you have ever seen a World War
II bomber like the B-25
or the B-17,
or if you have ever seen or been on an old commercial airplane
like a DC-3,
then you are familiar with something called a radial
engine. Many planes of the WWII era used very large, very
powerful radial engines to drive their propellers.
In this edition of HowStuffWorks,
you will learn how the radial engine works, what makes it
different from other engine configurations and why it fits in
so well with airplanes.
The Basic Idea If you have read How Car Engines
Work, then you are familiar with the concept behind the
four-stroke engine. Car engines arrange pistons in three
typical patterns, as shown here:
(Click on image to see
animation) Inline - The cylinders are
arranged in a line in a single
(Click on image to see animation) V
- The cylinders are arranged in two banks set at an
angle to one
(Click on image to see
animation) Flat - The cylinders are
arranged in two banks on opposite sides of the
Each different configuration has different smoothness,
manufacturing cost and shape characteristics that make it more
suitable for certain vehicles.
A radial engine is yet another configuration for the
pistons, as we will see in the next section.
Inside a Radial Engine The radial engine
idea is very simple -- it takes the pistons and arranges them
in a circle around the crankshaft, as shown here:
Image courtesy Baris
You can see in the illustration that this is a
five-cylinder engine -- radial engines typically have anywhere
from three to nine cylinders. The radial engine has the same
sort of pistons, valves and spark plugs that any four-stroke
engine has. The big difference is in the crankshaft.
Instead of the long shaft that's used in a
multi-cylinder car engine, there is a single hub -- all
of the piston's connecting rods connect to this hub. One rod
is fixed, and it is generally known as the master rod.
The others are called articulating rods. They mount on
pins that allow them to rotate as the crankshaft and the
Applications Radial engines have several
advantages for airplanes:
They can produce a lot of power. A typical radial engine
in a B-17 has nine cylinders, displaces 1,800 cubic inches
(29.5 liters) and produces 1,200 horsepower.
Radial engines have a relatively low maximum rpm
(rotations per minute) rate, so they can often drive
propellers without any sort of reduction
Because all of the pistons are in the same plane, they
all get even cooling
and normally can be air-cooled. That saves the weight of
Radial engines reached their zenith
during WWII. There are some radial engines around today, but
they are not that common. Most propeller-driven planes today
use more traditional engine configurations (like a flat
four-cylinder) or modern gas turbine
engines. Gas turbines are much lighter than radial engines
for the power they produce.
One place where you can still see the influence of the
radial engine concept is in the two-cylinder engine of a Harley
The engine from a Harley can be thought of as
two cylinders from a radial
It can be thought of, in a way, as two pistons from a
radial engine. In a Harley, both pistons share a single
connection point to the crankshaft, like in a radial engine.
For more information on radial engines and related topics,
check out the links on the next page.