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I am wondering what are advantages and disadvantages of these two designs, e.g. Rotax 912 vs Lycoming O-235, compared to each other. At first glance they are pretty similar (both are 4-cylinder, horizontally opposed, with carburetors, naturally aspirated, with similar horsepower output etc.) but there are also some differences (engine displacement, maximum RPM, preferred fuel, oil system and consumption etc.) Can someone explain pros and cons of both designs?

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The biggest difference between these two engine designs is that the rotax contains a reduction gearbox which allows it to turn faster and hence develop more power per cubic inch of displacement, and the lycoming is a direct drive engine limited to a rotating speed that maintains the prop tips subsonic. This gives the rotax better power-to-weight ratio but causes it to run hotter and shortens its time-between-overhauls. In addition, the gearbox itself requires overhauling which adds to the overhaul costs. In practical terms, air-cooled & geared engines for GA aircraft have not proven as popular as direct-drive engines in similar applications; in fact it is common to replace geared engines with direct drive engines with larger displacement to improve reliability and reduce rebuild costs.

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  • $\begingroup$ Another difference stemming from the higher heat produced by the Rotax is that it has to be liquid cooled (partially) like an automotive engine. $\endgroup$
    – Dean F.
    Dec 16, 2020 at 4:48
  • $\begingroup$ Hmmm. I've seen Lycoming/Continental to Rotax conversion kits but not the other way around, what have I missed? $\endgroup$
    – Jpe61
    Dec 16, 2020 at 12:20
  • $\begingroup$ Thank you frr your answer! Do you know what are pros and cons of using small cylinders and high max rpm (Rotax) vs big cylinders and low max rpm (Lycoming)? $\endgroup$
    – Konrad
    Dec 21, 2020 at 9:22
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    $\begingroup$ small cylinders and high RPM mean the engine weighs less but wears out faster and is subject to catastrophic failures due to overheating/cooling system issues. big cylinders and low RPM mean the engine is heavier but lasts longer, and less subject to overheating/burnt valves. $\endgroup$
    – niels nielsen
    Dec 21, 2020 at 18:18
  • $\begingroup$ And no change in power output / fuel consumption / efficiency? $\endgroup$
    – Konrad
    Dec 27, 2020 at 13:27

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