Split-single engine

The split-single engine (British English; twingle engine in U.S. English) is a type of two-stroke internal combustion engine where two cylinders share a single combustion chamber. The first production split-single engine was built in 1908, and various designs were used on several motorcycles and cars until the mid-1950s, with Puch continuing with their version until 1970. During this time, the design was occasionally used for engines with four or more cylinders. The split-single uses a two-stroke cycle (i.e. where every downward stroke produces power) with the following phases: 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 minimizing the loss of 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 split-single engines used a single combustion chamber (spanning both cylinders), with larger engines (such as with four-cylinders) continuing to share one combustion chamber per cylinder pair. Initial designs of split-single engines from 1905 to 1939 used a single Y-shaped or V-shaped connecting rod. Externally, these engines appeared very similar to a conventional single-cylinder two-stroke engine, with one exhaust, one carburetor in the usual place behind the cylinders, and one spark plug.