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A multi-cylinder engine is a reciprocating internal combustion engine with multiple cylinders. It can be either a 2-stroke or 4-stroke engine, and can be either Diesel or spark-ignition. The cylinders and the crankshaft which is driven by and co-ordinates the motion of the pistons can be configured in a wide variety of ways. Multi-cylinder engines offer a number of advantages over single-cylinder engines, chiefly with their ability to neutralize imbalances by having corresponding mechanisms moving in opposing directions during the operation of the engine.
A multiple-cylinder engine is also capable of delivering higher revolutions per minute (RPM) than a single-cylinder engine of equal displacement. This is true for two reasons. First of all, the stroke of the pistons is reduced. This decreases the distance necessary for a piston to travel back and forth per each rotation of the crankshaft, and thus limiting the piston speed for a given RPM. Secondly, in an engine with multiple cylinders, the piston mass is reduced. This reduces stress on internal components at higher RPM's. Typically, the more cylinders an engine has, the higher the RPM's it can attain for a given displacement and technology level, at a cost of increased friction losses and complexity. Peak torque is also reduced, but the total horsepower is increased due to the higher RPM's attained.
Although there are 1, 3 and 5-cylinder engines, almost all other inline engines are built with even numbers of cylinders, as it's easier to balance out the mechanical vibrations. Another form of multiple-cylinder internal combustion engine is the radial engine, with cylinders arranged in a star pattern around a central crankshaft. Radial engines are most commonly used as aircraft engines, and in basic single-row configuration are built with odd numbers of cylinders (from 3 to 9). An odd number of cylinders is necessary in a four stroke radial, since the firing order is such that every other cylinder fires as the crankshaft rotates. Only with an odd number of cylinders will all cylinders evenly fire in this manner in two crankshaft revolutions (first the odd cylinders, followed by the even cylinders). "Twin-row" or "multi-row" radials are also built, which is basically two or more single-row radials connected front-to-back and driving a common crankshaft. In this "twin row", or "multi-row" configuration, the total number of cylinders will be an even number, although each row still has an odd number. For example, a typical single row radial such as the Wright Cyclone has 9 cylinders. The twin row Wright Twin Cyclone is based on this engine and thus has two banks of 9 cylinders, for a total of 18, an even number.
Configurations of two-cylinder engines include:
- Straight-twin engine, or "parallel twin" with two cylinders in parallel sharing a crankshaft.
- V-twin engine, or "V2" engine, two cylinders in V configuration, sharing a crankshaft.
- Flat-twin engine, a flat engine with 2 opposed cylinders on the same crankshaft angled 180° to each other.
Configurations of three-cylinder engines include:
- Straight-three engine, also called the inline-triple, inline-3, or I-3 engine, the most common three-cylinder internal combustion engine.
- V3 engine, seen on some two-stroke racing motorcycles.
- W engine with three cylinders (W3), such as the Anzani 3-cylinder fan engines, ("W"and radial three-cylinder engines of 1905 to 1915, used in the Blériot XI which first flew the English Channel among other applications).
Configurations of four-cylinder engines include: the most common 4-cylinder engine
- Inline-four engine, an engine with 4 cylinders in a straight line.
- V4 engine, an engine with 4 cylinders arranged in a V configuration.
- Flat-four engine, a flat engine with 4 cylinders horizontally opposed to each other at 180° apart.
- Square four engine (U4 engine), two banks of 2 cylinders, each driving a single crankshaft and geared to a common output shaft.
Configurations of five-cylinder engines include:
- Straight-five engine, an engine with 5 cylinders in a straight line.
- VR5 engine, an engine with 5 cylinders staggered slightly, allowing cylinder bore centerlines to be closer together, reducing overall engine length.
Configurations of six-cylinder engines include:
- V6 engine, a V engine with six cylinders.
- VR6 engine, a compromise between a V6 and straight-6 engine, shorter than a straight-6 and narrower than a V-6.
- Straight-six engine, an engine with six cylinders aligned in a straight line.
- Flat-six engine, a flat engine with two banks of 3 cylinders horizontally opposed at 180° apart.
Configurations of eight-cylinder engines include:
- V8 engine, an engine with 8 cylinders arranged in a V configuration.
- Straight-eight engine, an engine with 8 cylinders in a straight line.
- Flat-eight engine, a flat engine with 8 cylinders horizontally opposed to each other at 180° apart.
- W8 engine, an engine with 8 cylinders in four banks of 2, arranged in a "W" configuration.
Configurations of ten-cylinder engines include:
- V10 engine, a V-engine with ten cylinders.
- Straight-10 engine, an engine with ten cylinders aligned in a single row.
Configurations of twelve-cylinder engines include:
- Straight-12 engine, a straight engine with twelve cylinders.
- V12 engine, a V engine with twelve cylinders.
- Flat-twelve engine, a flat engine with twelve cylinders.
- W12 engine, a W engine with twelve cylinders in either four groups of 3, or three groups of 4.
- Victor Albert Walter Hillier, Peter Coombes, Hillier's Fundamentals of Motor Vehicle Technology, Book 1 (2004), p. 47.