Engine that powers a motorcycle
A motorcycle engine is an engine that powers a motorcycle. Motorcycle engines are typically two-stroke or four-stroke internal combustion engines, but other engine types, such as Wankels and electric motors, have been used.
Although some mopeds, such as the VéloSoleX, had friction drive to the front tire, a motorcycle engine normally drives the rear wheel, power being sent to the driven wheel by belt, chain or shaft. Historically, some 2,000 units of the Megola were produced between 1921 and 1925 with front wheel drive, and the modern Rokon, an all terrain motorcycle with both wheels driven, has been produced since 1960.
Most engines have a gearbox with up to six or even 7 ratios. Reverse gear is occasionally found on heavy tourers, for example the Honda GL1600, and sidecar motorcycles, such as the Ural. The rider changes gears on most motorcycles using a foot-pedal and manual clutch, but early models had hand-levers. More recently, some have automatic or semi-automatic gearboxes, and some using CVT transmission. 
Outside the United States, engine capacities typically ranged from about 50 cc to 650 cc; but in Europe since 1968 motorcycles with larger capacities have become common, ranging as high as the Triumph Rocket 3's engine. In the United States, V-twin engined motorcycles with capacities of 850 cc or more have been the norm since the 1920s.
History.
The first motorcycles were powered by steam engines. The earliest example is the French Michaux-Perreaux steam velocipede of 1868. This was followed by the American Roper steam velocipede of 1869, and a number of other steam powered two and three wheelers, manufactured and sold to the public on through the early 20th century.
Using frames based both on the earlier boneshaker and the later—and in many ways completely modern—safety bicycle design, these early steam motorcycles experimented with a variety of engine placement strategies, as well as transmission and options. While today nearly every motorcycle has its engine in the center of the frame; this became standard only around 1900-1910 after nearly every possible engine location was tried. The origins of the scooter engine arrangement (including the shaft drive) can be traced back to the 19th century but the current layout became generally adopted after the Great War. The modern equivalent became universally popular in the 1950s and remains much the same today.
The Otto cycle gasoline internal combustion engine was first used on an experimental two-wheeler created by Gottlieb Daimler to test the practicality of such an engine in a vehicle. This motorcycle, the Daimler Reitwagen, is credited as the "world's first motorcycle" by many authorities, partially on the assumption that a motorcycle is defined not as any two-wheel motor vehicle, but a two-wheel "internal combustion engine" motor vehicle. 
The Oxford English Dictionary, for example, defines the word motorcycle this way. 
The steam cycles were also simply neglected and forgotten by many historians, even as the Michaux-Perreaux waited forty years on display in the National Motor Museum, Beaulieu.
In recent years, a surge in interest in clean energy has put many new electric powered two wheelers on the market, and they are registered as motorcycles or scooters, without the type of powerplant being an issue. Diesel motorcycles were also experimented with briefly in the 20th century, and are again the subject of interest due to fuel economy and the needs of military logistics. The USMC has ordered a new diesel motorcycle, the M1030 M1, that can use the same fuel, JP-8, as the rest of their armored vehicles, aircraft, cars and trucks.
The overwhelming majority of the motorcycles produced and used in the world today have small displacement air-cooled single-cylinder engines, both two- and four-strokes. In the wealthier parts of the world, Europe and Japan, larger displacements and multiple cylinders are common alongside small-displacement bikes required by various licensing and rider experience requirements, and so a very diverse range of sizes, cylinder numbers, configurations, and cooling systems are seen on the road. Many developed countries have graduated licensing, where a rider is licensed for a period of time to ride only smaller-displacement motorcycles before being allowed to ride larger ones. In the United States, there are no such mandates, and so the mix is skewed even further to the largest displacements, consumer demand drives manufacturers to offer their largest motorcycles to that country, and to export far fewer sub- models to the American market.
Types.
