A V-twin engine, also called a V2 engine, is a two-cylinder piston engine where the cylinders share a common crankshaft and are arranged in a V configuration.
Although widely associated with motorcycles (installed either transversely or longitudinally), V-twin engines have also been used for industrial engines and in several small cars. The V-twin design dates back to the late 1880s.
One of the first V-twin engines was built by Gottlieb Daimler in 1889. It was used as a stationary engine, for boats and in the Daimler Stahlradwagen ("steel-wheeled car"), Daimler's second car.  The engine was also manufactured under licence in France by Panhard et Levassor. 
An early V-twin engined motorcycle was produced in November 1902 by the Princeps AutoCar Company in the United Kingdom.  The following year, V-twin motorcycles were produced by Eclipse Motor & Cycle Co in the United Kingdom (the XL-ALL model),  Glenn Curtiss in the United States, and NSU Motorenwerke in Germany. 
Peugeot, which had used Panhard-built Daimler V-twins in its first cars,   began producing its own V-twin engines in the early 20th century. This Peugeot engine powered a Norton motorcycle that won the first Isle of Man TT race in 1907. 
Most V-twin engines have a single crankpin, which is shared by both connecting rods. The connecting rods may sit side-by-side with offset cylinders, or have fork and blade connecting rods which avoids the twisting forces caused by having offset cylinders.
Some notable exceptions include a 180° crank pin offset used by the 1935 Moto Guzzi 500cc,  a dual-crankpin configuration used by the 1983 Honda Shadow 750,  and the 75° crank pin offset (45° offset in the United States) used by the 1987 Suzuki VX 800. 
Although any 'V angle' (the angle between the two banks of cylinders) between zero and 180 degrees is theoretically possible for a V-twin engine, in practice angles smaller than 40 degrees are rarely used. The most common V angle for a V-twin engine is 90 degrees, which can achieve a perfect primary balance (if the correct counterweight is used) like most Ducatis, most Moto Guzzis, the Honda RC51, Suzuki TL1000S and TL1000R.   However, this arrangement results in an uneven firing order, with the second cylinder firing 270 degrees after the first cylinder, then a 450 degrees interval until the first cylinder fires again. 90 degree engines are sometimes called L-twin (like the "L" in TL1000R or TL1000S) rather than V-twin.
The alternating longer and shorter gaps between firings produce a characteristic V-twin alternating engine noise "phutphut phutphut phutphut phutphut".
When a V angle of less than 90 degrees is used, perfect primary balance can only be achieved if offset crankpins are used. If not, balance shafts are usually required to reduce the vibration.  Vehicles which use engines with V angles of less than 90 degrees include:
Vehicles which use engines with V angles of greater than 90 degrees include the 1934 Moto Guzzi 500cc (120 degrees)  and the 1940-1948 Zündapp KS 750 (170 degrees). 
As per other motor vehicles, the terms longitudinal engine and transverse engine are most often used to refer to the crankshaft orientation relative to the frame.  [ self-published source? ]  However, some companies use the opposite terminology, stating that a "transverse" V-twin engine has the cylinders mounted on each side of the motorcycle (therefore with the crankshaft running in line with the frame) and that a "longitudinal" V-twin engine has the cylinders at the front and rear.  The latter terminology is used by the Italian manufacturer Moto Guzzi. 
To avoid such ambiguity, some people use descriptions of "transverse crankshaft engine", "longitudinal crankshaft engine",   or "transversely mounted cylinders". 
The most common arrangement is to mount the engine with the crankshaft oriented transversely to the frame. The advantage of this mounting is that the width of the motorcycle can be smaller than a longitudinally-mounted V-twin.  A disadvantage of this configuration for air-cooled engines is that the two cylinders receive different air-flows and cooling of the rear cylinder tends to be restricted  (although the uneven cooling isn't as pronounced as a parallel-twin engine, where the inner faces of the cylinders are not exposed to any airflow). Some transverse V-twins use a single carburettor in the middle of the V-angle to feed both cylinders. While this avoids the need for two carburettors, it creates further cooling problems for the rear cylinder by placing its hot exhaust port and pipe at the back of the cylinder, where it may be exposed to less cooling airflow.
