Most two-strokes in the last 50 years have used the Schnurle Loop principle, which, as you say, used the crankcase. The mixture isn't actually compressed there, but it uses the principle that while a piston is going upwards and compressing the mixture, it leaves a vacuum behind it in the crankcase. If you can seal off the crankcase from the atmosphere, then you can use the suction and compression in the crankcase (in opposite phase to the cylinder) to insert the mixture into the cylinder. There is a passage created between the top of the crankcase and the cylinder, that comes out in (usually two or more) ports in the sides of the cylinder. The carburettors are connected to the crankcase. As the piston goes up the vacuum behind it sucks in some mixture. As it comes down it squashes the mixture out of the crankcase and up through the transfer ports into the cylinder. The piston then comes up again, closing off the transfer ports, compresses the mixture, bang, and off we go again.
There are other ways of doing it, like on the BSA Bantam, but they cannot approach the efficiency of the Schnurle loop.
For real fun, put a 3 cylinder 750cc two stroke engine in a fairly rudimentary frame, connect basic brakes, and call it a Kawasaki H2 Mach IV. Oh, yes, I had one of those...
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Some good two stroke information.
The second one has an excellent animation at the foot of the page if you give it a few seconds to load.
encyclopedia.thefreedictionary.com/Two-stroke%20cy...e
www.vf750fd.com/blurbs/stroke.html
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BTW the second article referred to above quotes: "...once the expansion chamber goes into resonance, there will be a HUGE, almost instantaneous increase in power"
Damned right!
I have some video footage somewhere of me racing my Class One kart. For lap after lap, the guy behind me is breathing down my neck at the end of long straight, but every time we exit the chicane at the end of it, usually side by side, when we reach a certain part of the traffic I disappear up the road like an exocet. Eventually, he gets past on the straight, but having left the chicane, and from some distance behind, I overtake him like his engine just stopped. Kapow! Gone!
Both of our engines were identical (class regs), as was our gearing, as was our tyre size (class regs), as was our total weight (both right down at the class minimum). The only difference was that as a regular at the track in question (Shenington, near Edge Hill), I had my expansion pipe set to perfection for this manoeuvre.
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How did a spell checker get 'traffic' out of 'track'? Bizarre!
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SjB,
Thanks for the links, great explanation and animations.
StarGazer
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It is possible to design a two stroke with valve gear very similar to a four stroke type engine. The cam(s) would rotate at crank speed. The piston in a two stroke can perform this valving function, so it would be design overkill to employ four stroke type valving on a two stroke. Perhaps the inlet would be supercharged to perform the below piston pumping action of a more typical two stroke.
Logically, the piston cannot act as valve in a four stroke engine, as valve operation must take place at half of crank speed.
IMO, it is a shame that two strokes haven't become popular in motor cars; they can be much simpler, with a much reduced parts count, and potentially increased reliability. Also, there are some interesting two stroke layouts, for example, the Hooper stepped piston engine, which is, I think, still under development, and the much older, brilliant, Deltic engine.
number_cruncher
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Thanks all, clearly explained and informative. I was never a big fan of two strokes, but I owned a couple and the one thing I did appreciate was the simplicity and lightness of the engines. removal and strip down was a doddle, perticularly on a single where there was only one of everything.
Isn't there an optimum engine size for strokers, beyond which they loose their performance advantage over four strokes?
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Number Cruncher, do you know what became of the Orbital engine - that did have potential?
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\"Nothing less than 8 cylinders will do\"
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Indeed, I remeber it being featured on the cover of an issue of Car Design and Technology in the early nineties, with the Australian Orbital company working in collaboration with Ford.
I haven't heard anything much since, but that means little! - I don't have wide or strong industry connections, I know a few relatively isolated and specialised researchers.
number_cruncher
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Ralph Sarich invented the orbital engine in the early '70s.
These links are quite informative;
www.taletyano.com/MftM/Illustrations.htm
www.geocities.com/kiwiengineer2002/orbital.html
For more choice type " ralph sarich orbital" into Google
To quote an online encyclopaedia
"The advantage is that there is no high-speed contact area with the engine walls, unlike in the Wankel where edge wear is an ongoing engineering problem"
IMO an invention that deserved more success.
Hawkeye
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Stranger in a strange land
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Here is a link to a page that has some explanation...
gofree.indigo.ie/~vire7/2stroke.htm
Two stroke cars. Hmm, Daihatsu, Suzuki, Saab, Trabant. Any others?
Pete M
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Wartburg Knight.
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In my sub-division there must be at least 500 motorised tricycles, all of 'em blue-smokes. Hate the damn things.
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Unless I'm very much mistaken, the Telegraph ran a story about ten or eleven years ago about how direct injection and valves would tranform the two-stroke into a clean, economical and powerful engine design.
At the time, a Jaguar spokesman said that they were of the opinion that their next engine design would be the last four stroke they ever made.
I don't know what happened, but the suggested design certainly makes sense, as you'd be able to have a wet sump, so fewer emissions.
Imagine a V12 Jag with expansion chamber at the lights, going "RING a ding ding ding dingg.."
V
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Don't forget the two stroke that was meant to power the Ford KA.
It didn't work, so they had to fit a Mark2 Escort engine instead!
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Not forgetting 2-stroke 'uniflow' diesels, which were quite popular in large vehicles at one time. Foden, Detriot Diesel, Commer TS3 and the Napier Deltic engines fitted to the locomotives of the same name. The newish class 59 main-line freight locos used by EWS and quarry companies are 2-stroke.
On 2-stroke diesels a mechanical supercharger supplies an excess of compressed air to inlet ports in the cylinder wall which are uncovered by the piston towards the bottom of its stroke. The compressed air then blows the spent combustion gases out through conventional exhaust valves in the cylinder head. As the piston rises the inlet ports are covered and the exhaust valves close, trapping the clean air in the cylinder to be compressed ready for the next spray of fuel to be injected.
For a given power output, uniflow 2-strokes were smaller and lighter than conventional 4 stroke diesels, but tended to be thirsty and dirty through incomplete combustion of fuel.
Cheers, Sofa Spud
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"Unless I'm very much mistaken, the Telegraph ran a story about ten or eleven years ago about how direct injection and valves would tranform the two-stroke into a clean, economical and powerful engine design."
I think Bimota must have read that same article right before they created the Vdue. Of course Bimota promptly folded after creating it. 8-)
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Don't forget the DKW/Auto Union, which actually sired the Saab and the Wartburg.
Had a Knight myself decades ago - one of those motors of which you can have fond memories, even though it was a lot of grief.
While on the subject, what about the Bond Minicar, with its slow but faithful Villiers 197cc? Or the little Berkeley sports car with the 328cc Excelsior twin - looked a bit like a baby E-type and went like sawdust off a shovel...
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One of the most impressive bits of two stroke engineering I've seen is sitting in my 1/10th scale R/C car.
2.49cc (.15ci) air cooled 2 stroke single.
Fuel: Nitromethane/methanol/castor mix
15mm bore x 13.6mm stroke
1.2 hp @ 29,000 RPM
Max continuous revs: 32,000 RPM
Nominal time between rebuilds: About 20 tanks of fuel. :(
Waheyyyy!!!
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Got loads of little enines here but how about 0.020 Cubic inch or 0.3CC turns 5 x 3 inch prop at around 12,000RPM now 40odd years old! Also 0.75 Cu Inch which is 12.5CC and turns a 14 x 6 inch prop at around 10,000RPM.
Jim
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