Almost all production motorcycles have gasoline internal combustion engines. Both four-stroke and two-stroke engines are used, but strict emission laws have led to far fewer two-strokes. A few have used Wankel rotary engines, but no Wankel bikes are currently in production. Motorcycle engines can be air cooled or liquid cooled, and optionally include oil cooling as well. Some scooters use batteries and an electric motor. The 2009 TT races introduced a new category 'TTX' (renamed as TT Zero) for electric bikes using fuel-cells or batteries.
Motorcycle engines can be mounted transversely; with the crankshaft aligned perpendicular to the frame or longitudinally, with the crankshaft parallel to the frame. Motorcycles with transversely mounted engines are more suited to have chain or belt final-drive. Motorcycles with longitudinally mounted engines are more suited for shaft final-drive.
Motor scooters often have the engine as part of the rear suspension, so the engine is not fixed rigidly to the main frame. Instead, the combined engine-transmission-swingarm assembly is pivoted to follow the road surface and is part of the "unsprung weight". The chain final-drive of scooters runs in an oil-bath within the engine casings. "Step-through" motorcycles may have a rigidly fixed engine, or may have a scooter-type arrangement.
Two-stroke and four-stroke.
Two-stroke engines have fewer moving parts than four-stroke engines, and produce twice the number of power strokes per revolution.
Fuel economy is better in four-strokes due to more complete combustion of the intake charge in four-stroke engines.
Nevertheless, two-strokes have been largely replaced on motorcycles in developed nations due to their environmental disadvantages. Cylinder lubrication is necessarily total-loss and this inevitably leads to a smokey exhaust, particularly on wide throttle openings. Two-stroke-engined motorcycles continue to be made in large numbers, but mostly off-road motocross low-power mopeds, small scooters and step-through underbones where they still compete strongly with four-strokes (including the highest-selling motorcycle of all time, the 50 cc Honda Super Cub). The major markets of two-stroke motorcycles are in dirt bikes and developing nations.
Cylinder heads (four-stroke).
Cylinder head design has a significant effect on the efficiency of combustion, and hence the power output of the engine. The head may be flat, in which case the combustion chamber resides within the cylinder and/or a depression in the piston crown, but usually a "dome" within the cylinder head provides most of the combustion volume. Motorcycles use poppet valves in a range of designs: side valve, overhead valve (OHV) with pushrod operation, overhead cam (SOHC), and double overhead cam (DOHC). A cylinder with desmodromic valves may have three or even four camshafts. An OHC or DOHC cylinder head will have at least two valves per cylinder (1 inlet and 1 exhaust), but multi-valve engines may have three (2 inlet and 1 exhaust), or four (2 inlet and 2 exhaust), or even five (3 inlet and 2 exhaust). Cylinder heads are the hottest part of the engine and require adequate cooling, typically air cooling, oil cooling or liquid cooling.
Some motorcycles such as Harley-Davidsons, Moto Guzzis and BMWs become identifiable by their cylinder-head types, namely airhead, panhead, oilhead, and even knucklehead. The Ducati desmos head enables higher rpm to be achieved without detrimental valve float also known as valve bounce.
Valve control in four-strokes.
In a side-valve engine, the valves are operated from the "underhead" cam without special valve gear. Overhead valve (OHV) engines have valves operated by pushrods. Single- and double-overhead camshaft (SOHC and DOHC, respectively) engines have overhead camshafts typically operated by chain, belt, gear train or bevel gear drive.
The earliest motorcycle engines had exhaust valves operated in the same way as a side-valve engine, but they had overhead intake valves held closed by a weak spring. The intake valve was opened by the suction caused by the intake stroke. As the need for mechanical control of the intake valve arose, the design was modified by adding another cam to the camshaft and using a pushrod and rocker arm to operate the intake valve.
Honda equipped the CBR400F with REV, described as "revolution responding type valve pausing mechanism", in 1983. This system enabled switching over the number of valve operations per cylinder between low and medium speed revolution range and high speed revolution range. In 2002, Honda introduced HYPER VTEC in the VFR800 Interceptor. In 2006, Kawasaki introduced VVT in the Concours 14.
