this. Many older Cessna 172/182s wouldn't be fitted with EGTs. They certainly weren't factory standard in the 50s, 60s and 70s. All this leaning of mixture is done manually, by turning a knob screw in the cockpit. Another risk is shock cooling. This isn't really an issue with Cessna singles, but can be a real problem with heavier twins like a Piper Chieftain (PA31-350). They easentially run two bigger versions of a very similar engine.

sokoloff 10y ago
When lean of peak, leaner is cooler, so the overheating risk you're describing is generally from cylinders operated "not lean enough" if the intent was to operate lean of peak.
- gonzo 10y ago
  Well, 50deg LoP is the same EGT as 50deg RoP. But, in general, leaner is hotter, not cooler. At a ROP setting, the extra fuel that is not burned lands on many of the top end components including the exhaust valve. As it evaporates off the surface or out of the fuel/air charge, it cools the component or charge. Also, peak cylinder pressures are higher when run RoP. http://www.gami.com/paulferraris_leanofpeaksaga.pdf http://www.gami.com/paulferraris_leanofpeaksaga.pdf In any case, the Cessna 172S Lycoming IO-360-L2A (engine) handbook says, "Continuous operation at mixture settings lean of peak EGT is prohibited"
  - sokoloff 10y ago
    In the context of cylinder head temperature (cylinders overheating is what phillc73 mentioned), 50LoP is markedly cooler cylinder temps than 50RoP. See the blue line in the graph here: https://www.advancedpilot.com/redbox.html https://www.advancedpilot.com/redbox.html or the CHT line on the graph in Lycoming's (in)famous "Experts are Everywhere" document: http://users.kymp.net/kotkanik/documents/SSP700A.pdf http://users.kymp.net/kotkanik/documents/SSP700A.pdf (note that in the Lycoming document, leaner mixtures are on the left and in the APS red box graph, leaner mixtures are on the right; Lycoming graphs BSFC and APS graphs 1/BSFC) > But, in general, leaner is hotter, not cooler. That is not borne out by the data shown in graphical form in the links above (which agree with each other and match my experience operating big-bore Continental engines). Only while richer than 25 rich of peak EGT is your statement correct. Lean of peak, leaner is in fact cooler, which was my original claim. > Also, peak cylinder pressures are higher when run RoP. From a cylinder longevity standpoint, this is a bad thing, of course. It's particularly a bad thing when cylinders are run very hot (over say 425F) because aluminum loses a significant percentage of its strength in that temperature regime: https://www.nap.edu/openbook/NX006900/xhtml/images/p20003245g11001.jpg https://www.nap.edu/openbook/NX006900/xhtml/images/p20003245...
    - phillc73 10y ago
      Also keep in mind that these are air-cooled engines. Due to the configuration, it may very well be the case that one or both rear cylinders are not receiving the same cooling effect as the front two. Therefore, even if you're running lean of peak, it's very, very important not to overdo it On a Cessna 182 I used to run, I could lean it to 50-52 litres per hour, using a digital fuel flow gauge (not standard factory equipment). If I leaned to 48 litres per hour, the EGT gauges clearly showed a rise in temperature for certain cylinders. If I was leaning without using these gauges for assistance, it would be very easy to make a mistake and overdo it, or not lean enough and waste fuel/money.
    - FabHK 10y ago
      That Lycoming paper is great, btw!
      - 10y ago

sokoloff 10y ago

When lean of peak, leaner is cooler, so the overheating risk you're describing is generally from cylinders operated "not lean enough" if the intent was to operate lean of peak.

gonzo 10y ago
Well, 50deg LoP is the same EGT as 50deg RoP. But, in general, leaner is hotter, not cooler. At a ROP setting, the extra fuel that is not burned lands on many of the top end components including the exhaust valve. As it evaporates off the surface or out of the fuel/air charge, it cools the component or charge. Also, peak cylinder pressures are higher when run RoP. http://www.gami.com/paulferraris_leanofpeaksaga.pdf http://www.gami.com/paulferraris_leanofpeaksaga.pdf In any case, the Cessna 172S Lycoming IO-360-L2A (engine) handbook says, "Continuous operation at mixture settings lean of peak EGT is prohibited"
- sokoloff 10y ago
  In the context of cylinder head temperature (cylinders overheating is what phillc73 mentioned), 50LoP is markedly cooler cylinder temps than 50RoP. See the blue line in the graph here: https://www.advancedpilot.com/redbox.html https://www.advancedpilot.com/redbox.html or the CHT line on the graph in Lycoming's (in)famous "Experts are Everywhere" document: http://users.kymp.net/kotkanik/documents/SSP700A.pdf http://users.kymp.net/kotkanik/documents/SSP700A.pdf (note that in the Lycoming document, leaner mixtures are on the left and in the APS red box graph, leaner mixtures are on the right; Lycoming graphs BSFC and APS graphs 1/BSFC) > But, in general, leaner is hotter, not cooler. That is not borne out by the data shown in graphical form in the links above (which agree with each other and match my experience operating big-bore Continental engines). Only while richer than 25 rich of peak EGT is your statement correct. Lean of peak, leaner is in fact cooler, which was my original claim. > Also, peak cylinder pressures are higher when run RoP. From a cylinder longevity standpoint, this is a bad thing, of course. It's particularly a bad thing when cylinders are run very hot (over say 425F) because aluminum loses a significant percentage of its strength in that temperature regime: https://www.nap.edu/openbook/NX006900/xhtml/images/p20003245g11001.jpg https://www.nap.edu/openbook/NX006900/xhtml/images/p20003245...
  - phillc73 10y ago
    Also keep in mind that these are air-cooled engines. Due to the configuration, it may very well be the case that one or both rear cylinders are not receiving the same cooling effect as the front two. Therefore, even if you're running lean of peak, it's very, very important not to overdo it On a Cessna 182 I used to run, I could lean it to 50-52 litres per hour, using a digital fuel flow gauge (not standard factory equipment). If I leaned to 48 litres per hour, the EGT gauges clearly showed a rise in temperature for certain cylinders. If I was leaning without using these gauges for assistance, it would be very easy to make a mistake and overdo it, or not lean enough and waste fuel/money.
  - FabHK 10y ago
    That Lycoming paper is great, btw!
    - 10y ago