How do engines work?

Guest starring squids and space shuttles.

This topic is exciting (one would hope that, as an engineer, writing about engines is fun for me). The first time I finally got engines and could visualize the moving parts was so dang cool, and hopefully by the end of this article you’ll have that a-ha moment, too. In later articles I will explore more advanced concepts based on these.

Engines are machines that turn power into motion.
Cephalopods, like the squids pictured above, use jet propulsion to get around by pushing water through their siphons. Public domain.

Why start with squids?

Squids move by filling themselves with water and then squeezing it out very fast through a siphon. The jet of water pushes the squid by generating forward thrust — in other words, squids use jet propulsion.

Once you understand how squids move, rocket engines will be a breeze. The short video below has clips of these lovely creatures in action, as well as a narrator that goes into more detail on the topic.

A three minute video about squids.

How are squids like rockets?

Rocket engines use the same principles of jet propulsion. Also known as reaction engines, they produce thrust by pushing out exhaust. The reaction starts when a spark ignites the fuel and as it burns the exhaust is forced out through a nozzle. Like a squid’s siphon, it’s perfectly shaped to get the maximum thrust possible.

Diagram of a rocket motor (engine and motor are interchangeable terms).

Reaction engines can generate enough thrust to push a 4.4 million pound space shuttle, for example. But note: a lot of that weight is fuel that burns away. By the time the shuttle gets into space, it’s much lighter. And even though space has no air, rocket engines can keep burning because the propellant (labeled “grain” in the diagram above) contains an oxidizer.

What about cars?

Car engines are more complicated because unlike rocket engines, which just generate thrust in one direction, car engines use various mechanisms to turn the pressure from combustion into back-and-forth motion (which is then turned into circular motion for wheels).

There are many different types, but four-stroke engines are the best way to explain things. It has four distinct phases. During these, the piston moves up and down while the two valves at the top take turns letting stuff in/out of the cylinder.

Animation of the four cycles of an engine.
  1. Intake. The piston moves down and sucks a mixture of fuel and air into the cylinder (through the intake valve).
  2. Compression. Both valves close so the fuel & air are trapped while the piston compresses upward.
  3. Combustion. A spark plug ignites the fuel. It combusts, expands, and forces the piston back down.
  4. Exhaust. The piston pushes the spent fuel/air out through the open exhaust valve.

I’ve described a gasoline engine alongside an animation is of a diesel engine. Don’t worry, though. The only difference is that gasoline needs the spark but diesel doesn't, it combusts by compression alone.

The piston’s linear motion is turned into circular motion with a crankshaft, like this:

Animation of four pistons turning a crankshaft. Public domain.

Engines have been developed and improved by humans for thousands of years. Though energy sources and exact design may change, the underlying concept of turning power into motion probably won’t change anytime soon.

Five hundred million years ago, baby squids had mastered the same skill that would take humans into space.


Diagram of a Stirling piston engine. I don’t even know how to caption this.
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