Motorcycle Engine Types – Part I
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.