Types
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Types
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. 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.
Number of cylinders
Motorcycles have mostly, but not exclusively, been produced with one to four cylinders, and designers have tried every imaginable layout.
Single
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 with the 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
Two-cylinder motorcycles are called "twins." The three most common arrangements are
- The "V-twin" where the cylinders form a "V" around the crankshaft, which is oriented transversely (i.e., perpendicular to the direction of travel).
- The inline twin or straight-two, which is common in classic British motorcycles and Japanese motorcycles. It is 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 maximize airflow cooling.
- 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.
