Motorcycle Engine Types
In the more than 100-year history of motorcycle design there have been a dozen or more engine types. But as with most technology, those that didn't meet the tests of cost-effectiveness and customer acceptance have fallen away. Here are some basic facts and a few of the winners. Except for some scooters and mopeds, motorcycle engines are of the internal combustion type, and what they combust is gasoline. Gasoline and air are sucked or injected into a chamber, called the cylinder, when a round piston on a rod moves downward. The piston moves part way back up the cylinder, compressing the gas-air mixture and a spark plug produces a spark across a small gap between two metal prongs. That spark ignites the fuel and, as it burns, the product gases expand explosively, pushing the piston back down. The force is transferred via the crankshaft and/or chain or belt to the rear wheel and the bike moves forward. How many cylinders there are -- and how they're arranged -- determines the basic engine types. Single Cylinder Just what the name says, this type has a single chamber with one piston. Used almost exclusively in off-road bikes, they produce loads of torque at lower rpm (revolutions per minute). Unfortunately, since there's nothing to counter-balance the movement of the single piston, they also produce a lot of vibration. The sound produced is something only a motocross fan could love. Twin Cylinder The dual cylinder type comes in a variety of configurations, but most today are V-shaped - the pistons are angled at 90 degrees (there are several exceptions). They produce more power than single cylinder engines, owing to the greater displacement and amount of fuel combusted. Displacement is, roughly, the volume of the cylinder above the piston. They also tend to vibrate much less because, as the number of cylinders increases, the more forces in a particular direction are offset. Some variations involve putting the cylinders side-by-side or opposed. The latter is called a 'Boxed' or 'Boxer' design and they produce good power with low vibration. Unfortunately, the cylinders protrude out the sides, making cornering difficult and generating the need to protect the riders legs against heat. Triumph has used the design off and on over the years since WWII. The Triple Though much less common, this three-cylindered design has excellent power and vibration characteristics. But the number of moving parts is much greater and it increases the number of carburetors or fuel injectors required. Again showing their interest in less-common engine designs, Triumph has produced 'triples' and they go like the wind. One of the advantages of more cylinders is the ability to produce higher rpm. That produces more power, while weighing less than four cylinder designs. When selecting a bike styling, comfort, stability and speed are all going to be important. But central to any choice is the engine onboard. Which you choose will be determined, in part, by the kind of riding you intend to do. Off-road you'll want lots of torque at low rpm to climb hills and accelerate quickly through sharp turns. Single or twin is going to be your preferred type. Standard bikes used for commuting around town will eliminate the single, and may even nudge you toward a four cylinder. Four Cylinder Used in autos for decades, the design came later to motorcycles. Not surprisingly, they're heavier than two cylinder designs and consume more fuel. But in exchange they produce much smoother power, which makes for a more pleasant ride on those long trips. They don't produce as much torque at low rpm (revolutions per minute) as twin or single cylinder types, but they make up for it by having lots of headroom - high rpm with low vibration are easier to achieve. There are several sub-types, just as there are in auto engine designs. Inline-Four In this configuration, all four cylinders are lined up with the rods/pistons in parallel. Narrower engine compartments are possible this way, but vibration is harder to minimize, so they often have offset mechanisms to improve the balance. Used most often in Standard or Traditional types, they're sometimes used in larger sportbikes, as well. Kawasaki and Suzuki both have 600cc designs in this category that perform well. V-4 The classic small engine for autos, this design made its way into larger motorcycles decades ago. Harley-Davidson may not have been the pioneer, but they brought the use of V-4's to a high art. Today, they have lots of competition from Honda, BMW and many others. When there are more cylinders, there is a lower net force in any single direction, and therefore less vibration over all. They also have lots of headroom to produce good power at higher rpm. Smooth, they make for a good performer with a classic sound. On the downside, they are inevitably heavier and don't produce as much torque at the lowest rpm. Some larger sportbikes use the design, but it's more commonly found in cruisers and, of course, touring bikes. The quiet, water or oil-cooled designs lend themselves to reliable performance over long trips. Six Cylinders The design is uncommon, owing to the large number of moving parts and the sheer size. But they produce massive power and a smooth, low-vibration engine. Found only among the largest displacement engines, 1000cc or more, they produce good power through every gear. They produce a lot of heat, though, mandating fan-cooled radiators. But the top-end speed can't be beat. Honda has experimented with the design in a few commercial models, and you can find a used one without too much trouble. Be prepared to spend more for repairs or parts if you do your own repairs. Not only are there more parts, they're harder to find. When selecting a bike styling, comfort, stability and speed are all going to be important. But central to any choice is the engine onboard. Which you choose will be determined, in part, by the kind of riding you intend to do. If you can find one, a two-cylinder Boxer can be fun and provide a smooth ride, but the cornering and space limitations are likely to become an annoyance after a while. For that satisfying low-rumble you need more than just big pipes, you need a big engine to drive them. But the sound is secondary. For a smooth cruiser you'll want at least an in-line four with good balance. For a touring bike, anything less than a V-4 is likely to be unsatisfying before long.