How Radial Engines Work

B-25 bomber's are equipped with radial engines.  See more pictures of radial engines.
Photo courtesy NASA

If 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 article, 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

(Click on image to see animation) Inline - The cylinders are arranged in a line in a single bank.
(Click on image to see animation) Inline - The cylinders are arranged in a line in a single bank.

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) V - The cylinders are arranged in two banks set at an angle to one another.
(Click on image to see animation) Flat - The cylinders are arranged in two banks on opposite sides of the engine.

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:

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Image courtesy Baris Mengutay

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 pistons move.


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 gearing.
  • 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 water-cooling.

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 Davidson motorcycle.

The engine from a Harley can be thought of as two cylinders from a radial engine.

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.

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