The split-single (Doppelkolbenmotor to its German and Austrian manufacturers), is a variant on the two-stroke engine with two cylinders sharing a single combustion chamber.
The split-single system sends the intake fuel-air mixture up one bore to the combustion chamber, sweeping the exhaust gases down the other bore and out of the exposed exhaust port.
The rationale of the split-single two-stroke is that, compared to a standard two-stroke single, it can give better exhaust scavenging while minimising the loss of unburnt fresh fuel/air charge through the exhaust port. As a consequence, a split-single engine can deliver better economy, and may run better at small throttle openings.
A disadvantage of the split-single is that, for only a marginal improvement over a standard two-stroke single, the "Twingle" has a heavier and costlier engine. Since a manufacturer could produce a standard twin-cylinder two-stroke at an equivalent cost to a Twingle, it was perhaps inevitable that the latter should become extinct.
There have been "single" (i.e. twin-bore) and "twin" (i.e. four-bore) models. Unusually for a motorcycle engine, some Twingles have the carburettor mounted on the front of the engine, beneath the exhaust.
In the 60-year history of this arrangement there were two important variants, earlier versions have a single, Y-shaped or V-shaped connecting rod and these look much like a regular single-cylinder two-stroke engine with a single exhaust, a single carburettor in the usual place behind the cylinders and a single sparkplug. Racing versions of this design can be mistaken for a regular twin-cylinder, since they had two exhausts or two carburettors but these are actually connected to a single bore in an engine with a single combustion chamber. Some models, including those in mass-production, used two spark-plugs igniting one combustion chamber.
After World War II, more sophisticated internal mechanisms improved mechanical reliability and led to the carburetor being placed in front of the barrel, tucked under and to the side of the exhaust. This is the arrangement which was used on the Puch 250 SGS and sold in the United States by Sears from the 1950s through the early 1970s under their own Allstate brand, with the engine being referred to in Sears literature as the Twingle.
The first split-single engine was the Lucas, built in the UK in 1905. It used 2 separate crankshafts connected by gears to drive 2 separate pistons, so that the engine had perfect primary balance.
In 1912 Italian engineer Adalberto Garelli patented a split single engine which used a single connecting rod and long wrist pin which passed through both pistons. His company, Garelli Motorcycles, produced a 346 cc version for use in motorcycles for road use and for racing. Production continued until 1926, by which time Garelli was increasingly concentrating on the military market. Garelli motorcycles remained in business until the late 1990s, but they did not further develop or produce these engines.
The Trojan two-stroke, as used from 1913 in the Trojan car in the UK, was independently invented but would now be described as a split-single. Photos of a 1927 "twin" model at the London Science Museum show the internals. The "fore-and-aft" layout of the cylinders means that the V-shaped connecting rod has to flex slightly with each revolution. Unlike the German/Austrian motorcycle engines, this engine was water-cooled.
Trojan also made another split-single engine later with the cylinders arranged in a 'V' formation. The unusual 'V6' design had two split-single sets of cylinders (4 cylinders total) on one bank of the V and two scavenge blower cylinders on the other bank of the V.
After World War I ended, Austrian industry struggled to recover. Italian engineer Giovanni Marcellino arrived at the main factory of Puch in Graz to wind up operations. Instead of liquidating the factory, he settled in the town and, in 1923, designed and began production of a new split-single with asymmetric port timing, taking inspiration from industrial opposed-piston engines. As arranged on a typical motorcycle, Marcellino's design had the pistons one behind the other, unlike Garelli's pistons, which were side-by-side. The new system allowed better cylinder filling and a longer power stroke. To avoid flexing of the connecting rod, the small-end bearing of the cooler intake piston was arranged to slide slightly fore-and-aft in the piston. In 1931 Puch won the German Grand Prix with a supercharged split-single, though in subsequent years they were overshadowed by the split-singles of DKW.
After World War II, Puch split-single production and racing were restarted in 1949 with an improved system of one connecting rod hinged on the back of the other. These engines typically use the forward piston to control both intake and exhaust ports, with the interesting result that the carburettor is at the front of the engine, under and to the side of the exhaust. The rear piston controls the transfer port from the crankshaft to the cylinder. Increasingly, these models were fitted with an oil mixing pump, fed from a reservoir incorporated in the petrol tank. Some also have a twin-spark plug ignition system firing an almost figure-eight shaped combustion chamber. Sears marketed considerable numbers of the Puch SGS split-single fitted with both these innovations as the "Allstate 250" or "Twingle" in the US. The improvements tamed, if not virtually eliminated, the previous problem of two-stroke plug fouling. A total of 38,584 Puch 250 SGS motorcycles were produced between 1953 and 1970. Puch gave up racing in the 1950s, and split-single production ended around 1970, but the machines themselves remain well-regarded and collectable.
In 1931 split-single engines produced by Ing Zoller were used in motorcycle racing and helped DKW dominate smaller motorcycle racing classes between the wars.
In 1932 the Mallory Special car driven at Indianapolis by Duray used a Duray 16-cylinder two-stroke using a split-single configuration.
In 1935, the Monaco-Trossi Grand Prix car was built with a 16-cylinder radial engine using a split-single configuration.
The Ehrlich Motor Company (EMC) was a split single 350 cc motorcycle engine built in the UK from 1947 to 1952. After 1948 the engine also was fitted with an oil pump controlled by the throttle, which dispensed two-stroke oil into the fuel at a variable rate depending on throttle opening (and presumably engine speed/load), instead of having to pre-mix oil in the fuel.
Iso Autoveicoli designed their Isetta bubble car 1953–56 around a 236 cc (14.4 cu in), 7.1 kW (9.5 hp) split-single two-stroke motorcycle engine.
The German TWN motorcycle company (originally part of Triumph Motorcycles in the UK) experimented with split-singles in 1939 and started producing two models when production resumed in 1946. They used a Y-shaped connecting rod, so the pistons are "side-by-side", making the engine little different visually from a regular two-stroke, with the carburettor in the usual place behind the inlet cylinder bore.
The BDG125 125 cc was made from 1946 to 1957, the BDG250 250 cc from 1946 to 1957, the Cornet 200 cc from 1954 to 1957 (12v electrics and no kickstart), the Boss 350 cc from 1953 to 1957 and the Contessa scooter 200 cc from 1954 to 1957. The bulbous shape of the exhaust of the Cornet and Boss is a two-stroke TWN feature, not linked to the split-single-engine. All TWN motorcycle production ceased in 1957.
For modern vehicle taxation purposes the split-single suffers no penalty and offers no advantage, as only the swept volume is considered, not the number of cylinders or spark plugs. This remains true even if the two pistons are not the same size and have different strokes (mechanically possible, if rarely used). This simple calculation was not always the case (see Tax horsepower, as used in the UK and some European countries in the 1920s and 1930s).
Lubrication weaknesses of the early "side-by-side" versions with the carburettor in the "normal" place behind the cylinder, were substantially the same as with all other two-strokes running on the same "petro-oil" mixture. However, they were greatly eased in the later ones, since the cool, lubricated mixture is delivered straight onto the hot (exhaust side) of the hotter, exhaust piston from the carburettor at the front of the engine under the exhaust.
Pollution from the original 1920s and 1930s total-loss lubrication versions was similar to other forms of two-stroke; however, the post-war split-singles from Puch (marketed in the US by Sears, Roebuck and Co.) were amongst the first to be fitted with pumped oil injection, making them substantially cleaner.