Motorcycle Engine Types Explained
An explantion of the many powerplant variations
The relative freedom of space and layout when designing a motorbike lends itself to there being plenty of piston layouts in use.
Inline (straight) engines
Just one piston, producing power a quarter of the time. A single-cylinder engine produces the most power for the space it takes up of any engine type, but is naturally prone to harsh vibrations which need to be counterbalanced. As basic and cheap as you can get, which is why they're also used in things like lawnmowers, and remote control cars...
Similarly to single-cylinder engines, parallel-twins have the entire weight of the pistons moving up and down at once, so they suffer from the same issues with vibration. However, because there are two pistons firing at opposite times, the power delivery is evened out. Parallel-twins are used extensively by classic British motorcycle manufacturers such as Triumph, BSA and Royal Enfield.
The first of the sort-of balanced engines, an inline-triple is balanced around the crankshaft, but does rock end-to-end slightly. This makes it smoother than the parallel-twin, while still being shorter than an inline-four, making it easier to fit into the compact space of a motorcycle. The inline-triple is most likely to be encountered in a modern Triumph motorcycle, as well as some small economy cars.
This is the cylinder layout most likely to be found in a car, which is important for us as it means that lots of the R+D done by car companies transfers over. The inline-four has the same perfect primary balance as the inline-three, but without the rocking motion, making it smoother. The inline-four was first installed in a motorcycle in 1905 and to this day is the most common engine type in non-cruiser motorcycles.
The inline-six is the first engine type to have completely perfect balance and as such is known for legendary smoothness, and is a precursor to the V6 popularly used in automobiles. The straight-six was raced by Honda in the RC series of motorcycles, the first of which was just 249cc, making each combustion chamber just 41.5ml. For reference, a shot glass is 44ml.
V (Vee) layout engines
To follow the V-layout terminology, the V-twin has two cylinders arranged into two banks of one. Unlike the parallel-twin, the v-twin has good natural balance but also an uneven firing order, giving them an attractive "burbling" exhaust sound at the expense of uneven power delivery which can make a motorbike less stable through fast corners. Ducati's "L-twin" engine is counted as a V-twin, just sort of turned over slightly to resemble an "L" rather than a "V". Moto Guzzi famously mount V-twins transversely which avoids issues of keeping the trailing cylinder cool.
The V4 is a shorter alternative to the inline-four that comes with many of the same advantages. Power delivery is smoother than a V-twin due simply to the engine making power more of the time. Being basically two V-twins stuck together, it is also more compact than the long inline-four, making it easier to fit into the motorcycle frame. The downside is high cost of design and manufacture. Having two banks of cylinders means having to have two intake systems, exhaust lines, cylinder heads etc compared to the inline-four. The V4 is most commonly employed by Honda who mount them transversely.
Most people would consider eight cylinders to be too many even in a car nowadays, let alone a motorcycle. V8s are separated into single and cross plane types. A single plane V8 is just two inline-fours sharing a crankshaft. This is really cheap and simple to make but suffers from the same imperfect balance as the inline-fours do. The cross-plane type solves the balance issues of the inline-fours, but requires internal counterweights which slow the engine revving up and down.
Flat (Boxer) engines
The flat-layout engines are commonly referred to as the "boxer" engine layouts because the pairs of pistons look like a boxer's gloves moving together and apart at the same time. The flat-two has the same even firing rate as the parallel-two, but doesn't suffer from the same issue of poor natural balance, as the two pistons are always moving opposed to each other. The flat-two is most often installed on BMWs including the BMW R1200RT sport-touring bike ridden by the UK police.
Compared to an inline-four, the flat four doesn't suffer from the same vibration issues due to retaining the perfect power delivery rate and improving on the smoothness. It doesn't have the same R+D poured into it from the automotive world, but some very popular cars such as the VW Beetle employed an inline-four despite being more expensive to develop and manufacture than the inline-four. Honda began using a 1000cc flat-four to introduce their category-defining Gold Wing lineage of bikes in 1975.
The flat-six has the same perfect balance and smoothness of the inline-six, while making the block shorter and wider, thus more able to easily fit into a motorcycle frame. Sticking with Honda, the flat-six was introduced to the Gold Wing in 1988 – a model which stayed in production for 13 years. To this day, you're still most likely to find one in a Honda Gold Wing or the spin-off, the Honda Valkyrie.
Wankel rotaryView Wankel rotary bikes
Never quite taken seriously, and not just because of the inventor's name. Felix Wankel conceived the idea of an internal combustion engine with no pistons in the late 1920s, but it wasn't until 1957 that a working prototype would be constructed. The design of the rotary engine hasn't had the same massive amount of resources devoted to it as piston engines, and as a result suffers from issues with reliability, low torque output, poor fuel economy and even worse emissions. On the other hand, they generally produce more power-to-weight and power-to-space than piston engines. The only decent production run was the Norton Commander which ceased in 1992 and was the last time a rotary-powered motorbike has gone on sale to the public.
ElectricView Electric bikes
From toothbrushes to Teslas, electric motors are in most things these days and motorcycles are no exception. With exceptions, electric bikes usually don't require a gearbox or therefore a clutch, but any weight saved there is surely taken up by the armada of batteries required to get any sort of range. This could possibly be improving in the future as lots of money is being spent on electric automobiles for which the technology is likely to be transferable. Already, electric bikes are in a different league in terms of running costs with some governments offering subsidies for purchase and tax exemptions, on top of a "filling" cost-per-mile equivalent to a petrol bike that gets about 400MPG.