Motorcycle engines can be compared by many criteria.
Displacement is defined as the total volume of air/fuel mixture an engine can draw in during one complete motorcycle 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 average person this is the "size" of the engine. Motorcycle engines range from less than 50 cc (cubic centimeters), commonly found in many mopeds and small scooters, to 8200cc, a Chevrolet V8 engine.
Number of cylinders
Motorcycle engines 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.
One-cylinder motorcycle engines 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-cylinder motorcycle engines are called "twins." The two most common arrangements are
- "V-twin" where the cylinders form a "V" around the crankshaft, which is oriented transversely (i.e., perpendicular to the direction of travel).
- Common in classic British motorcycles and Japanese motorcycles, is the inline twin also known as the parallel twin. In this design the cylinders are side by side vertically above the crankcase. If not vertical they are generally nearly so in order to maximize airflow cooling.
BMW’s opposed twin:
The angle in the V-twin engines varies from around 45 degrees to 90 degrees. Typical of the 45 degree are the Harley Davidson and Vincent engines which due to their firing order tend to vibrate more which give them their distinctive feel and sound. Ducati and Moto Guzzi make V-twin engines with cylinders arranged at a 90 degree angle to minimize 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 is inherently balanced, producing very low vibration levels without the use of counterbalance shafts. The 90-degree Vee, like Ducati and Moto Guzzi use, has perfectly counterbalancing primary forces and minor secondary imbalances acting in a plane perpendicular to the centerline of the Vee as viewed from the side. That also produces very low vibration levels. Narrow angle V twins vibrate. Sunbeam produced an air cooled inline twin driving a prop shaft.
Narrow-angle V-twin engines dominate the cruiser and chopper motorcycle world.
Three-cylinder engine 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-cylinder motorcycle engines are 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 used the same concept in its water-cooled RZ500 two-stroke.
Since the advent of Honda's CB750 straight-four engine, straight-fours have dominated the non-cruiser street motorcycle segments.
Honda has produced a five-cylinder engine for racing, the RCV, but no five-cylinders exist for commercial production motorcycles.
Six cylinder motorcycle 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 KZ 1300. Nowadays the most famous six cylinder engine is the boxer used on the Honda Valkyrie series and Honda Gold Wing.
More than six cylinders
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. But no major motorcycle manufacturer uses more than six cylinders. In the mid 90's Daimler-Chrysler manufactured a limited number of Tomahawk concept bikes featuring a Dodge Viper's V-10 engine.
Cooling: Water, Air or Oil
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. The radiator has a small fan attached to it 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-cooled motorcycles have no "cooling system," as such. As air blows past the engine case, it disperses heat. The cylinders on these bikes are designed with heat sinks (fins) to aid in this process. Air cooled bikes are cheaper, simpler and lighter than their water-cooled counterparts, but unless the ambient temperature is cold, they may overheat if the bike stands still, as in traffic.
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 vs. Two stroke
As applied to motorcycles, two stroke engines have some advantages over equivalent four-strokes: they are lighter, mechanically much simpler, and produce more power when operating at their best. But four-strokes are cleaner, more reliable, and deliver power over a much broader range of engine speeds. They use the Otto Cycle: Induction-Compression-Power-Exhaust. In developed countries, two-stroke road-bikes are rare, because - in addition to the reasons above - modifying them to meet contemporary emissions standards is prohibitively expensive. Almost all modern two-strokes are single-cylinder, air-cooled, and under 600 cc.
In Europe there are a lot of water cooled 125 cc two-strokes and off road motorcycles that are also two-strokes with water-cooling.
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.
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 Intake (CDI) is now standard on all bikes, as opposed to the previous technology, magnetos.
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