Discussion in 'General Auto Conversion Discussion' started by Winginitt, May 15, 2019.
There isn't any cheap brand new two-strokes anymore.
I don't see many technical issues that require filling by a 2-stroke.
For the B&S variants, a modified version of the 810cc is flying on SD-1s and Minisport claims this engine (they call it the SE33) provides 33 HP and weighs 71 lbs. It seems likely that the adaptations can be performed by a handy homebuilder and that all-up costs would be reasonable (almost surely less than $2000). Now, I have my doubts about 33 continuous HP from that mill, but it seems reasonable to believe it could make it for takeoff and make 28 HP all day.
Above that, we can have a 1/2 VW of 32-42 realistic HP for $3200-$4600 and weighing 85 lbs (add a few pounds and $600 for starter and alternator). In a box and ready for the prop to be bolted on.
I don't see a hole for the 2-stroke to fill. Are they available for lower cost, will they be as reliable, and is there a huge weight difference after we add in the PSRU and the added fuel?
It does seem logical that precisely-timed EFI could do a lot to reduce 2-stroke fuel consumption, I'm not sure about enhancing reliability. A 2-stroke engine like that would have attractive HP/weight. But, on the cost front, there would need to be a cheap "lump" to be developed.
Maybe the best thing to improve the reliability of a two stroke would be a variable pitch prop that kept the engine loaded at all times to stop it leaning out?
AJ, I have also thought of carbon replacements for some parts. Many could use a mould pulled from the stock part. Intakes could be made from composite or aluminium tube for a big saving over a stock cast one. The trick being to adequately brace the carb flange.
My question was not so much about cost, but that there seemed to be a big debate on the small 4 strokes involving FWF weight, power-to-weight, the weight of the redrive, doing without a starter, and the fact that a 75 pound 4-stroke putting out a reliable 28 consinuous HP was at the bottom end of yielding acceptable performanace for most "airplane" style LSA's.
Believe me I'm not a strong 2-stroke promoter by nature. But the power-to-weight, and power-to-size advantage cannot be discounted. I'm talking about BELOW the threshold of the VW/HKS/O-100, and an aircraft where you don't want a 75 pound engine behind your head. On small light minimalist aircraft in the ultralight range, where 30-40HP at the lightest weight possible would open up posibilities that heavier engines could not be workable, I'm guessing the little buzzsaw is probably the way to go.
But the stigma and "bad rap" of the 2 stroke is still front and center, and since I'm not an expert on them at all, I wanted to see if and how it is possible to use a 2-stroke aircraft engine where you can have the same faith in as a 4-stroke.
The Rotax two-strokes are about the same weight as a 40hp 1/2 VW. Maybe more, with block, gear box and extra heavy tuned muffler system. Around 100 pounds for the 446-503, I think.
The advantage is the gear box provides for a much larger prop at slow ultralight speeds.
The faster ultralights can use direct drive four stroke.
I understand some of the weights and power levels. I'm definitely not talking about Rotax. I am talking, for example about the Polini that everyone is in love with currently, or the Simonini equivalent. They apparently have much better metallurgy, better fuel consumption, and have addressed most of the seizure/catastrophic failure issues that gave Rotax a bad name many years ago.
The Polini makes 36HP at under 40 pounds, with a starter and PSRU included. If you had a small experimental, like the Debreyer Pelican, or a Cri-Cri-ish small minimal go-kart kind of thing, the difference between 38 or 39 pounds and 70 to 75 pounds (half-VW) is a much larger portion of the aircraft weight than an SD-1 or Volksplane. In the case of the Pelican or the Cri-Cri, 30 or 35 pounds difference hanging on the frornt or back end of the airframe is huge.
If we leave cost out of it, my impression is that a 2-stroke using modern EFI and good engineering (incl a well engineered redrive) could probably provide a lot of thrust/lb and good reliability. I don't think they can match a 4-stroke yet--the paramotors still crump out with some frequency. In cases were light weight is worth paying a high price, a 2-stroke seems to offer good potential for meeting the requirement. Something like the very sophisticated engine in the Husqvarna TE300i (separate fuel and oil injection, liquid cooling, etc) would probably make a nice airplane engine--just bring money.
IMO, the cost issue with the 2-strokes is due to the present lack of mass-market applications for 30-40 HP 2-strokes engines. Tens of thousands of B&S and even new VW cases/bits get cranked out annually to meet the needs of price-sensitive consumers who require high reliability. That yields a very different per-unit price than small-batch hand production of a few dozen airplane engines every year for folks willing to pay a premium (per HP).
The Polini Thor is 40 pounds without the radiator and liquid. (those weights not listed) https://www.polinithor.com/en/polini-thor-250-thor-250-ds-2/
Usually high rpm (8000) doesn't last many hours. They don't say what the TBO is.
I simply like the sound of low rpm.
Hi VB and all,
The Thor 250 dual spark has features that I think desirable in a flight worthy two stroke. Most important is liquid cooling; Coated bore, no liner, dual ignition, a counter balance shaft, gear reduction with a centrifugal clutch and electric start. With this type of application very small front to back dimension is required so rearrangement of intake, exhaust and cooling for fixed wing is desirable. It is flying on real airplanes and so far the feedback is good. Cruise consumption is just over 2 gal/hr. Can a four stroke match it at a reasonable price? https://www.ktm.com/en/news/int/ktm-unveils-worlds-first-2-stroke-fuel-injection-enduro-machines/ It seem to be doing fine with a carb. What can be done with a four stroke conversion is TBD. Given enough money, an equivalent fourstroke could be engineered but it would cost more than a 500hp crate V-8. The V-2s are flying airplanes and other flying machines so the weights/power/cost are useable. Go for it.
I have an injected 2-stroke outboard : 75 hp, so larger than what's being discussed here, but the technolgy would scale down:
It's smaller and lighter (and noisier) than the four-stroke alternatives.
It has a very fancy oil injection program, so surely you could program it to handle overrun/descent conditions.
It seems to run at about 100:1 fuel/oil ratio on average for me (programmed to use Evinrude's fiendishly expensive oil).
Of course, it's direct salt-water cooled, which saves a lot of weight and bulk, while keeping temps well under control : not an option for aircraft.
Maybe you could bolt some cooling fins to the head for direct cooling augmentation. My brother did that to our Cat 30 pony engine and stopped the overheating.
A two stroke with an up to date EFI system could give The Rotax 100 hp engines real competition. It is done on outboard, snowmobile and now dirt bike engines. Hirth could do it tomorrow. Why they haven't is beyond my understanding.
To compete with the Rotax 912, a Hirth two stroke would need ASTM LSA certification. Perhaps they don’t think that it is worth the trouble.
Hmm, the OP started this thread about auto conversions but we've ended up on 2- strokes. How about a compromise?
The OP didn't mention auto engines in post 1.
That 671 Detroit diesel vibrated the iPad mini in my hands.
You're right, he didn't say auto engine. But he did say "conversion engine," and made the post in the "Auto Conversions" area. So I'm posting an auto conversion of sorts.
We built something similar (for dyno testing) about 20 years ago.
The rear half unbolted to make it a power pack - but still too heavy for aircraft use
You're right, but that's maybe because we still don't have an industrial engine conversion area. I thought he said it was an industrial engine.
OK, here is a nice industrial engine I would like to build.
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