Transverse V-twin engines have been used by Harley-Davidson, Ducati and many recent Japanese motorcycles, such as the Suzuki SV650. Some Ducati V-twin engines have been marketed as "L-twin" engines, due to the front cylinder being vertical and the rear cylinder being horizontal, thus forming an "L" shape.
A less common arrangement is to mount the engine longitudinally. An advantage of this arrangement is that both cylinder heads can protrude into the air stream, so they can each receive the same amount of cooling (for air-cooled engines).  Also, the transmission being located behind the engine is easier to fit within a typical motorcycle frame and, for shaft-drive motorcycles, a 90° bevel gear is not needed at the start of the driveshaft.
As per all longitudinal engines, a disadvantage is that the torque reaction will twist the motorcycle to one side (such as on sharp acceleration/deceleration or when opening the throttle in neutral) instead of shifting the weight balance between the front and rear wheels. However, many modern motorcycles reduce this effect by rotating flywheels or alternators in the opposite direction to that of the crankshaft.  
Longitudinal V-twin engines have been used by the Honda CX series and several Moto Guzzi motorcycles.
Most cars are powered by engines with three or more cylinders, however several small cars have been produced with V-twin engines particularly during the period from 1912 to 1920 when cyclecars were made by many companies (due to a favourable tax position). Almost all of these used proprietary engines, either adapting the larger motorcycle engines used for sidecar work (large singles or V-twins), or using engines specifically made for cyclecars such as those made by JA Prestwich Industries ('J.A.P.' engines) or F.E. Baker Ltd ('Precision' engines).
In 1912 Humber produced a light car called the Humberette with a Humber-made V-twin side-valve engine of 998cc. The engine had a directly attached clutch, 3-speed gearbox and prop shaft output to a rear differential. A water cooled version of this engine was made available in 1914, but WW1 ended Humberette production in 1915. 
From 1911-1939, various Morgan 3-wheelers three-wheeled cyclecar models were powered by V-twin engines. Production of three-wheeler models then resumed with the 2012–present Morgan 3-Wheeler. Also in the United Kingdom, Birmingham Small Arms Company (BSA) produced several cars powered by their V-twin motorcycle engines. These were produced from 1921-1926 (four-wheel models) and 1929-1936 (three-wheel and four-wheel models). 
Several manufacturers have produced models inspired by the original Morgan three-wheeled car, such as the 1978–present Triking Cyclecar (using a Moto Guzzi engine),  the 2006–present Ace Cycle Car (using a Harley-Davidson engine)  and the 1990–present JZR Trikes kit car (using engines from several manufacturers). 
Mazda's first car, the 1960-1966 Mazda R360 rear-engined kei car, was powered by the 356 cc (21.7 cu in) Mazda V-twin engine.   The 1961-1962 Mazda B360 front-engined light commercial vehicle used a 577 cc (35.2 cu in) version of this engine.
Various V-twin engines have been produced for industrial uses such as pressure washers, lawn and garden tractors, tillers, generators and water pumps. The engines are usually air-cooled with a 90° V angle. Depending on the application, the engine's orientation can be either have a horizontal or vertical crankshaft.
Manufacturers of commercial V-twin engines include Briggs & Stratton with its Vanguard, Professional and Intek V-twin series,  Honda with its V-twin series engines,  Kawasaki with its FD, FH, FR, FS, and FX series,  Subaru with its EH series,  Tecumseh with its OV691EA and TVT691 engines,  and Kohler. 
A V6 engine is a six-cylinder piston engine where the cylinders share a common crankshaft and are arranged in a V configuration.
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A flat engine is a piston engine where the cylinders are located on either side of a central crankshaft. Flat engines are also known as horizontally opposed engines, however this is distinct from the less common opposed-piston engine design, whereby each cylinder has two pistons sharing a central combustion chamber.
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But the engine, now fitted lower, in front of the rear axle, had progressed to become a narrow vee-twin with cylinders at 20°, and giving an estimated 1.6hp at 700 rpm.
