Single-cylinder engine

A single-cylinder engine, sometimes called a thumper, is a piston engine with one cylinder. This engine is often used for motorcycles, motor scooters, motorized bicycles, go-karts, all-terrain vehicles, radio-controlled vehicles, power tools and garden machinery (such as chainsaws, lawn mowers, cultivators, and string trimmers). Single-cylinder engines are made both as 4-strokes and 2-strokes.

Compared with multi-cylinder engines, single-cylinder engines are usually simpler and compact.¹ Due to the greater potential for airflow around all sides of the cylinder, air cooling is often more effective for single cylinder engines than multi-cylinder engines. This reduces the weight and complexity of air-cooled single-cylinder engines, compared with liquid-cooled engines.

Drawbacks of single-cylinder engines include a more pulsating power delivery through each cycle and higher levels of vibration.² The uneven power delivery means that often a single-cylinder engine requires a heavier flywheel than a comparable multi-cylinder engine, resulting in relatively slower changes in engine speed. To reduce the vibration level, they often make greater use of balance shafts than multi-cylinder engines,³ as well as more extreme methods such as a dummy connecting rod (for example the Ducati Supermono).⁴⁵ These balancing devices can reduce the benefits of single-cylinder engines regarding lower weight and complexity.

Most single-cylinder engines used in motor vehicles are fueled by petrol (and use a four-stroke cycle),⁶⁷⁸ however diesel single-cylinder engines are also used in stationary applications (such as the Lombardini 3LD and 15LD).

A variation known as the split-single makes use of two pistons which share a single combustion chamber.

Early motorcycles, automobiles and other applications such as marine engines all tended to be single-cylinder. The configuration is almost exclusively used in portable tools, along with garden machinery such as lawnmowers.⁹ Single-cylinder engines also remain in widespread use in motorcycles, motor scooters, go-karts, auto rickshaws, and radio-controlled models. From 1921 to 1960, the Lanz Bulldog tractor used a large horizontally mounted single-cylinder two-stroke engine.¹⁰ However, they are rarely used in modern automobiles and tractors, due to developments in engine technology.

Single-cylinder engines remain the most common engine layout in motor scooters and low-powered motorcycles. The Honda Super Cub (the motor vehicle with the highest overall sales since its introduction in 1958) uses a 49 cc (3.0 cu in) four-stroke single-cylinder engine. There are also several single-cylinder sportbikes (such as the KTM 690 Duke R), dual-sport motorcycles (such as the BMW G650GS) and the classic-styled Royal Enfield 500 Bullet.¹¹¹²

The Moto3 class in the MotoGP World Championship have used four-stroke 250 cc (15.3 cu in) single-cylinder engines since the class replaced 125 cc (7.6 cu in) two-strokes in 2012.

Engines of other sorts, like the beam engine and certain types of Stirling engine, operate using one cylinder and thus can also be considered single-cylinder engines.