What's a Twingle?
Two of the most popular Sears models (175 and 250) used an engine design blessed (cursed?) with the nickname "twingle". Even Sears used this designation in some of their catalog ads. While most motorcycle engineers reserve the twingle moniker for a two-cylinder engine modified to fire both cylinders at the same time, it is arguable that the two-stroke split single engine might also fit under that umbrella.
The point of the split single design, as developed by Puch in the late 1920's, was to increase the efficiency of the two-stroke engine over then current designs using deflector head pistons. Remember, the Schnurle porting arrangement hadn't been invented yet, and rotary valves would have to wait another 30 years before becoming practical.
In the split single design, the intake charge enters the crankcase below the skirt of the front piston. The charge receives primary compression and goes up the three transfer ports in the rear cylinder. The mixture then must travel up the rear cylinder, sweep the bathtub shaped combustion chamber and try to chase the exhaust out the twin ports in the front cylinder.
Because of the crank/rod/piston assembly's design, asymmetrical port timing is achieved. The front piston is attached to the main rod, which has a large roller bearing at the crankpin end. The main rod also has a boss near the big end that has a bushing to attach the rear (slave) rod. This arrangement (when viewed from the left side of the engine) rotates counterclockwise and the exhaust ports close slightly before the transfers do, and also open slightly before the transfers open. This is not possible with a standard piston-port engine.
This all sounds fine so far, and it is for the most part, if you value characteristics such as low hydrocarbon emissions, steady idle quality and very good torque characteristics for a low speed small displacement engine. But if you desire peak horsepower, high rpm capability and fast engine acceleration, look elsewhere. This engine is built like a tank and runs like one too. Rpm's can't go too high because of several factors: a tortuous path for the air/fuel mixture to follow; a huge combustion chamber to burn the mixture in; long, heavy pistons running through a stroke twice the size of the bore; and long, skinny (but tough forged steel) connecting rods.
If you have built hot two-strokes like the RD350 Yamaha, you would laugh if you saw a split single piston. But I would like to see the rev-happy Yamaha engine pull around my 550 lb (total wet weight) Allstate 250 with Globe sidecar.
The split single design had it's time in the sun, but the sixties horsepower race blew right by it. Now it is up to us techno-troglodytes to appreciate a design that is a wonderfully functional anachronism. Call it a twingle if you must, but now you should understand why it really is a split single.