By 1934 Guzzi offered a range of 175, 250 and 500cc models including full touring machines. The next year they raised the ante once again, challenging the all-vanquishing Norton at the legendary Isle of Man TT, basically a course the British racer owned lock, stock and single barrel thanks to a phenomenal rider, Scotsman Jim Guthrie. Moto Guzzi went to a Brit for riding skills, one Stanley Woods. They gave him a new racer featuring a 120-degree V-twin with offset cranks firing at 180 degrees with bevel gears and shafts driving the SOHC, good enough for 44 hp at 7500 rpm and 112 mph, on equal standing with the Norton. It had an ace up its sleeve so to speak in that it incorporated a type of pivoted-fork rear suspension while the frontend was a springer, a design that had never won a Senior TT due to its handling deficiencies, or so was thought. Guzzi had done some tweaking in that department as well. It also came equipped with a massive twin-leading shoe front brake, a 4-speed gearbox, and alloy wheels, another innovation to cut down unsprung weight. When the dust had settled and the calculations determined, the wreath of victory went to Woods and Moto Guzzi, leaving Norton as they say, gobsmacked. Not only that, the Guzzi had smashed the track lap record. The next day Moto Guzzi was world famous.
At some point in the motorcycle's development, the company changed the crankpin offset from 45 to 75 degrees in hope of creating a smoother-running motor. But just as production began, American Suzuki engineers decided that the new offset resulted is less mid-range power as well as a too-sanitized exhaust note, one that didn't sound very V-Twin-like. Presto, now the US models come with the 45-degree offset, while the rest of the world gets the 75-degree staggered crankpins.
The Zundapp (sic) was powered by an air-cooled 170-degree V-twin that was very similar in design that was very similar in design to the BMW boxer twins
The transversely mounted [cylinder] V-twin, as used to good effect for many years by Moto Guzzi, slots easily into the frame, and has excellent cooling as both heads are stuck out into the wind. It also provides the perfect set-up for using shaft drive.
Ducati's engines, which are longitudinal (they are positioned lengthwise in the frame) most obviously display the "L" configuration, but Moto Guzzi's engines, which are transverse (arranged croswise in the frame), are also at 90 degrees.
Just as importantly, the V7 became an instant technology trendsetter thanks to its innovative transverse, air-cooled V-twin engine with shaft drive.
We could, of course write a book about Moto Guzzi's transverse V-Twin.
Ducati 750 Sport with its clip-on handlebars and racing setup, is for those who want to do their touring stretched out prone! Engine is a longitudinal V-twin. ..The unique 90∘longitudinal engine produces enormous low and mid-range torque...Moto Guzzi 850T...An 850-cc 90° transverse V-twin engine...
According to my source at Moto Guzzi Technical Services, "The Guzzi engine is a 90-degree 'L' twin, actually, because the cylinders are oriented at 90 degrees, instead of a typical V twin that has a smaller angle ( 60-degree, 77-degree, etc.). It is called 'transverse' because the engine is mounted with the crankshaft oriented front to back instead of left to right. Because of this you cannot run a chain or belt drive directly to the rear wheel like in most motorcycles. This is why you have a separate gearbox that bolts to the engine and transfers the power to the rear wheel via the drive shaft. This is how it is done on the Moto Guzzi and a BMW.
...the boxer had three unique innovations that would remain throughout its years in development:The engine design included transversely mounted cylinders, which were cooled by exposure to the passing air.
In some construction layouts the transverse width is the same as a single-cylinder engine, which allows very narrow frames and bodywork with small frontal areas.
An air-cooled big-bore V-twin in particular can get very hot, especially the rear cylinder, which is not exposed to as much cooling air as the front.
Though the Valkyrie also has a longitudinal crankshaft, this torque reaction has been eliminated by making some of the components, such as the alternator, spin the opposite direction of the engine.
By arranging the rest of the engine internals to rotate in the opposite direction to the crankshaft their forces are cancelled out without having to resort to the weight, complexity and friction associated with two crankshafts.