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Apologies, I’m not very familiar with aircraft engines, why can’t we simply use high octane unleaded gasoline, like most turbocharged car engines? Are the cylin

by ericd 5y ago

Apologies, I’m not very familiar with aircraft engines, why can’t we simply use high octane unleaded gasoline, like most turbocharged car engines? Are the cylinder pressures significantly higher in airplane engines?

sokoloff 5y ago
Airplane engines are overwhelmingly air-cooled, fixed timing (after starting), pilot-controlled fueling, and expected to produce 100% of rated power for 5 minutes and then 65-75% of rated power for hours on end. Automotive engines are overwhelmingly water-cooled [providing a far more stable and lower cylinder head temperature], variable/computer-controlled timing often with knock sensing [providing the means to adjust timing dynamically when anything beyond light pinging is detected], closed-loop fuel control, and rarely producing more than 20% of their rated power. Aircraft engines need to stay together with a cylinder head temperature of 460ºF, fixed timing, and fuel mixtures that are set by the pilot. It's a far more demanding (and less controlled) environment. Some additional refs that might be interesting: https://www.avweb.com/features/pelicans-perch-43detonation-myths/ https://www.avweb.com/features/pelicans-perch-43detonation-m... https://www.avweb.com/features/pelicans-perch-55lead-in-the-hogwash/ https://www.avweb.com/features/pelicans-perch-55lead-in-the-... https://www.avweb.com/features_old/pelicans-perch-31those-fire-breathing-turbos-part-1/ https://www.avweb.com/features_old/pelicans-perch-31those-fi...
- maxcan 5y ago
  Small nitpick but while aircraft engines cannot catastrophically fail at 460F, no one should be flying them that hot. Its basically guaranteed to shorten the TBO on the engine that way. When I was flying a turbocharged cirrus (notoriously hot running engine) we always kept the cylinder heads to 380F. It limited our climb performance but was so much better for the engine.
  - sokoloff 5y ago
    I agree. I use 380°F as my soft limit for a TNIO550B and take more aggressive action at 395°F. Nevertheless, it’s a certification requirement that the engine have appropriate detonation margin at the manufacturer’s declared CHT redline and TCM/CMI has chosen to put their redline around 75°F higher than I’m comfortable in normal operations.
    - maxcan 5y ago
      I just went under contract for my first turboprop.. the numbers there are nutty. The engine can sustain 840C, it can go to 870 for less than 20 seconds and it can even sustain 1000C for up to 5 seconds without triggering any kind of exceedance.
Blackthorn 5y ago
We can and do, if the engines are upgraded with goodies like electronic ignition (yeah, all those old aircraft engines are using fixed timing magnetos). But a lot of aircraft engines just aren't. And if you're in a certified plane instead of an experimental plane, good luck ever getting it. Basically any engine made by a European engine manufacturer takes "mogas" without incident, and prefers it because lead causes issues. But to the sibling comment's point, those engines also have goodies like single lever power (so no direct mixture or rpm control). That changes the parameters a lot. Edit: to add on, if you're in an experimental plane, you can do all sorts of things to your engine. You can buy aftermarket electronic fuel injection and ignition kits with variable timing, and reclaim a good deal of fuel economy in the process. Most engines that these kits support will then quite happily run along with a fuel like 94UL. Some people have even converted automobile fuel ignition controllers for the purpose (I hear the Ford V8 ECU is pretty popular for this).
- slownews45 5y ago
  Yeah, don't diamond note you CAN use leaded, but recommends AGAINST it?
- ericd 5y ago
  Thanks for the rundown! So with unmodified engines, the lead mainly just helps make extra sure there’s no knocking, which is very important because the engine won’t compensate (any issue midair would be very bad)?
jabl 5y ago
Aviation gasoline is much higher octane than the "premium" (or however it's branded) high octane gasoline at your local gas station. Typically premium gasoline is about 98 RON / 88 MON, whereas 100LL aviation gasoline is 100 MON. The big news here is that GAMI has come up with a formula that provides the required octane without lead, and also otherwise fulfills the various requirements for aviation gasoline.

