The split-single is a variant on the two-stroke engine with two cylinders sharing a single combustion chamber
Combustion chamber
A combustion chamber is the part of an engine in which fuel is burned.-Internal combustion engine:The hot gases produced by the combustion occupy a far greater volume than the original fuel, thus creating an increase in pressure within the limited volume of the chamber...


There have been "single" (i.e. twin-bore) and "twin" (i.e. four-bore) models and several important internal developments, the last of them being obvious externally too, with the carburettor (uniquely amongst motorcycles) moving to the front of the engine under the exhaust.

Principle of Operation

The split-single system sends the intake
An intake , or especially for aircraft inlet, is an air intake for an engine. Because the modern internal combustion engine is in essence a powerful air pump, like the exhaust system on an engine, the intake must be carefully engineered and tuned to provide the greatest efficiency and power...

 fuel-air mixture up one bore to the combustion chamber
Combustion chamber
A combustion chamber is the part of an engine in which fuel is burned.-Internal combustion engine:The hot gases produced by the combustion occupy a far greater volume than the original fuel, thus creating an increase in pressure within the limited volume of the chamber...

, sweeping the exhaust gases down the other bore and out of the exposed exhaust port. The split-single two-stroke thus delivers better economy than the common forms of two-stroke and runs better at small throttle openings, at the cost of a heavier engine.

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 cylinder
Cylinder (engine)
A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...

s 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 carburettor being placed in front of the barrel, tucked under and to the side of the exhaust. This is the arrangement seen in the United States and marketed by Sears as the Twingle.

Garelli invented 1912

The first split-single engine was patented in 1912 by Italian engineer Alberto Garelli. His company, Garelli Motorcycles
Garelli Motorcycles
Garelli Motorcycles is an Italian moped, and motorcycle manufacturer. It was founded in 1919 by Alberto Garelli.At age 22, Adalberto Garelli received a degree in engineering and dedicated his work to developing and perfecting the 2-stroke engine for Fiat. Garelli quit in 1911 due to Fiat's lack of...

, 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 remains an important manufacturer to the present day, but they did not further develop or produce these engines.

Trojan invented 1913

The Trojan two-stroke, as used from 1913 in the Trojan car
Trojan (automobile)
Trojan was a British automobile manufacturer. Cars with the Trojan marque were made from 1914 and 1974.-Early history:The company was founded by Leslie Hayward Hounsfield who went into business as a general engineer in a small workshop called the Polygon Engineering Works in Clapham, South London...

 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
Science museum
A science museum or a science centre is a museum devoted primarily to science. Older science museums tended to concentrate on static displays of objects related to natural history, paleontology, geology, industry and industrial machinery, etc. Modern trends in museology have broadened the range of...

 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.

Puch 1923-1970

After the First World War ended, Austrian engineering industry struggled to recover. Italian engineer Giovanni Marcellino arrived at the main factory of Puch
Puch is a manufacturing company located in Graz, Austria. The company was founded in 1889 by the industrialist Johann Puch and produced automobiles, bicycles, mopeds, and motorcycles.-Pre 1919:...

 in Graz
The more recent population figures do not give the whole picture as only people with principal residence status are counted and people with secondary residence status are not. Most of the people with secondary residence status in Graz are students...

 to wind up operations. Instead of liquidating the factory, however, 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 the Second World War, Puch split-single production and racing were re-started 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 early two-stroke 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 previously notorious problem of two-stroke plug fouling. A total of 38,584 Puch 250 SGS
Puch 250 SGS
The Puch 250 SGS was a motorcycle manufactured by the Austrian Steyr Daimler Puch AG in Thondorf near Graz.The motorcycle was powered by a split-single two-stroke engine...

 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.

DKW 1931-1939

In 1931 split-single engines produced by Ing Zoller were used in motorcycle racing and helped DKW
DKW is a historic German car and motorcycle marque. The name derives from Dampf-Kraft-Wagen .In 1916, the Danish engineer Jørgen Skafte Rasmussen founded a factory in Zschopau, Saxony, Germany, to produce steam fittings. In the same year, he attempted to produce a steam-driven car, called the DKW...

 dominate smaller motorcycle racing classes between the wars.

TWN 1946-1957

The German TWN
Triumph (TWN)
Triumph-Werke Nürnberg AG or TWN, was German bicycle and motorcycle company. In 1886, Siegfried Bettmann founded the Triumph bicycle factory in Coventry, England, and in 1896 he founded a second bicycle factory in his native Nuremberg, Germany, under the same Triumph name...

 motorcycle company (originally part of Triumph Motorcycles
Triumph Motorcycles
Triumph Engineering Co Ltd was a British motorcycle manufacturing company, based originally in Coventry and then in Solihull at Meriden. A new company, Triumph Motorcycles Ltd based in Hinckley gained the name rights after the end of the company in the 1980s and is now one of the world's major...

 in the UK) experimented with split-singles in 1939 and started producing two models when production resumed in 1946. They used mostly Garelli's original 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.

Regulation implications

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
Tax horsepower
The tax horsepower or taxable horsepower was an early system by which taxation rates for automobiles were reckoned in some European countries, such as Britain, Belgium, Germany, France, and Italy; some US states like Illinois charged license plate purchase and renewal fees for passenger...

, as used in the UK and some European countries in the 1920s and 1930s).


Lubrication is the process, or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each another, by interposing a substance called lubricant between the surfaces to carry or to help carry the load between the opposing surfaces. The interposed...

 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 is the introduction of contaminants into a natural environment that causes instability, disorder, harm or discomfort to the ecosystem i.e. physical systems or living organisms. Pollution can take the form of chemical substances or energy, such as noise, heat or light...

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.

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