The vast majority of motorcycles are powered by some kind of reciprocating piston internal combustion engine. Each of these pistons is contained within a cylinder. The number of cylinders and their orientation varies, and these variations yield the different engine types we see in use. Generally speaking, the design, layout, and number of cylinders a motorcycle engine has is meant to help reduce vibrations and allow the engine to run more smoothly, to fit into the frame a certain way to achieve specific motorcycle dimensions or weight distribution, or a combination thereof. Each engine layout has its own characteristics, benefits, drawbacks, and ideal applications. Let's jump in and get the low-down on the common engine types used in motorcycles.

Single Cylinder

The most basic of engine designs is the single cylinder engine. Also known as "thumpers," single cylinder engines are remarkably simple and compact, as well as inexpensive to produce. They're typically smaller displacement and used in lower-powered motorcycles, though manufacturers like KTM and Husqvarna produce single-cylinder engines with crazy power output for their size. They are predominantly air-cooled, since air can easily flow around the cylinder, though water-cooled varieties exist. Power delivery can be a bit more uneven, resulting in significant vibration, especially at higher revolutions, and require heavier flywheels or counterbalancing to reduce vibration. They are ideal for applications where simplicity is important, such as off-road, dual-sport, and enduro style motorcycles as well as street bikes derived from those.

Once we get into multiple cylinder engines, there are basically three engine configurations; inline or straight engines, V engines, and flat engines.

Let's start with the straight/inline engines:

Parallel-Twin

Also known as straight-twin, inline-twin, and vertical-twin (not to be confused with V-Twin), Parallel-Twin engines are generally compact for their displacement. Since their cylinders share a common crankshaft, and there’s typically only one cylinder head for both pistons, they’re mechanically almost as simple as singles, making them inexpensive to manufacture. Vibration is still a  problem without significant counterbalancing or alterations made to the crankshaft firing order. Parallel-Twin engines are found in many street motorcycles from British and Japanese manufacturers, ranging from inexpensive beginner-friendly bikes like the Ninja 400, to high-end roadsters like the Triumph Thruxton.

Triple

Also known as straight-three or Inline-Three engines, Triples sit between Parallel-Twin and Inline-Four engines. They are smaller than Inline-Fours, but the additional cylinder over a Parallel Twin allows them to be more balanced, and prone to less vibration, thus smoother. The additional cylinder adds complexity, but the trade off for more power and smoothness over a Twin is usually worth it. Triples are fairly popular engines among naked roadsters from Triumph (Speed/Street Triple) and Yamaha (MT-09/XSR 900), and even the Triumph Tiger adventure motorcycle where the crankshaft design and firing interval is such to give both a smooth ride for on-road use and better rider feedback for off-road riding.

Inline Four

Also known as a straight-four, Inline-Four engines have all four cylinders mounted in a straight line across the crankshaft. This maintains a degree of mechanical simplicity, such as only needing a single cylinder head and exhaust manifold for the entire engine, which makes them relatively easy and inexpensive to manufacture. Inline-Fours are also virtually in perfect balance, and since they do not need the heavy counterweights that might be necessary to reduce vibrations in one, two, or three cylinder engines, they can be made so that they rev up and down very quickly, allowing them to be able to deliver tremendous power for their size. This makes them great for use in sport bikes where quick response and power delivery is desired.

Straight Six

The biggest and most complex of the inline engines, the Straight Six builds on the Inline Four, with the additional cylinders further balancing the engine (see the trend: more cylinders = better balance and smoothness), creating an engine with perfect primary and secondary balance. That said, Straight Six engines are physically larger, more complex, and more expensive to manufacture than their smaller brethren, and are generally limited to large tourers, like the BMW K1600.

Let's move on to the V engines:

V-Twin

The quintessential American motorcycle engine. The cylinders in a V-Twin share a common crankshaft, and are arranged in their eponymous "V" shape. The angle of the "V" varies by design and manufacturer, with the most famous design, that from Harley-Davidson, positioning the cylinders at a 45º angle from each other. The V configuration reduces the overall engine size and weight compared to a Parallel-Twin of the same displacement. Many V-Twin motorcycle engines are placed longitudinally (relative to the dimensions of the engine, not the crankshaft) in the frame which does have the drawback of the rear cylinder receiving less air for cooling than the front cylinder. Ducati and Suzuki get around this with a 90º Twin that is tipped forward, and others like Honda with their CX500, and Moto Guzzi with their - well - everything, literally go the other direction and use a transverse mounting, so the cylinders stick out from the sides of the frame. V-Twins are the standard for cruisers, but can also be seen in sportier or more standard attire, like those from Ducati, Suzuki’s SV650, and Moto Guzzi.

V-Four

The V-Four engine shares some similarities with both V-Twins and Inline-Fours. Like an Inline-Four, its design is capable of being in balance to reduce vibrations. Like a V-Twin, the "V" configuration allows for a shorter overall length, and thus a more compact engine compared to a similarly sized straight engine. The "V" configuration lends some additional mechanical complexity and cost, with a second cylinder head and exhaust manifold being required. Like an Inline-Four, the engine's balance allows for a high-revving engine, perfect for sport and race bikes, to such an extent that many MotoGP teams have replaced their Inline-Four engines with more compact V-Fours.

The last of the common engine configurations used in motorcycles is the Flat-Twin, or Boxer engine:

Flat-Twin

Also known as horizontally-opposed Twins or Boxer Twins, Flat-Twins are an interesting engine configuration that places the cylinders at 180º degrees from each other, on opposite sides of the crankshaft, and each piston moves in and out simultaneously. Given the even firing order and the fact that the forces of the piston movements essentially cancel each other out, this produces excellent engine balance. A longitudinal mounting for Flat-Twins is typical, which places the cylinder heads sticking out the sides of the frame. With a longitudinal mounting, the cylinder heads are equally exposed to the air, and thus evenly cooled, and it also provides for a low center-of-gravity for smooth and easy handling. The drawback is that the exposed cylinder heads are susceptible to scraping the ground during hard cornering (or crashes) as well as to debris and obstacles while riding. Boxer Twins are almost exclusively found in BMW and Ural motorcycles.

Flat Four and Flat Six

We'll be lumping these together because there's only one major motorcycle that uses them, the Honda Goldwing, and the only significant difference (beyond the internal changes, updates, and design improvements Honda has made over the years) is the number of cylinders. In keeping with the trend from other engine configurations, the benefits of additional cylinders in flat engines is additional balance, and thus an engine that is smoother running with less vibration. Thus, the Flat-Six is a smoother engine with less vibration than the Flat-Four. The downside, of course, is additional complexity and cost. Since the Flat-Four and Flat-Six engines are smooth-running, powerful, large, heavy, complex and expensive, they're best suited for a heavy luxury tourer, like the Goldwing.