True V-twin vs V-2
There is some ambiguity regarding what defines a V-twin which arose from 180 degree V engines (see Flat engine ). When the cylinders are arranged in a V pattern, it is generally accepted as being a V engine. However, some people differentiate using V-twin to mean the classic V design of a shared crank pin for each pair of cylinders and V-2 to indicate a boxer -style crank with a separate crank pin for each cylinder. This convention is not common, and applies only to two cylinder engines.
For example, while most V8 engines use shared crank pins for cylinder pairs, they are not referred to as V-octuplets. Most engine manufacturers do not use this convention and the 1983 Honda Shadow 750 is known as being the first V-twin with an offset-dual-pin crankshaft. [ 1 ]
Generally, any two-cylinder motorcycle engine with its two cylinders at an equidistant opposite angles from the center rotation of the crankshaft is referred to as a V-twin. The V-twin engine is not designated by a specific angle, but people associate the 45° Harley-Davidson engine with the classic V-twin configuration. While any motorcycle can have a v-twin engine, cruiser style motorcycles most commonly use this configuration.
For example, Honda offers a 52° engine in several of their cruiser motorcycles. But other angles can be seen such as the 75° Suzuki and KTM. the 80° Honda CX-500. the 47° Vincent. and the 60° Aprilia. The engine manufacturer SS offers a 45° and a 56.25° engine for use in custom choppers.
A 90° twin engine, such as the signature Ducati engine and recent Aprilia engines — with the front cylinder approximately parallel to the ground and the rear cylinder vertical — is generally referred to as a L-twin engine. There is no technical distinction between V-twin and L-Twin engines. These are merely names used by convention.
From an engineering perspective, the 90° angle is preferred as it acts as a counterweight to balance the engine.
The terms longitudinal engine and transverse engine are most often used to refer to the crankshaft orientation, [ 2 ] [ 3 ] however, some sources, most prominently Moto Guzzi. use the terminology in the opposite way.
A Moto Guzzi Technical Services representative tried to explain to LA Times columnist Susan Carpenter that Moto Guzzi engines are called ‘transverse’ because the engine is mounted with the crankshaft oriented front to back instead of left to right. [ 4 ] In spite of this, it it could be assumed that those who call V-twin motorcycle engines transverse when they are mounted with the crankshaft front-to-back and the cylinders sticking out the sides are saying that to them, the engine’s axis is the line passing from one cylinder to the other, at a right angle to the crankshaft, rather than going by the crankshaft’s axis. [ 5 ] [ 6 ] [ 7 ] [ 8 ] [ 9 ] [ 10 ] [ 11 ] Highly technical sources, such as V. Cossleter’s Motorcycle Dynamics . [ 2 ] or Gaetaeno Cocco’s Motorcycle Design and Technology [ 12 ] are careful to not simply use the terms longitudinal engine or transverse engine, but rather to specify that they mark the engine’s orientation based on the crankshaft, and so they will say transverse crankshaft engine or longitudinal crankshaft engine.
Transverse crankshaft mounting
The engine can be mounted in transverse crankshaft position as on Harley-Davidsons. Ducatis and many recent Japanese motorcycles. This transverse position gives the motorcycle a reduced frontal area. The main disadvantage of this configuration is that the rear cylinder and the front cylinder will receive different air-flows making air cooling somewhat problematic especially for the rear cylinder.
Cooling problems are somewhat mitigated by having all four sides of each cylinder exposed to air flow. This differs from a parallel-twin cylinder engine which has a distinct front, back, and sides, but the inside of each cylinder is not exposed to airflow as the cylinders are typically joined together with a cam chain running up through the block in-between the cylinders.
Some cooling strategies of transverse-crankshaft 90° V-twins
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