Motorcycle engine
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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.
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[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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.[1] 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.
[edit] 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
[edit] 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.
[edit] 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.
[edit] 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 known as the Benelli Sei. Laverda fielded a transverse 1000cc V-Six in endurance racing, and Honda made a 250 cc straight Six GP Bike. The six-cylinder engine is currently used by Honda in the boxer engine of the Rune, Valkyrie and Gold Wing.
[edit] Eight
Galbusera built a V8 in 1938, and Moto Guzzi experimented over a period of two years with its dual overhead cam 500 cc V8 (the Otto Cylindri) in the 1950s. Some custom and one-off motorcycles use more than six cylinders. For example, the Boss Hoss motorcycle uses a Chevy V-8 motor (5,700 cc and 6,000 cc). In the 1990s Daimler-Chrysler manufactured a limited number of Tomahawk concept bikes featuring a Dodge Viper's V-10 engine. Australian company Drysdale have built short runs of 750 cc V8 superbikes and 1L V8 roadgoing motorcycles, both with engines specifically developed for the purpose. No major motorcycle manufacturer has used eight or more cylinders although Honda made the 'almost' V8 oval piston NR750 road bike and NR500 GP bike (eight con-rods etc.)and Morbidelli has shown two V8 prototype road bikes, but has yet to get off the ground
[edit] Engine cooling
[edit] Liquid
Liquid-cooled motorcycles have a radiator (similar to the radiator on a car) which is the primary way their heat is dispersed. Coolant is constantly circulated between this radiator and the cylinders when the engine is running. While most off-road motorcycles have no radiator fan and rely on air flowing over the radiators from the forward motion of the motorcycle, many road motorcycles have a small fan attached to the radiator which is controlled by a thermostat. Some off-road motorcycles are liquid cooled and anti-dirt protection is attached to the radiator. The cooling effect of this fan is enough to prevent the engine overheating in most conditions, so liquid-cooled bikes are safe to use in a city, where traffic may frequently be at a standstill.
Emissions regulations and the market demand for maximum power are driving the motorcycle industry to liquid-cooling for most motorcycles. Even Harley-Davidson, a strong advocate of air-cooled motors, has begun producing a Revolution liquid-cooled engine.
[edit] Air
Most air cooled motorcycles take advantage of air blowing past the cylinder and cylinder head while in motion to disperse heat. Frequent, sustained stationary periods may cause over-heating. Some models (mostly scooters) are equipped with fans that force the air to go past the cylinder block, which solves the problem of city driving. The cylinders on air cooled bikes are designed with fins (heat sinks) to aid in this process. Air cooled bikes are cheaper, simpler and lighter than their water-cooled counterparts.
[edit] Oil
Some manufacturers use a hybrid cooling method where engine oil is circulated between the engine case and a small radiator. Here the oil doubles as cooling liquid, prompting the name "oil-cooling." Suzuki has produced many "oil-cooled" motorcycles. Modern BMW R-series flat-twin motorcycles, such as the R1150GS, use air and oil cooling.
[edit] Two stroke
Many motorcyclists[who?] over the years have been convinced that two-stroke engines were better suited to motorcycles than four-strokes: they are mechanically simple, easier to cold start, and when operating at their best produce significantly more power from lighter mechanicals, having twice as many powerstrokes at the same engine revolutions. Their fuel inefficiency, in their carburated versions, seemed a small price to pay. However, they have been largely replaced (actually, outlawed) in developed nations due to the environmental disadvantages, and have largely been relegated to small scooters and step-thru's carburated.
A four-stroke powerband is of broader range than a two-stroke, making such machines easier to control. However modern two-stroke engines, or at least those powering dirt/road race bikes and the last of the 'hot' street bikes, have some form of exhaust power-valve system providing a similar powerband range. Also, in a large capacity two-stroke machine (usually over 300 cc) there is significantly more bottom end power. meaning that a two-stroke can provide good power similar to low revs on a four-stroke before going into a two-stroke's top end and powerband, giving a more controllable and predictable ride.
Almost all modern two-stroke bikes are single-cylinder, water-cooled, and 250 cc or under (although many two-stroke enduro bikes are larger then 250 cc, most ones being around 300 cc). In Europe and Asia there are many 125 cc two-stroke street bikes, in some cities these are the most widely used form of personal-transportation. Enduro (offroad) motorcycles in Europe are generally powered by 125cc or 250cc two-stroke motors. Most mopeds have 50 cc two-stroke engines though some jurisdictions allow sizes up to 180 cc or there about for underbones.
[edit] Diesel
Only very small numbers of diesel engined motorcycles have ever been built. The improved fuel efficiency is offset by the increased weight, reduced acceleration and potential difficulty of starting, at least in colder climates. Enfield India built a few from 1965 onwards but is no longer doing so.[2] In November 2006, the Dutch company E.V.A. Products BV Holland announced their first diesel-powered motorcycle, its Track T-800CDI, using an 800 cc three-cylinder Daimler Chrysler diesel engine.[3].
Several armies are moving to an all-diesel engine fleet to reduce the fire risks of petrol and the need to provide two different fuels. This includes their despatch riders as well, encouraging the market for diesel motorcycles. Interest in biofuels is also likely to encourage future developments for small Diesels.
Diesels are also available in both two and four-stroke versions.
[edit] Valve control
Honda equipped the CBR400F with HYPER VTEC (or REV:Revolution-modulated valve control) in 1983. The system enabled to switch over the number of valve operations per cylinder between low and medium speed revolution range and high speed revolution range. In January 2002 HYPER VTEC evolved into Spec II and in December 2003 SPEC III was introduced.
[edit] Other components
Fuel injection and computer engine management systems are now normal on middle range and larger motorcycles and are increasingly being incorporated onto the smaller machines, partly driven by better emission control and lower maintenance but mostly by manufacturing cost considerations. Ignition systems moved from magneto in the 1950s to battery-coil-contact breaker (points), and these were increasingly superseded by Capacitor Discharge Ignition (CDI) from the 1980s. Small, single cylinder motorcycles abandoned the Flywheel Magneto system with contact breakers to similar flywheel driven solid-state systems at about the same time.
- Turbo and Superchargers. Superchargers (blowers) were common in the GP's, until they were banned (which didn't help the two-stokes, as pre Ernest Degners new technology, they needed the help against the four-strokes). The big four also made a turbo-ed bike, Honda made two......, mainly as an exercise in technical expertise and later discontinued for more conventional methods. Bolt on (well nearly) blowers are available to put on street bikes - and they are essential for drag bikes and land speed record streamliners etc. Most sports bikes now use some sort of 'ram-air' system where, as road speed increases, more and more air is forced through ducts in the fairing to pressurise the airbox - not to be confused with the original Ram-Air system where an air scoop/cowl was fitted to the top of the cylinder heads of Suzukis' two-strokes to aid cooling.
[edit] References
- ^ [1] V-twin Design Engineering (Retrieved 26 November 2006)
- ^ "Diesel motorbikes". Journey to Forever. http://journeytoforever.org/biodiesel_bikes.html. Retrieved 2007-06-28.
- ^ "The first commercially-available diesel motorcycle". Gizmag.com. November 20, 2006). http://www.gizmag.com/go/6493/. Retrieved 2007-06-28.
