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To somewhat expand on the other answers: The only reason there are cars that "require 93 octane but can run 87 with lower power" is that they have knock sensors
by lutorm 5y ago
To somewhat expand on the other answers: The only reason there are cars that "require 93 octane but can run 87 with lower power" is that they have knock sensors and ECUs that can limit ignition timing to avoid knock. This is a sophisticated system and nothing like it is available in any aviation engine, being designs from the 50s (except maybe some new experimental engines.)
The effect of running too low octane fuel in an aviation engine is not "slighly less power", but likely detonation and catastrophic engine failure.
And that's why it's a violation of FAA regs to run aircraft on non-approved fuels.
- tuxinator 5y agoJust because something is old doesn't mean it should get a free pass on continuing to use leaded fuel though... there must be other reasons why leaded is still used besides simple knock sensors and ignition timing.
- Baeocystin 5y agoExhaust valve seats. If you don't use appropriately hardened seats, you need lead to help cushion the wear between the exhaust valves and their seats in the head. If your perfectly reasonable next question is, why aren't standard aircraft engines just built with the appropriate seats, well, then... The answer is low market volume coupled with high regulatory hurdles. There aren't any villains here, just written-in-blood safety regs coupled with a small enough market that there is a lot less money available to fight inertia.
- mateo1 5y agoExactly. Flying for sport is expensive as it is today because safety has to be a priority (as it should). Add a requirement for new engines or major overhauls on old planes and the cost will spiral out of control. Besides, leaded fuel was a problem when literally every single car on the street was burning it. The total volume of fuel consumed by aviation piston engines is a joke compared to that.
- ericpauley 5y agoIn the case of Lycomings at least these valve guides have been hardened on cylinders produced since 1998, without having to certify a new type. On the Lycoming engines approved for 94UL I'm not aware of a restriction that these new guides must be installed for unleaded use.
- User23 5y ago> just written-in-blood safety regs To be clear, this is literally true. Those safety lessons were paid for in blood.
- userbinator 5y agoAlso, aviation engines are in general pushed far closer to their operational limits than a regular car engine. The latter might see full throttle and need to generate its rated power for a few percent of its life. Failures are not too dangerous. The former needs to generate rated power for a far larger fraction, and is also held to much higher reliability standards. Hence the requirement for high octane fuel and massive displacements relative to power output.
- lutorm 5y agoThe difference in operational regime is true, although it's my impression that automobile engines are pushed through some very severe tests as part of their development. For example, this movie, https://youtu.be/GNPB3RtHN2M https://youtu.be/GNPB3RtHN2M, claims that KIA tests their engines for 300 straight hours at full throttle. As for reliability, I'm not convinced. In practical terms, I think automobile engines are far more reliable than aircraft engines. When was the last time your automobile engine broke an exhaust valve, cracked a cylinder head, or had a magneto come apart?
- userbinator 5y agoWhen was the last time your automobile engine broke an exhaust valve, cracked a cylinder head, or had a magneto come apart? Exactly. An automobile engine is subjected to far less stresses in normal operation. 300h at full load is considered an extreme torture test for one, while a typical aircraft engine is meant to run for over 1000h between overhaul ( https://en.wikipedia.org/wiki/Time_between_overhauls https://en.wikipedia.org/wiki/Time_between_overhauls ) and spends a significant amount of that time at full load --- that's normal operating conditions.