Unit construction.
Engines and gearboxes were originally separate items, the gearbox being driven by a "primary chain". Later designs combined the two elements into a single "unit construction". Some manufacturers, such as Norton and Enfield continued with "pre-unit" construction until recently, and Harley-Davidson still makes pre-unit engines today.
In the early 1960s, BSA introduced new short-stroke unit construction 500 and 650 cc parallel twins to replace their existing pre-unit long-stroke engines. Triumph modified their large pre-unit 500 and 650 cc parallel twins into unit-construction engines. The smaller Triumph 350 and 500 cc twins had been unit construction from their introduction in 1957. The decision by Lucas to discontinue making dynamos and magnetos, and to produce only alternators and coil ignition systems, was a significant factor in forcing the redesign.
Traditionally, engines and gearboxes had separate lubrication systems with different grades of oil; but modern practice is to use the same oil for both.
Cylinders and configuration.
Generally, motorcycle engines have between one and six cylinders. The majority of motorcycle engines are configured as singles, parallel twins, triples, fours and sixes; and all these may be inline or transverse. Vee engines include V-twins and V-fours, and horizontally opposed engines include flat-twins, flat-fours and flat-sixes. Wankel engines are either single-rotor or twin-rotor. Bigger bikes tend to have more cylinders for smoothness and increased power. Modern singles range in capacity from 50 cc to 660 cc, twins from 175 cc to 1,800 cc, triples from 380 cc to 2,300 cc, and so on.
Single.
Single-cylinder engines ( "singles" or "thumpers") have the cylinder vertical, inclined or horizontal, the last type most common in step-through motorcycles. Single-cylinder engines require both a larger flywheel and a heavier-duty gearbox than multicylinder engines. Small singles are cheap to build and maintain and are suitable as cheap utility motorcycles.
Until the mid-1960s, road-racing machines, like the UK's Matchless, AJS and Norton, tended to be large singles, but since then multicylinder racers have become the norm. Off-road and smaller dual-sport bikes tend to use single-cylinder engines. The simplicity of these engines, often relying on a single carburetor, make them relatively easy to maintain and repair in remote locations with few tools.
Compared to lightweight off-road singles, the modern category of heavier off-road adventure bikes tend to forgo such simplicity, and instead have larger and more powerful parallel-twin, flat-twin, or V-twin engines.
Twin.
Starting with Edward Turner's 1937 Triumph Speed Twin design, and until the mid-1970s, the parallel-twin was the most common British motorcycle type. Parallel-twins are usually mounted transversely, with the cylinders side by side above the crankshaft, and with exhaust pipes at the front, in the cool airstream. Longitudinal, or inline, twins include the 500 cc Sunbeam S7 and S8. There are three crankshaft configurations for this engine: 360°, 180°, and the newer 270°. Parallel twins usually have only two main bearings.
In a V-twin engine the cylinders form a "V" around the crankshaft. A Vee-angle of 90°, as used by Ducati and Moto Guzzi, can give perfect primary and secondary balance, with a pleasingly irregular firing order. A lesser angle gives a more compact motor, but one which is prone to vibration, such as 42° (Indian), 45° (Harley-Davidson), 52° Honda, and 60° (Aprilia). Most V-twins have a single crankpin shared by side-by-side connecting rods so that the cylinders are slightly offset, but a variation is to have a single crankpin with "fork &amp; blade" con-rods, to keep the cylinders in line. Non-90° V-twins may have offset crankpins to try to reduce vibration.
V-twins may be mounted either longitudinally with the cylinders protruding either side, like the Honda CX500 and Moto Guzzi, or transversely, such as Harley-Davidson, Ducati, or Aprilia. Transverse V-twins can raise difficulties in cooling the aft cylinder, and in siting the airbox, battery, aft carburetor, and aft exhaust pipe.
In a flat-twin, or boxer, engine the cylinders are horizontally opposed. The boxer has perfect primary