- 1 Principle of operation
- 2 Characteristics
- 3 History
- 4 See also
- 5 References
Principle of operation
The split-single uses a two-stroke cycle (i.e. where every downward stroke produces power) with the following phases:
- Pistons travel upwards, compressing the fuel-air mixture in both cylinders. A spark plug ignites the mixture (in the right side cylinder in the animation) when the pistons is near the top of the cylinders.
- Pressure from the ignited air-fuel mixture pushes both pistons downwards. Near the bottom of the travel, an exhaust port becomes exposed (in the left side cylinder in the animation), causing the exhaust gases to exit both cylinders. At the same time, the intake port is exposed on the other cylinder, causing a fresh air-fuel mixture (which has been pressurised in the crankcase by the downward movement of the pistons) to be drawn in for the next cycle.
The advantage of the split-single engine compared to a conventional two-stroke engine is that the split-single 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 single-cylinder engine, a split-single engine is larger, heavier and more expensive. Since a manufacturer could produce a conventional two-cylinder engine at similar cost to a split-single engine, a two-cylinder engine is usually a more space- and cost-effective design.
Most engines used a single combustion chamber (i.e. two cylinders), however some engines used two combustion chambers (i.e. four cylinders) or more. Unusually for a motorcycle engine, some split-single engines have the carburettor mounted on the front of the engine, beneath the exhaust.
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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.
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.
Lucas (invented 1905)
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.
Garelli (invented 1912)
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.
Trojan (invented 1913)
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. By 1935, a four-cylinder version of the Puch split-cylinder design used produced 10 kW (14 hp) and was used in motorcycles.
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.
Duray Indianapolis 500 1932
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 1953–56
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.
|Wikimedia Commons has media related to Split-single engines.|
- "Triump TWN". www.classicmotorcycles.org.uk. Archived from the original on 17 January 2012.
- "Split-single torque and economy rival that of a four-stroke". www.sammymiller.co.uk.
- Walker, Mick (11 October 2018). Mick Walker's European Racing Motorcycles. Redline Books. ISBN 9780953131136 – via Google Books.
- The Garelli 350 cc split-single stayed in production until 1926 Archived 2012-05-06 at the Wayback Machine and made a major impact in racing.
- Trojan Two-stroke 1927 Science Museum photo of split-single with V-shaped Connecting Rod.
- "Trojan Museum Trust". www.trojanmuseumtrust.org. Retrieved 2016-11-11.
- Puch's two-stroke double-piston engines Archived 2007-10-09 at the Wayback Machine asymmetric port opening of 1923 Puch Marcellino design, inspired by industrial opposed-piston engines.
- Walker, Mick (2000), Mick Walker's European Racing Motorcycles, ISBN 9780953131136, retrieved 2011-08-28
- "Cylinders with two pistons used in motorcycle engine". Popular Mechanics. Hearst Magazines: 843. December 1935. Retrieved 12 July 2019.
- "Allstate 250" 1966 from Sears. Note the mistake in this diagram, labeling of the "front" and "rear" pistons has been reversed.
- Friedrich F. Ehn: Das große Puch-Buch. Weishaupt, Graz 1993, ISBN 3-900310-49-1 (German). 38,584 Puch 250 SGS were produced from 1953 to 1970.
- Allstate "Twingle" Sears sold tons of them ... survived better than almost any other bike of that era.
- Adopted by Ing Zoller in 1931 the concept [of the Split Single Engine] was to make DKW the dominant racing motorcycle in the Lightweight and Junior classes during the pre war years.
- TWN split-single BDG250 A radical two stroker in 1953? Business Standard Motoring, Aug 2008.
- TWN Bike Review includes sketch of split-single "side-by-side" piston layout.