Motorcycle engines vary enormously, but the typical kind sits immediately under the fuel tank, in between (or partly forwards of) the rider's legs/feet, driving the rear-wheel via a roller chain or drive shaft which is external to the engine. The scooter type of engine is further back, has an integrated final drive driving a small rear wheel.
Types
Almost all commercially available motorcycles are driven by conventional gasoline internal combustion engines, increasingly four-strokes in all size ranges. Some are still air-cooled (forced with a fan in some cases) but water-cooling is more common. The mid-range and large two-strokes seen in the 1970s and 1980s have almost disappeared, particularly as emission laws were introduced. There are a few small scooter-type models using batteries and an electric motor. Van Veen, Hercules, Norton, and Suzuki produced quite small numbers of motorcycles propelled by Wankel rotary engines. The 2009 TT races included a new category 'TTX' for electric bikes using either fuel-cells or batteries
Most motorcycle engines have the primary working member or crankshaft across the frame (transverse mounting). Others are arranged to turn a shaft-drive to the rear wheel and the crankshaft is longitudinal, along the frame.
A sub-type of motorcycle, the scooter, has the engine as part of the rear suspension, so it is not fixed to the main frame. Such engines pivot to follow the road surface and are partly "unsprung weight". The final drive of scooters is much shorter than that of regular motorcycles and is contained within the engine casings in an oil-bath, this design is only suitable for machines with small wheels, or is fully automatic using belts and expanding/contracting pulleys, ala DAF variomatic cars. The engines of the motorcycles known as underbones or "step-throughs" may be of either kind.
Displacement
Engine displacement is defined as the total volume of air/fuel mixture an engine can draw in during one complete engine cycle. In a piston engine, this is the volume that is swept as the pistons are moved from top dead center to bottom dead center. This is the "size" of the engine. Motorcycle engines range from less than 50 cc, commonly found in many mopeds and small scooters, to a 6,000 cc engine used by Boss Hoss in its cruiser style motorcycle BHC-3 LS2. Many state laws in the U.S. define a motorcycle as having an engine larger than 50 cc, and a moped as a vehicle with an engine smaller than 60 cc.
Number of cylinders
Small motorcycles normally have a single cylinder, many smaller and mid-range motorcycles have twin cylinders and most medium to large motorcycles have four cylinders. However, no generalisations can be made, as there are a few large singles and twins. Three cylinders have been widely used and there have been some six-cylinder machines. Many different layouts have been used with vertical cylinders the most popular. There are some horizontally opposed and V layouts.
Single
Single cylinder engines are known as "singles," and in larger capacities as "thumpers" (for the sound they make). In most singles, the cylinder points up and slightly forward with the spark plug on top, but another arrangement is a horizontal cylinder such as used by Honda in its C series singles. Single cylinder engines deliver more torque at lower revolutions per minute than a similar engine with more cylinders, so are better suited for off road motorcycles. Split Single two - strokes have been used by DKW and Puch where a smaller secondary cylinder and piston moves the fuel around.
Two
Two-cylinder engines are known as "twins." The three most common arrangements are:
- The parallel twin as in most common British and many Japanese motorcycles. Engines of this design typically have the cylinders side by side vertically above the crankcase, maximizing airflow cooling. Logitudinal twins include the 500 cc Sunbeam S7 and S8.
- The V-twin where the cylinders form a "V" around the crankshaft, which is oriented transversely on models such as on the Honda CX500 and most Moto Guzzi motorcycles, or longitudinally. V-twins can also be separated into two types, one having a shared crankpin (a normal con-rod is inserted between a forked con-rod thus sharing the single crankpin and keeping the cylinders in line, which gives the distinctive Harley-Davidson sound; and the other with two crankpins, with the cylinders staggered, which nearly every one else uses.
- The opposed twin in which the cylinders protrude sideways into the cooling air stream.
