Parallel Twin

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. 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.

; 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.

Perfect primary balance is possible with a 180 degree straight-twin engine, however the design creates a rocking couple which requires use of a balance shaft to reduce the vibration. A 180 degree straight-twin engine has a secondary imbalance (similar to an inline-four engine), however the lower reciprocating mass means that this often does not require treatment.

A 180° crankshaft engine suffers fewer pumping losses than a 360° twin, because displacement of the crankcase is relatively unchanged as the pistons move.

In the 1960s, Japanese motorcycle manufacturers favoured the use of 180 degree crankshafts, since the increased smoothness allowed higher rpm and thus higher power outputs. For example, the 1966 Honda CB450 180 degree crankshaft engine has a similar power output to contemporary British 360 degree crankshaft engines, despite having a smaller displacement of 450 cc compared with 650 cc. Both the 1973 Yamaha TX500 and the 1977 Suzuki GS400 had a 180 degree crankshaft and a balance shaft. Since 1993, most Honda straight-twin motorcycle engines use 180 degree crankshafts.

Two-stroke engines typically use a 180 degree crankshaft, since this results in two evenly-spaced power strokes per revolution. The fundamental frequency of vibration is twice that of an equivalent single-cylinder engine, however the amplitude is halved. Two-stroke engines that do not use a 180 degree crankshaft include the 1972 Yankee.

; 270 degrees
In an engine with a 270 degree crankshaft, one piston follows three quarters of a rotation behind the other. This results in an uneven firing interval where the second cylinder fires 270 degrees after the first, followed by a gap of 450 degrees until the first cylinder fires again. This is the same pattern as a 90 degree V-twin engine, and both configurations have a similar 'pulsing' exhaust sound as a result. The pistons in a 270 degree straight-twin engine are never both stationary at the same time (as per a 90 degree V-twin engine), thereby reducing the net momentum exchange between the crank and pistons during a full rotation.

An imperfect primary balance is created in a 270 degree straight-twin engine, due to a combination of free force and rocking couple; a balance shaft is often used to compensate for this. The secondary balance of a 270 degree engine is perfect, however the configuration does result in an unbalanced rocking couple.

The first production 270 degree straight-twin motorcycle engines were fitted to the 1996 Yamaha TRX850 and Yamaha TDM. Later examples include the 2009 Triumph Thunderbird, 2010 Norton Commando 961, 2012 Honda NC700 series, 2014 Yamaha MT-07, 2016 Triumph Thruxton 1200, and 2018 Royal Enfield Interceptor 650 & continental gt ,.

Main bearings

Each cylinder in a straight-twin engine has a separate crank pin, unlike V-twin engines which can use a common crank pin for both connecting rods. Most vintage British straight-twin motorcycle engines (such as Triumph, BSA, Norton and Royal Enfield) had two main bearings. Beginning in the late 1950s, most Honda straight-twin engines had four main bearings. Subsequent straight-twin engines had four or occasionally three main bearings.

Usage in motorcycles

History

The world's first production motorcycle, the 1894 Hildebrand & Wolfmüller used a straight-twin engine. The cylinders lay flat and forward-facing, with the pistons connected directly to the rear wheel using a locomotive-style connecting rod. In 1903, the Werner Motocyclette became the second production motorcycle model, using a straight-twin engine with vertical cylinders. The Werner engine uses cast-iron cylinders with integral heads, side valves and has a displacement of 500 cc.

The 1938 Triumph Speed Twin was a successful straight-twin motorcycle which also led to straight-twin engines becoming more widely used by other brands. The engine was designed by Edward Turner and Val Page

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