How Opposed Piston Opposed Cylinder (OPOC) Engines Work

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.

In the internal combustion engines we've talked around so far, the pistons operate in parallel, with each cylinder aligned to the next and a separate piston in each one. But what if we could stick two pistons in one cylinder and coordinate their actions so that they face one another -- hence the term "opposed cylinder" -- but do not collide? Each of these cylinders would only take up half the length of the cylinder, so that it would only have to move half the distance of a cylinder in a standard engine, thus saving fuel yet still providing the same rotating effect on the crankshaft. And the crankshaft could pass through the center of the cylinder, perpendicular to the cylinder's long axis, so that both pistons could rotate the crankshaft as they moved in opposite directions. And they could pool their exhaust wastes in the center of the cylinder, so that the ends of the cylinder wouldn't have to be capped off to keep the noxious exhaust fumes from escaping before they needed to.

Wouldn't that be cool? You bet it would!

This is called an opposed piston, opposed cylinder (OPOC) engine. In the OPOC engine devised by Ecomotors for the Defense Advanced Research Projects Agency (or DARPA, and yes this means that early applications are likely to be military), the two pistons in the single cylinder are effectively interlaced, with each one divided into two parts and moving inside one another in opposite directions creating the compression stroke, so that the opposing ends of one part of each piston are closing together and compressing the fuel air mixture between them while the opposing ends of the other are moving apart to admit air in the gap to create the intake stroke. Since these two strokes are simultaneous, the whole action of the pistons takes only two back and forth motions, thus making this a two-stroke engine instead of the more conventional four-stroke engine. And because these two pistons in one cylinder perform the work of the two pistons in two ordinary cylinders, they do only the work that normally goes on in one cylinder but apply two cylinders worth of motion to the crankshaft. This gives the OPOC engine a high power density -- that is, a high ratio of power to the mass of the engine itself.

And here's something that really makes Ecomotor's OPOC engine stand out from the crowd: It's modular. You can use one, two or even three of them joined together with a gear arrangement that's scalable, from a one-cylinder engine (which in normal engine terms is really a two cylinder engine) up to a three cylinder (equivalent to a six stroke engine) and beyond. Just keep hooking the cylinders together to make your engine bigger and more powerful. And an OPOC engine is mechanically much simpler than a standard internal combustion engine. In the standard arrangement, a complex and precisely timed series of linkages is required to make sure the intake and exhaust valves are open when needed. That means the engine has an incredibly small number of moving parts. For instance, in a conventional internal combustion cylinder, a complicated mechanism is necessary to time the intake valve and exhaust valve so that they are open only when needed and are never open simultaneously. But in the OPOC engine, these "valves" are simply holes in the side of the cylinder, which are covered and uncovered by the sliding of the pistons themselves, thus removing the need for a complicated mechanism to make them open and shut. Ecomotors estimates that the number of moving parts in its engine has been reduced from 385 to 62, meaning that there is one heck of a lot fewer parts that need servicing and can go bad.

The upshot is that OPOC engines are simpler and thus less likely to break down. They're also more efficient, lose less energy while operating, and -- because they do the work of two pistons with only one -- can produce much more power than a standard internal combustion engine for only a portion of the gas. Is this the engine of the future? Probably. At least until that nuclear fuel cell comes along.