Rotax engines - more efficient than Lyco or Conti?

They are just a lot more efficient than their 1950s counterparts.

No they aren’t, they are just less powerful and therefore use less fuel. High RPM and reduction gear would tend to make the Rotax less efficient than a direct drive engine, but this disadvantage is offset by by having the 9.1:1 or 10.8:1 (in 100 HP form) compression ratio that smaller cylinders allow.

I work professionally with the engineering application of these engines and others, with a focus on efficiency and range, to include new engine and ancillary development. One of the people I work with developed the EFI system for the Rotax’s most prominent high altitude, long endurance application that started in the 1980s, so you can imagine the issue of Rotax efficiency may have come up from time to time.

I’ve found sticking to facts works well for me.

Well, 1.6l cylinder volumes in gasoline engines are just not a brilliant idea . Consequently when you want a bit of power from these you want minimum 100 octane at least. Reduction gears can be quite allright , see Yak and Sukhoi M 14 radials, never a problem and no noisy propellers – when having planetary gears and not just spur gears. With these M 14 P and F radials they get 360, 400, and 450 hp from 10 l capacity , no turbo, no avgas needed, just super 98 max. Big cylinders are great in Diesel engines, but in aircraft not the best idea – unless in airliners with two stroke Junkers type and multicylinders for smoother running. Any American V8s with similar fuel burn like modern European or Asian engines, same hp numbers ?? No way . . . . Vic

For sure the small displacement supercharged Russian 9-cylinder radials were one solution to getting up to 400 HP out of an engine running on Russian fuel. But I’d prefer not to own and maintain a 9 cylinder engine.

The auto industry is worldwide, I don’t think there are significant national differences anymore. That era was fun, but it’s gone. The US built car engine that was until this year somewhat different was the small block Chevy V8, which in its final versions focused quite effectively on power and torque density (compact size) versus specific output (power per liter). It was as efficient as anything else in the same power class. Also very inexpensive by comparison, so that anybody could drive a 500 HP car. Now Everyman Chevy driver doesn’t have pushrods but instead for a bit more money buys 670 horsepower with 460 lb-ft of torque.

vic wrote:

Yak and Sukhoi M 14 radials, never a problem and no noisy propellers – when having planetary gears and not just spur gears. With these M 14 P and F radials they get 360, 400, and 450 hp from 10 l capacity , no turbo, no avgas needed, just super 98 max.

You forgot to mention a TBO of 500 hours for M-14P and 100 (150?) hours for M-14PF, plus a very labour-intensive overhaul. Even if it makes 2×TBO, which is achievable with a good engine oil instead of Russian MS-20, it’s still a short-lived engine.

Not so, 750 hours from new, then 500 intervals hours for “checks” , whatever that means, nobody in the west knows exactly. And these hours came from communist times and state owned aircraft. Well, short lived might apply as well to American primitive turbo engines. Maybe you could tell me which components on M 14 appear shortlived ? But I have learned about a lot of trouble spots on American stuff here. Silvaire, question was mainly about efficiency of small engines higher revving vs. big blocks same power and gasoline fuel. My theory is cylinder volume with gasoline be better below 600 cc, less troubles and better efficiency. Vic

Said before: LOP , engine monitors and constant cruise operation means the main current limit for efficiency of the tractor Lycosaurus fuel-injected engines on our aircraft is compression ratio. A modern automotive engine is much more efficient than an old one…in a car environment (low-partial-power, constantly-changing, low-altitude operation). Not so in an aviation environment.