How Opposed Piston Opposed Cylinder (OPOC) Engines Work

Better internal combustion engines are on the way. And when we say better we mean lighter, more fuel efficient and less polluting. Take a look at our animation of how the OPOC engine works.
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Internal combustion engines pollute the air. Internal combustion engines rob the planet of precious and non-renewable resources. Internal combustion engines require fossil fuels that tie the United States economically to countries that we'd rather not be doing business with.

And internal combustion engines aren't going away anytime soon.

Oh, sure, you've heard about all the new technologies that should be replacing the internal combustion engine any day now, technologies like electric motors, hybrid power trains, hydrogen fuel cells and even cars that run on compressed air, but none of these technologies is ready to save the auto industry from the internal combustion engine quite yet. Electric motors are probably our best bet for the immediate future and there are even some cars on the market now that use them as a power source, but they take time to recharge, have a limited driving range, and they can't simply be fueled up in five minutes at the local service station. Besides, do you really want to get stuck in the middle of East Nowhere, Middle America, with a dead lithium-ion battery array and nobody around who has the foggiest notion how to recharge it? Hybrid power trains are already quite feasible, as the huge success of the Toyota Prius demonstrates, but they still contain internal combustion engines, so they don't really solve the problem. They just postpone the day when we'll finally need to get rid of this antiquated technology. Hydrogen fuel cell cars will be really amazing when they're available in vehicles that can be bought and driven by the average consumer. This should be, oh, about 20 to 30 years from now, around the time you'll invest in your first set of false teeth. And compressed air cars? Nobody really knows when those will be ready to hit the road, but it'll probably be a good while yet before you can refuel your car using a bicycle pump.

These technologies are important. Think tanks and auto manufacturers are researching them right now. The transportation your children's children use will depend on them. Someday one or all of these technologies will free the world from its out-of-control addiction to fossil fuels. But in the meantime what we really need is something that can realistically be ready for practical use within the next few years: a better internal combustion engine.

Here's the good news: Better internal combustion engines are on the way. And when we say better we mean lighter, more fuel efficient and less polluting. If we can't put internal combustion engines out to pasture quite yet, we can at least make them behave a little more politely while they're still galloping around on the public streets.

One of the most exciting new types of internal combustion engines is the opposed-piston opposed-cylinder engine, and if you can't remember all those tongue-twisting syllables you can just call it an OPOC engine. (Don't feel bad. Everybody else calls it that too.) OPOC engines aren't really new -- the idea's been around for a while -- but a company called Ecomotor is finally getting serious about building OPOCs that will be ready for consumer vehicles long before hydrogen fuel cells are the rage of the nation. And as proof that Ecomotors is offering serious technology that really could revolutionize the way we use gasoline in the near future, a fellow named Bill Gates has already invested in the company. Yes, that Bill Gates, and nobody can say that the co-founder of Microsoft doesn't know a thing or two about the practical aspects of cutting-edge technology.

But what exactly is an OPOC engine and how does it differ from the internal combustion engines that all of us love and hate? To answer that question, we'll first give you a refresher course in standard car engines, and then we'll show you how OPOCs do pretty much the same thing but just a little differently -- and a little better.

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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.

When Two Pistons Face Off, They Both Win

Workers assemble engines for Porsche 911 cars at the Porsche plant in Zuffenhausen, Germany. A complex task, no doubt. Ecomotors estimates that the number of moving parts in its engine has been reduced from 385 to 62, making it much easier to service.
Workers assemble engines for Porsche 911 cars at the Porsche plant in Zuffenhausen, Germany. A complex task, no doubt. Ecomotors estimates that the number of moving parts in its engine has been reduced from 385 to 62, making it much easier to service.
Marco Prosch/Getty Images

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.

Author's Note: How Opposed Piston Opposed Cylinder (OPOC) Engines Work

I'm not one of those guys who grew up with my head under the hood of a car taking the engine apart and putting it back together again just to see if I could do it. More likely you'd find me at the keyboard of a computer, programming in languages like BASIC and C, or writing books about why controlled fusion power was the energy source of the future. (I'm still waiting on that one.) But when I started writing about cars, it was only natural for me to gravitate toward writing about automotive technologies that were out on the bleeding edges, ways of powering and using cars that were so advanced, you'd think they might have driven straight out of a movie like Blade Runner or Minority Report. I don't know about you, but I get this tingly feeling up and down my spine when I learn about something that's new, exciting and does things in a way that people (in this case auto engineers) have never done them before.

Opposed piston-opposed cylinder (OPOC) engines may not sound as bleeding edge as, say, flying cars or 1981 DeLoreans with flux capacitors to help them travel through time, but by the time I finished researching this article I realized that they were every bit as exciting. (Okay, maybe not quite as exciting as that flux capacitor thing.) OPOC engines are the product of a lot of ingenious thought by brilliant engineers who weren't willing to accept that the way internal combustion engines have always been done is the only way that they can be done. Yes, OPOCs have been around for a long time -- the early prototypes of the OPOC engine go back to the 19th century -- but automotive engineers, with a little help from the military's cutting-edge research wing DARPA (the Defense Advanced Research Project Agency), are finally getting their moment in the sun and nobody could be more excited than I am.

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  • Ecomotors International. "Clean, Efficient and Lightweight Propulsion Systems for a Better World." (March 7, 2012)
  • DeMorro, Christopher. "OPOC Engine Is Smaller, Lighter, and 50% More Efficient Than Turbodiesels." Gas 2. (March 7, 2012)
  • Ecomotors International. "EcoMotors International's Opposed-Piston, Opposed-Cylinder Engine Promises to Revolutionize Commercial Vehicle Design with Powerful, Lightweight, Fuel Efficient, Low Emissions Engines." (March 7, 2012)\
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