Motorcycle engine

A motorcycle engine propels the motorcycle. The motor typically sits immediately under the fuel tank, in between and just forward of the rider's legs.

Types of Engine

Four Stroke Engine

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Engine configuration

Almost all commercially available motorcycles are driven by conventional gasoline internal combustion engines, but some small scooter-type models use an electric motor.

Gasoline engines can be compared by many criteria.

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. To the layperson 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 6000 cc engine used by Boss Hoss in its cruiser style motorcycle BHC-3 LS2. Many state laws in the US define a motorcycle as having an engine larger than 50cc, and a moped as a vehicle with an engine smaller than 50cc.

Number of cylinders

Motorcycles have mostly, but not exclusively, been produced with one to four cylinders, and designers have tried every imaginable layout. Engines with more cylinders provide more power for the same displacement, and feel smoother to ride. Engines with fewer cylinders are cheaper, lighter and easier to maintain.

Single cylinder

One-cylinder motorcycles are known as "singles," and in larger capacities as "thumpers" (for the sound they make). In some singles, the cylinder points up and slightly forward, spark plug on top but the most common arrangement is a horizontal cylinder such as used by Honda in its C series singles, the highest volume motorcycle of all time with over 40 million units. They are most common in all configurations..

Two cylinders

Two-cylinder motorcycles are called "twins." The two most common arrangements are

1. the "V-twin" where the cylinders form a "V" around the crankshaft, which is oriented transversely (i.e., perpendicular to the direction of travel).
2. also common especially in classic British motorcycles and Japanese motorcycles, is the inline twin or straight-two, known as a parallel twin when the cylinders share a common crank pin. In this design the cylinders are side by side vertically above the crankcase. If not vertical they are generally nearly so in order to maximise airflow cooling.

BMW's famous opposed twin

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 due to 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 famous BMW flat-twin ("boxer twin") engine, 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. Narrow angle V twins vibrate. Sunbeam produced an air cooled inline twin driving a propshaft.

Narrow-angle V-twin engines dominate the cruiser motorcycle segment.

Three cylinders

Three-cylinder designs are unusual — they are referred to as "triples" and are normally inline triples in layout. The British Hinkley built Triumph and Italian Benelli as well as Japanese Yamaha are three motorcycle manufacturers who have used triples in their large displacement motorcycles. The Italian firm Laverda was also renowned for their 1000cc and 1200cc triples.

On the other hand, in the two-stroke world, triples were more common. In the 1970s Kawasaki had its 250, 350, 500, and 750 triples which were known for their power (but maybe not rideability) and Suzuki had 380, 550, and 750 triples of which the last one was water cooled and thus gained the nickname "Water Buffalo" or "Kettle". All the others were air cooled. Honda also produced a water cooled V-3 two-stroke.

Four cylinders

The Honda GL1000 flat-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.

Five cylinders

Honda has produced a five-cylinder engine for racing, the RCV, but no five-cylinders exist for commercial production motorcycles.

Six cylinders

Six cylinder engines are uncommon, and usually found only on the biggest motorcycles. Two of the best six cylinder examples are the Honda CBX and the Kawasaki KZ1300. Nowadays the most famous six cylinder engine is the boxer used on the Honda Valkyrie series and Honda Gold Wing.

More than six cylinders

Galbusera built their V8 in 1938, and Moto Guzzi experimented over a period of two years with their dual overhead cam 500cc V8 (The Otto Cylindri) in the 1950's. A number of custom and one-off motorcycles use more than six cylinders. For example, the Boss Hoss motorcycle uses a Chevy V-8 motor(5700 and 6000 cc). In the mid 90's 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. But no major motorcycle manufacturer uses more than six cylinders.

Cooling: Water, Air or Oil

Water

Water-cooled motorcycles have a radiator (exactly like 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 dirt oriented motorcycles have no radiator fan, and rely on air flowing over the radiators from the forward motion of the motorcycle, many road going motorcycles have a small fan attached to the radiator which is controlled by a thermostat. The cooling effect of this fan is enough to prevent the engine overheating in most conditions, so water-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 water-cooling for most motorcycles. Even Harley-Davidson, a strong advocate of air-cooled motors, has begun producing a Revolution water-cooled engine.]

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.

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.

Four stroke or Two stroke

Stock four-strokes are cleaner then carburated two-strokes; provided the four-stroke hasn't worn to the point where it starts 'burning oil' (e.g. carter oil entering the combustion chamber, either from leaking valves, head gasket, cylinder-wall/piston or a combination thereof). Some are more reliable as their RPMs can be kept comparatively low. A for-strokes powerband is of broader range then a two-stroke, making such machines easier to control. However modern two-stroke engines, or at-least dirt bikes, have some form of exhaust power-value system providing a similar powerband range.

As applied to motorcycles, two stroke engines have some advantages over equivalent four-strokes: they are lighter, mechanically much simpler (therefor cheaper to build and rebuild), and produce more then twice the power at the same weight (due its forced induction), when operating at their best. In the USA, two-stroke road-bikes are rare, because modifying them to meet contemporary emissions standards has been prohibitively expensive -- although this is debatable. Envirofit has developed a retrofit full-injection system for existing 125cc engines though.

Almost all modern two-strokes bikes are single-cylinder, air-cooled, and under 600cc. In Europe there are a lot of water cooled 125cc two-strokes street and 125-250cc offroad motorcycles. Also most mopeds and scooters have 50cc two-strokes engines, often bored to 60-80cc with widely available kits.

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.

Other components

Fuel injection and computer engine management systems are now usual on large and expensive bikes, and starting to propagate to smaller bikes, driven by increased demands for emission control and improved performance. Capacitor Discharge Ignition (CDI) is now standard on all bikes, as opposed to the previous technology, magnetos.