Motorcycle Engine Types – Part II

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.

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.

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