The angle in the V-twins varies from around 45 degrees to 90 degrees. Typical of the former are the Harley-Davidson and Vincent engines which because of their firing order tend to vibrate more. Ducati and Moto Guzzi make V-twins with cylinders arranged at a 90 degree angle to quell primary vibrations. Some Moto Guzzi motorcycles have V-twins oriented transversely: one cylinder to the left, one to the right.
- The Tandem Twin where the cylinders are longitudinal, and have two cranks geared together such as Kawasaki's KR250 road bike and KR250 and KR350 GP Bikes.
The parallel twin engine configuration was made famous by Edward Turner's Triumph Speed Twin design as used on the Bonneville.
In the flat-twin (boxer) engine, which is used by BMW Motorrad, Ural and historically by Douglas, the cylinders are horizontally opposed, protruding from either side of the frame. The boxer is the only twin-cylinder arrangement that has inherent primary balance without a rocking couple, producing very low vibration levels without the use of counterbalance shafts.
Sunbeam produced an air cooled inline twin driving a propshaft.
Narrow-angle V-twin engines dominate the cruiser motorcycle segment.
Three
Three-cylinder designs are referred to as "triples" and are normally inline triples in layout. The British Hinckley-built Triumph, mostly transverse but also the 2,300 cc longitudinal Rocket III, Italian Benelli and Japanese Yamaha XS750 are three motorcycle manufacturers who have used triples in their large displacement motorcycles. The Italian firm Laverda made a few 1,000 cc and 1,200 cc triples. BMW made the K75 Longitudinal 750 cc triple with the cylinders parallel to the ground. BSA made the 'Rocket-3' transverse 750 cc and 'old' Triumph the 750 cc 'Trident'
Two-stroke triples were somewhat more common historically. Kawasaki produced 250, 350, 400, 500, and 750 triples, which were known for their power (but maybe not rideability) in the 1970s while Suzuki produced 380, 550, and 750 triples (the last being water cooled). Motobecane made 350 cc and fuel injected 500 cc triples with 3 into 4 pipes in the early seventies. Honda produced the water cooled V-3 two-strokes MVX250 and NS400. There have been various race bike triples such as Kawasaki KR750, Suzuki TR750 transverse 3's, and Proton/Modenas KR3, Honda NS500 V-3's
Four
Four-cylinder engines are most commonly found in a transverse-mounted inline four layout, although some are longitudinal (as in the earlier BMW K100). V-4 and boxer designs (as in the eariler Honda Gold Wing) have been produced. One of the more unusual designs was the Ariel Square Four, effectively two parallel-twin engines one in front of the other in a common crankcase – it had remarkably little vibration due to the contra-rotating crankshafts. Honda use V4 engines in the ST series and VFR series. As for two-strokes, there were four cylinders in the smaller classes such as Kawasaki's 125 cc KR3 square 4 and Yamaha's 250 cc RD05 V4. Yamaha later raced transverse four TZ500/700/750's and vitually all the bikes in the last decade of the two-stroke GP500 era were fours (first squares then Vees) ie. Honda, Kawasaki, Cagiva, Suzuki, Yamaha - Kawasaki also experimented with a trapezeoidal four the 602S. Yamaha made the V4 RD500LC, and Suzuki the RG400 and RG500 square four road bikes.
Since the advent of Honda's CB750 straight-four engine, straight-fours have dominated the non-cruiser street motorcycle segments.
Five
Honda has produced five-cylinder engines for racing, the RC211V 990 cc V-five and the RC148 and RC149 125 cc straight Fives, and Verdal made a radial Five 750 racer in 1912. There have also been five-cylinder rotaries that are installed in the front or back wheel, but no five-cylinders are currently available for commercial production motorcycles.
Six
Six-cylinder engines are rare and found only on the biggest motorcycles. Two easily recognisable examples in recent times have been the Honda CBX, the Kawasaki KZ1300 and Kawasaki's Voyager XIII. Bennelli also made 750 cc and 900 cc Straight sixes kn
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