How Opposed Piston Opposed Cylinder (OPOC) Engines Work

Two Cylinders for the Price of One

Chances are that your car's engine has either four or six cylinders in it. (If you have more than six cylinders then you're driving a real muscle car and probably aren't shopping around quite yet for something that will make the internal combustion engine obsolete.) An engine cylinder is just what it sounds like -- a cylindrical hole in the engine into which you can place a moveable tube, called a piston. And it's that piston, when combined with gasoline, air and a spark plug that provides the motive power that makes your car go zooming down the road. That's the quick-and-dirty version of the story, anyway.

The cylinders in an automobile's internal combustion engine are capped so that the gases held in the area between the top of the piston and the top of the cylinder can't escape. However, there are also two valves at or near the top of each cylinder that can be opened and closed mechanically. These are designed, respectively, to allow air and gasoline into the cylinder (the intake valve) and to release exhaust from the cylinder (the exhaust valve) after the engine's combustion process is complete. These valves open and close in a manner carefully timed with the piston's motion so that the exhaust is released before a new supply of fresh air flows in.

It's the motion of the piston that drives the car. Pistons slide neatly up and down in the cylinder because that's what they're designed to do. Most cars use a four-stroke (or Otto cycle) engine, in which there are four stages to the piston's motion. In the first, called the intake stroke, the intake valve opens and the piston moves downward. The vacuum created by the downward moving piston sucks air along with a small amount of gasoline into the upper portion of the cylinder. Once the mixture has filled the available space left by the descending piston, the intake valve closes and the piston rises again in the compression stroke, squeezing the air-fuel mixture into a tight mass packed with so much potential energy that it qualifies as an explosive. (Fortunately there's very little gasoline in the mix, so we're not talking thermonuclear-weapon quality explosive but something more like a cherry bomb.) Then comes the part of the process that really gives the engine its kick: the combustion stroke, where the spark plug flashes and ignites that potential energy like a firecracker in a tin can, pushing the piston back down again. Finally, in the exhaust stroke, the exhaust valve opens, and the piston rises back to the top of the cylinder, pushing out the useless, gassy residue of the explosion of combustible materials. As soon as the exhaust valve closes, the process begins all over again.

While the piston rises and falls, it turns the crankshaft, a long, rotating rod that converts the up and down motion of the pistons into the circular motion that makes the car's gears and wheels spin. In most standard engine arrangements (there are quite a few), cylinders come in pairs, so that the downward motion of one piston during one stroke creates the upward stroke of the other, a cycle that could theoretically go on forever ... or at least until the gasoline runs out. This isn't exactly perpetual motion, but if you think about it you might ask how the motion of the pistons got started in the first place. The answer is that the four-stroke cycle usually begins with a short burst of rotational energy to the crankshaft from an electric starter motor, but early cars got up and running because some lucky driver had to turn a hand-operated crank to rotate, yes, the crankshaft. (Now you know why they call it that.) Aren't you glad you weren't driving cars back then?

This four-stroke cycle was invented in the 19th century -- in fact, variations on it go back to the steam engine -- and there are lots of variations on it. Let's see if we can come up with one that uses half as many cylinders yet gets just as much power.