A straight-twin engine, also known as an inline-twin, vertical-twin, or parallel-twin, is a two-cylinder piston engine whose cylinders are arranged in a line along a common crankshaft.
Straight-twin engines are primarily used in motorcycles; other uses include automobiles, marine vessels, snowmobiles, jet skis, all-terrain vehicles, tractors and ultralight aircraft.
Various different crankshaft configurations have been used for straight-twin engines, with the most common being 360 degrees, 180 degrees and 270 degrees.
Terminology
The straight-twin layout is also referred to as "parallel-twin", "vertical-twin" and "inline-twin". Some of these terms originally had specific meanings relating to the crankshaft angle or engine orientation; however, they are often also used interchangeably.
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). "Vertical-twin" was used to describe engines with a crankshaft angle of 180 degrees, which causes the pistons to travel in opposite directions. The terms "straight-twin" and "inline-twin" were used more generically for any crankshaft angle.
For motorcycles, "inline-twin" has sometimes referred to either a longitudinal engine orientation (i.e. with the crankshaft in line with the chassis) or a U-engine (tandem twin) where the cylinders are arranged longitudinally in the chassis (although the two crankshafts are actually oriented transversely).
Design
Compared with V-twin engines and flat-twin engines, straight-twins are more compact, a simpler design and cheaper to produce. Straight-twin engines can be prone to vibration, either because of the irregular firing interval present in 180° crank engines or the large uncountered reciprocating mass in 360° crank engines. Inline-twins also suffer further from torsional torque reactions and vibration.
Crankshaft angle
The most common crankshaft configurations for straight-twin engines are 360 degrees, 180 degrees and 270 degrees.
360 degrees
In an engine with a 360 degree crankshaft, both pistons move up and down at the same time. However, the firing interval is offset between cylinders, with one of the cylinders firing during the first crankshaft rotation and then the other cylinder in the following rotation. This set up results an even 360 degree firing interval unlike other crank configurations in inline twin engines. The 360 degree engines can use a single ignition system for both cylinders, using a wasted spark system.
The imperfect primary balance is as per a single-cylinder engine of equivalent reciprocating mass. Early engines attempted to reduce vibration through counterweights on the crankshaft; however, later methods also included balance shafts and a separate weighted connecting rod. Compared with a single-cylinder engine, the more frequent firing interval (360 degrees compared with 720 degrees) results in smoother running characteristics, despite the similar dynamic imbalance.
From the 1930s, most British four-stroke straight-twin motorcycle engines used a 360 degree crankshaft, since this avoided the uneven intake pulsing of other configurations, thus preventing the need for twin carburettors. In the 1960s, even though Japanese motorcycles mostly switched to 180 degree crankshafts for engines sized from 250 to 500 cc, various smaller and larger engines continued to use a 360 degree crankshaft. Vibration was less of an issue for smaller engines, such as the 1965 Honda CB92 and 1979 Honda CM185. Larger engines, such as the 1969 Yamaha XS 650 and 1972 Yamaha TX750, often used balance shafts to reduce the vibration. The later 1978–1984 Honda CB250N/CB400N engines also used a 360 degree crankshaft. 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. In 2009 Fiat launched Multiair inline twin car engines that use 360 degree crankshaft which relied on balance shafts to reduce the vibrations.
180 degrees
In an engine with a 180 degree crankshaft, one piston rises as the other falls. In a four-stroke engine, the firing interval is uneven, with the second cylinder firing 180 degrees after the first, followed by a gap of 540 degrees until the first cylinder fires again. The uneven firing interval causes vibrations and results in a 'lumpy' power delivery. A 180° engine also requires a separate ignition system for each cylinder.