AFAIK no turbo engine, or any engine with a CR above about 8.5:1 has been approved. That rules out some 2/3 of the US fleet, IIRC. In Europe, probably a lot less but still a lot of planes.
@Peter my source on the O-540 was the Lycoming SB I linked above.
Here is a list of compression ratios for Lycoming, with most at 8.5:1, including the O-540-A.
https://www.lycoming.com/sites/default/files/SSP-110-2%20Certified%20Engines.pdf
SSP_110_2_Certified_Engines_pdf
Peter wrote:
AFAIK no turbo engine, or any engine with a CR above about 8.5:1 has been approved.
Why do they need it with such low compression ratios (compared to car engines)?
Is the fixed ignition timing of aircraft engines ludicrously advanced (again, compared to car engines)?
My comparison here is fiddling around with a programmable distributor for my Spitfire. I can run a more aggressive advance curve without detonation (and thus get better throttle response) on 95UL than I can with 91UL. I have not measured the compression (it is modified) but I would guess around 10:1.
Graham wrote:
Why do they need it with such low compression ratios (compared to car engines)?
Large cylinders in a spark ignition engine, that are made large to reduce weight for a given engine displacement. With large cylinders it takes longer to burn the charge, and the longer it takes, the more time there is for unburied fuel to break down chemically and ignite spontaneously.
Low rpm engines need to be big because they need to make a lot of torque (hp = torque x rpm) and engine dimensions are basically proportional to torque.
Car engines are small because they rev them high
This recent YT from Skywagon University has a good explanation why the SkyLark Cessna which ran at 3300 RPM ran into trouble. Apparently pilots were flying it at reduced power settings, which the engine/gearbox/propeller did not like.
@RobertL18C – thanks for that data. Bit disappointed to see that my O540-L3C5D isn’t approved for anything except 100LL, even though it is apparently the same as the J3C5D which IS approved, for everything that matters (in particular, the compression). I wonder why that is? Maybe because it’s designed for use with a turbo? But in my TR182 installation it’s just turbo-normalized so that ought not to matter.
I was puzzling about these Lycoming Service Instructions SI1070AB asking myself, why from a certain engine type several variants are free to use anything “down” to pump station gas, but other variants are not. E.g. the IO-360 or the IO-540 show numerous variants where this is the case. It seem to be same compression ratio, horsepower and (in these examples) fuel-injected engines, so I can’t really identify a technical reason for why some of these are free to burn anything, and some are not.
Does this only address the (possible) fact, that only the depicted engines have been tested? Or is there more into it?
Does anybody have more information about whether the other engines may be expected to be on the “release list” sooner or later, or are there serious concerns that some of these engines will never fly on anything less than 100 octane?
@UdoR could you point to some specific examples? The fuel octane requirement variations I’m aware of in Lycoming O-360s and O-540s (same size cylinders for all, injected or not) are all related to either compression ratio, turbocharging and/or angle valve versus parallel valve cylinder heads. And smaller cylinder engines need less octane for the same CR. It would be interesting to look at some examples that were very non-obvious if you were to point them out.