Last Call: Hare Racing Adventure Edition

Just what the hee-heck do you think is going on here?
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Well, that there is what’s known as a “Twingle” and a.k.a “split-single”and it’s one of the most highly developed twingles ever made, the DKW ULd 250, a supercharged, two-stroke single cylinder engine. I aim to give you a little background on twingles, but in honor of today’s False Neutral
podcast let’s watch a little video showcasing the exhaust sound of a DKW Twingle

Broadly stated, a twingle is any arrangement of two pistons that runs like a single. This can include parallel (or even flat) twins with 720 degree timing (that is, they fire simultaneously), V-twins where the timing is 720 degrees plus or minus the V-angle, or two-stroke engines like the DKW where two pistons share a combustion chamber. Some would throw the parallel and flat twins out of the club because they have two crank pins. I will throw them out because I have never heard of that arrangement being built.
The earliest “split singles” were two strokes, and solved a problem with aspiration. One piston covers the intake ports, and the other covers the exhaust ports. This enabled the engine to have a longer power stroke, as the exhaust port could be even with the intake (transfer) port.
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Adalberto Garelli developed a split single in 1912 that stayed in production until 1926. (This period is what I like to think of as theCambrian Explosion of engine design, every arrangement was attempted.) His motor had transverse pistons that arrived at TDC simultaneously, riding on either side of the crankpin on a gudgeon. (Diagrams are hard to find, here’s a photo of a diagram)
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Later developments, notably by Puch (as Sears Allstate in the USA), Heinkel and DKW, placed the pistons in-line and on a y-shaped or lobed connecting rod that kept the exhaust piston a couple degrees behind the intake. DKW would even add a driven piston in the crankcase to supercharge the
plenum, known as a Ladepumpe motor.
For an in-depth story on the DKW supercharged split-singles, there’s no other place you need to go than http://www.odd-bike.com/2014/02/dkw-supercharged-two-strokes-force-fed.html
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Continuing on the idea of the semi-radial, common con-rod, single crankpin idea… tuners of the Harley Davidson XR750 are rumored to have made their flat tracker a twingle by firing the pistons one right after the other (Ba-Boom!) The resultant power pulse allowed the rear wheel to hook up during
the exhaust stroke the way a big single does. I say rumored because I can’t quite dig up the straight dope on these XR750s, just a lot of comments on message boards.