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 which is external to the engine (though sometimes invisible under a cover). The scooter type of engine is further back, has an integrated final drive driving a small rear wheel.

Contents 1 Types 2 Displacement 3 Number of cylinders 3.1 Single 3.2 Two 3.3 Three 3.4 Four 3.5 Five 3.6 Six 3.7 Eight 4 Engine cooling 4.1 Liquid 4.2 Air 4.3 Oil 5 Two stroke 6 Diesel 7 Valve control 8 Other components 9 References

[edit] Types

Almost all commercially available motorcycles are driven by conventional gasoline internal combustion engines, increasingly four-strokes in all size ranges. Most are still air-cooled (forced with a fan in some cases) but water-cooling is becoming increasingly popular. 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. Two manufacturers in the 1980s produced quite small numbers of motorcycles propelled by Wankel rotary engines, but these were neither clean, nor economical nor particularly reliable.

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. The engines of the motorcycles known as underbones or "step-throughs" may be of either kind.

[edit] Displacement

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 (cubic centimetres), 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 50cc, and a moped as a vehicle with an engine smaller than 60cc.

[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

One-cylinder motorcycles 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.

[edit] Two

Two-cylinder motorcycles are known as "twins." The three most common arrangements are:

1. 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.
2. The "V-twin" where the cylinders form a "V" around the crankshaft, which is oriented transversely (i.e., perpendicular to the direction of travel).^[1]
3. 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 parallel twin engine configuration was made famous by Edward Turner's Triumph Speed Twin design as used on the Triumph Bonneville.

In the BMW^[2] flat-twin ("boxer twin") engine, and used as well now by the Ural^[3] and historically by Douglas^[4], 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 unusual — they are referred to as "triples" and are normally inline triples in layout. The British Hinckley-built Triumph, Italian Benelli and Japanese Yamaha are three motorcycle manufacturers who have used triples in their large displacement motorcycles. The Italian firm Laverda made a few 1,000cc and 1,200cc triples.

Two-stroke triples were somewhat more common historically. Kawasaki produced 250, 350, 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). Honda produced a water cooled V-3 two-stroke.

[edit] Four

Four-cylinder engines are colloquially known as "four-bangers." They are quite similar to car engines and most commonly have a transverse-mounted inline four layout, although some are longitudinal (as in the earlier BMW K series). V-4 and boxer designs (as in the Honda Gold Wing series) 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. Yamaha and Suzuki used the same concept in their water-cooled two-stroke engines (RZ500 and RG500 respectively).

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 a five-cylinder engine for racing, the RCV, but no five-cylinders exist 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 and the Kawasaki KZ1300. The six cylinder engine is currently used in the boxer of the Honda Valkyrie series and Honda Gold Wing.

[edit] Eight

Galbusera built a V8 in 1938, and Moto Guzzi experimented over a period of two years with its dual overhead cam500cc 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 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 750cc 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.

[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.

[edit] Two stroke

Many motorcyclists 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 in the larger displacements and in developed nations due to the environmental disadvantages 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 bikes, have some form of exhaust power-valve system providing a similar powerband range.

Almost all modern two-stroke bikes are single-cylinder, water-cooled, and under 500cc. In Europe and Asia there are many 125cc 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 50cc two-stroke engines though some jurisdictions allow sizes up to 180cc 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.^[5] In November 2006, the Dutch company E.V.A. Products BV Holland announced that the first commercially available diesel-powered motorcycle, its Track T-800CDI, achieved production status using an 800 cc three-cylinder Daimler Chrysler diesel engine.^[6]

[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.

[edit] References