by ericd 5y ago

sokoloff 5y ago

Airplane engines are overwhelmingly air-cooled, fixed timing (after starting), pilot-controlled fueling, and expected to produce 100% of rated power for 5 minutes and then 65-75% of rated power for hours on end. Automotive engines are overwhelmingly water-cooled [providing a far more stable and lower cylinder head temperature], variable/computer-controlled timing often with knock sensing [providing the means to adjust timing dynamically when anything beyond light pinging is detected], closed-loop fuel control, and rarely producing more than 20% of their rated power. Aircraft engines need to stay together with a cylinder head temperature of 460ºF, fixed timing, and fuel mixtures that are set by the pilot. It's a far more demanding (and less controlled) environment. Some additional refs that might be interesting: https://www.avweb.com/features/pelicans-perch-43detonation-myths/ https://www.avweb.com/features/pelicans-perch-43detonation-m... https://www.avweb.com/features/pelicans-perch-55lead-in-the-hogwash/ https://www.avweb.com/features/pelicans-perch-55lead-in-the-... https://www.avweb.com/features_old/pelicans-perch-31those-fire-breathing-turbos-part-1/ https://www.avweb.com/features_old/pelicans-perch-31those-fi...

maxcan 5y ago
Small nitpick but while aircraft engines cannot catastrophically fail at 460F, no one should be flying them that hot. Its basically guaranteed to shorten the TBO on the engine that way. When I was flying a turbocharged cirrus (notoriously hot running engine) we always kept the cylinder heads to 380F. It limited our climb performance but was so much better for the engine.
- sokoloff 5y ago
  I agree. I use 380°F as my soft limit for a TNIO550B and take more aggressive action at 395°F. Nevertheless, it’s a certification requirement that the engine have appropriate detonation margin at the manufacturer’s declared CHT redline and TCM/CMI has chosen to put their redline around 75°F higher than I’m comfortable in normal operations.
  - maxcan 5y ago
    I just went under contract for my first turboprop.. the numbers there are nutty. The engine can sustain 840C, it can go to 870 for less than 20 seconds and it can even sustain 1000C for up to 5 seconds without triggering any kind of exceedance.

Blackthorn 5y ago

We can and do, if the engines are upgraded with goodies like electronic ignition (yeah, all those old aircraft engines are using fixed timing magnetos). But a lot of aircraft engines just aren't. And if you're in a certified plane instead of an experimental plane, good luck ever getting it. Basically any engine made by a European engine manufacturer takes "mogas" without incident, and prefers it because lead causes issues. But to the sibling comment's point, those engines also have goodies like single lever power (so no direct mixture or rpm control). That changes the parameters a lot. Edit: to add on, if you're in an experimental plane, you can do all sorts of things to your engine. You can buy aftermarket electronic fuel injection and ignition kits with variable timing, and reclaim a good deal of fuel economy in the process. Most engines that these kits support will then quite happily run along with a fuel like 94UL. Some people have even converted automobile fuel ignition controllers for the purpose (I hear the Ford V8 ECU is pretty popular for this).

slownews45 5y ago
Yeah, don't diamond note you CAN use leaded, but recommends AGAINST it?
ericd 5y ago
Thanks for the rundown! So with unmodified engines, the lead mainly just helps make extra sure there’s no knocking, which is very important because the engine won’t compensate (any issue midair would be very bad)?

jabl 5y ago

Aviation gasoline is much higher octane than the "premium" (or however it's branded) high octane gasoline at your local gas station. Typically premium gasoline is about 98 RON / 88 MON, whereas 100LL aviation gasoline is 100 MON. The big news here is that GAMI has come up with a formula that provides the required octane without lead, and also otherwise fulfills the various requirements for aviation gasoline.