Today, how many cylinders? The University of Houston's College of Engineering
presents this series about the machines that make our civilization
run, and the people whose ingenuity created them.
So how many cylinders should a car engine have?
Most of our cars have either four cylinders in a row, or cylinders in a
V-arrangement -- two or three on either side. So, why all this fancification?
Why not just one big cylinder?
Well, think about a piston, going back and forth in a cylinder, making a
crankshaft rotate. It briefly drives a shaft once every two revolutions.
Our car engines run in
four-stroke cycles.
Ignition occurs and the piston
pushes downward. Then it clears out exhaust as it goes back up. Next it
pulls in a new mixture of air and gasoline on its way down. Finally, it
goes up, compressing that mixture. Then another ignition, and the cycle repeats.
A single-cylinder engine speeds up on the first stroke; then it slows during
the rest of a four-stroke cycle's two revolutions. That would cause such an
engine to shake and vibrate.
So we need a big flywheel to keep it moving between ignitions. With more
cylinders and pistons, we can pin each piston's connecting rod to a different
angular location on the crankshaft -- then we time the explosions so that each
one kicks the rotation along during the two revolutions. And the flywheel can
be a lot smaller.
Karl Benz used a single-cylinder engine in his first 1885 car. Ford's first
Model T engine had four cylinders in a row. Some luxury cars of the 1920s
had inline engines with as many as eight cylinders. Engines with as many as
12 or more cylinders in a row have been used, but mainly in large marine and
stationary engines.
Of course smooth running is only one goal. More cylinders give less flywheel
weight, but they also mean greater manufacturing and upkeep costs. Then there's
compactness. The Duesenberg straight-8 was a favorite
of rich movie stars in the '20s. But it had a 12-foot wheelbase. Imagine
parallel-parking that beast.
The answer was the V-8 engine -- two rows of four forming a V. Even Karl Benz
experimented with a V-2 engine after he built his single cylinder motor.
A V-arrangement can even let two cylinders drive a common crank pin, pushing
it at different angular positions. And here complication increases: Engineers
have created all kinds of clever crankshaft designs to use with cylinders in
all kinds of positions -- V-4s, V-6s, flat-4s, flat-6s.
Airplanes imposed different design constraints. An inline engine offers little
frontal drag. The Wright Brothers used a straight-4 engine, but with a pretty
heavy flywheel. Then early builders went to engines with nine-cylinders, radiating
from a central hub. The pistons spun around the shaft and needed neither flywheel
nor cooling systems.
Many new technologies do settle on one best form. But some find more than one
good option, then keep jockeying among competitors. Just think about PC's vs.
Mac's, Classical vs. Country music -- just think about cylinders in their seemingly
endless arrangements.
I'm John Lienhard at the University of Houston,
where we're interested in the way inventive minds
work.
(Theme music)
See the Wikipedia entries on all the relevant topics. Search on words like
automobile engines, straight-4, flat-6, V-8, 4-stroke engine, etc. Google will
also send you to many simple and clear sites,
such as this one.
All photos by J. Lienhard. Toyota engines courtesy of Mike Calvert Toyota,
Houston, TX.
