Firstly, I'd like to say that I am in no way an engineer or mechanic, just a PPL who loves Rotax a lot.
I think you'll find if you run a Rotax at the same power as the Continental it will burn about the same amount of fuel.
Silvaire,
While you are undoubtedly more knowledgeable than me, I'm sorry but I still don't get this. What do you define as power? If HP, 100hp is 100hp. For an equal amount of HP, the fuel consumption is surely less on a Rotax?
There is also no doubt the old engines are more reliable than the Rotax. They have fewer bits and no gearbox. A geared engine running at a higher RPM will only ever be able to approach the MTBF of a simpler engine running at low revs.
IO540,
I respectfully disagree. The ignition system I described above has less bits than traditional mags. The oil system has no pumps, it operates off crankcase pressure. I think all in all, parts are about equal.
Regarding the gearbox, I have never personally heard one, but when a gearbox is close to failing, it makes a hell of a lot of racket (specifically a distinct clattering sound) and usually refuses to restart again on the ground. I think you will have a hard time finding in-flight gearbox failures. Also the very few gearbox failures on the 900 series engines were operator induced. The engine really hates it when prop rpm drops below 1,000. You can feel it running rather rough, but just as soon as you bring tha power over 1,000, it is as smooth as silk.
The fuel flow for a given IAS is mostly a function of the cockpit dimensions. The cross section is the biggest thing. A tandem 2-seater will burn probably about 1/3 less fuel than a side by side 2 seater. Most of the Rotax powered planes have really "intimate" cockpits compared to say a TB20 so no wonder they burn half the juice.
Great point
Years ago I saw some analysis on engine efficiency and the old engines beat every modern car petrol engine on SFC.
Quite interesting. Do you have a link?