So I've been building an ultralight

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danny_galaga:

--- Quote from: RandyT on November 22, 2024, 12:57:19 pm ---Yeah, I don't doubt that there are aircraft specific mods made to them. But when it comes to engine retrofits, as long as it has an output shaft of some nature, it could be made to work on just about anything, one way or another. The air-cooling presents the biggest challenge, which probably makes them more suitable to winter motorsports, where the cold air and a simple cooling fan at idle would probably be enough to keep it from self-immolating.

--- End quote ---

I mean the general format is not conducive..things like snowmobiles and jet skis tend to have inline engines because they are quite narrow..flat fours are quite wide and I'm.not sure you would fit this inside the bodywork. If going for crazy modified snowmobiles I would think a better fit would be an online four motorcycle engine like for instance the Suzuki GS 1000. Cheaper and more powerful than the Rotax 912 and potentially much easier to fit. It's a win/win/win 🙂

RandyT:

--- Quote from: danny_galaga on November 24, 2024, 06:53:47 pm ---I mean the general format is not conducive..things like snowmobiles and jet skis tend to have inline engines because they are quite narrow..flat fours are quite wide and I'm.not sure you would fit this inside the bodywork. If going for crazy modified snowmobiles I would think a better fit would be an online four motorcycle engine like for instance the Suzuki GS 1000. Cheaper and more powerful than the Rotax 912 and potentially much easier to fit. It's a win/win/win 🙂

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My thoughts were along the lines of an ultra-modified machine. After seeing it, it's clear that a drop in replacement would probably not be a thing. Hell, I have seen some YT videos of people running farm vehicles and full-sized cars with the Predator twin cylinder from Harbor Freight. I put one of these in my Terramite as well. There were several challenges to overcome, like mounting layout, the output shaft location and direction, exhaust, etc. but the end result was a more powerful and cheaper engine that has outperformed the original. Where there is a will, there is a way.

My only point was that a high-reliability, performant engine past it's prime for air use could still have a long and useful life for other applications where these qualities are sought and where failure won't equate to plummeting a few thousand feet to the ground :) . For what these cost, it would make sense to re-purpose them in order to reclaim some value. That is, of course, if complete rebuilds costing considerably less than new weren't an option.

danny_galaga:
Agreed..like I say, I think there's great potential for air-cooled VW cars. Once the engineering is sorted out, it could well be a drop in replacement. Adapting it looks simple, but would need a well thought out system. The 912 is more like a car engine than an aircraft engine, in that it operates in similar rev range. But it is still built from the ground up for aircraft. My thought is to drive the clutch where the prop is driven, and have a pair of 1:1 ratio gears to replace the existing approximately 1:2 ratio gears . The carbies can probably stay. Even though in general aircraft have different carbie requirements, the 912 uses Bing carbies, same as BMW motorbikes used to use.
Regarding cooling, the 912 has air cooled cylinders, oil cooling and liquid cooled heads. The cooling tends to work so well that in many European countries, they are flying with tape covering some of the radiator. In say a Beetle I reckon you could just put a radiator in the engine bay, without even a direct air flow and it would still be fine.

RandyT:
Cooling: Ok, that opens a lot more possibilities. When I hear "Air-cooled" I think of the older engines which had no radiators, oil-coolers, etc. whatsoever. That, of course, has changed over time, with most bikes, 4-wheelers and SxSs having at minimum oil coolers, with the latter even having actual radiators like a car. One of my 250cc scooters actually has two radiators in-line just to keep the heat down. I remember when I first got the machine, it had an overheating problem that I couldn't identify because I didn't know about the second radiator buried way down under the seat. Turns out that one had a dead electric fan out of the gate. Replaced it and no more overheating condition. :banghead:

Carbs: They sound like nice carbs. The only thing which might be fun is getting them adjusted properly for thicker air at ground level. Which leads me to another question: Do those engines tend to run lean on the ground, or is the air-fuel mix something which is controlled by some device or by the operator as necessary? I never really considered this before, but I would think something would need to happen to account for the difference.

danny_galaga:
The Bings, either by design or happy accident, lean out at altitude. 'Real' aircraft engines have a manual mixture control. Generally, below 5000 feet you just leave it on full rich. Above that you start leaning it, from memory in reference to an exhaust gas temperature gauge.
The other thing with aircraft is carby icing. At just the right altitude and humidity, ice will start to build up in the carby throat. You can imagine that wouldn't be good. On 'real' aircraft, if the engine starts to run rough and you suspect carby icing, you can pull a carby heat lever. That just bypasses the air filter and pulls air past the exhaust manifold. De ices pretty quickly, but you lose about 5% power. On my aircraft, and many others with a Rotax, there is a little water jacket around the throats. This means it's getting hot water from the cooling system and the throat is always warmer than otherwise. Because it's not feeding hot air into the engine, thus losing power, it's on permanently. The only drawback I see is that if your engine starts to run rough, you don't have that simple check of turning on the carby heat. Is it icing? Or something else? All you can do is drop altitude and see if it clears up.

Anyway, NONE of that matters for our hypothetical terrestrial uses, just thought you might find it interesting ☺️

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