Straight-twin engine
Straight-twin engines are primarily used in motorcycles; other uses include automobiles, marine vessels, snowmobiles, jet skis, all-terrain vehicles, tractors and ultralight aircraft.
[1] Some of these terms originally had specific meanings relating to the crankshaft angle or engine orientation; however, they are often also used interchangeably.
[1][2] In the United Kingdom, the term "parallel-twin" is traditionally used for engines with a crankshaft angle of 360 degrees, since the two pistons are in the same direction (i.e. parallel to each other).
This set up results an even 360 degree firing interval unlike other crank configurations in inline twin engines.
Early engines attempted to reduce vibration through counterweights on the crankshaft; however, later methods also included balance shafts and a separate weighted connecting rod.
From the 1930s, most British four-stroke straight-twin motorcycle engines used a 360 degree crankshaft,[11] since this avoided the uneven intake pulsing of other configurations, thus preventing the need for twin carburettors.
The 2008 BMW F series parallel-twin motorcycles also use 360 degree crankshafts, with a third "vestigial" connecting rod (acting as a counterbalance) and a rev limit of 9,000 rpm to reduce vibrations.
[13] In 2009 Fiat launched Multiair inline twin car engines that use 360 degree crankshaft which relied on balance shafts to reduce the vibrations.
A 180° crankshaft engine suffers fewer pumping losses than a 360° twin, because displacement of the crankcase is relatively unchanged as the pistons move.
Most vintage British straight-twin motorcycle engines (such as Triumph, BSA, Norton and Royal Enfield) had two main bearings.
The Werner engine uses cast-iron cylinders with integral heads, side valves and has a displacement of 500 cc.
The engine was designed by Edward Turner and Val Page, and was initially used in the 1933 Triumph 6/1 sidecar hauler (which won the International Six Days Trial silver medal and the 1933 Maudes Trophy).
[24] Following the trend created by the Triumph Speed Twin, the most common design used by British motorcycle manufacturers until the mid-1970s was a four-stroke straight-twin engine with a 360 degree crankshaft.
[35] Many large British motorcycles from 1945 to the 1960s used a straight-twin transverse engine (i.e. oriented with the crankshaft perpendicular to the frame), such as the Triumph Bonneville and Norton Commando.
Longitudinal engine straight-twin motorcycles are less common; however, examples include the 1930–1938 Dresch Monobloc[38] and the 1949–1956 Sunbeam S7 and S8.
[43] Another early straight-twin engined car was the 1898 Decauville Voiturelle, which used a pair of cylinders taken from a de Dion model mounted fore and aft and positioned below the seat.
From the 1950s, manufacturers of outboard motors had settled on the use of the basic inline engine design, cylinders stacked on top of each other with the crankshaft driving the propeller shaft.
[60][61][62] Design variations include two-stroke, four-stroke, petrol, diesel,[63] air-cooling, water-cooling, natural aspiration and turbocharging.
