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Sustainable Energy

The Two-Stroke Engine, Reconsidered

Once thought too polluting, the two-stroke engine makes a comeback in advance of stricter fuel efficiency standards.

EcoMotors International, a startup based in Troy, Michigan, has a new approach to an old idea–the two-stroke engine–which it says is up to 50 percent more efficient than most vehicle engines and pollutes far less than a conventional two-stroke engine.

Testing, testing: EcoMotors has built and tested five generations of its OPOC engine, and expects to start testing a sixth this month.

The company recently received a combined $23.5 million in investment from Bill Gates and Khosla Ventures. That money will go toward development of EcoMotors’s opposed piston, opposed cylinder (OPOC) engine. The engine uses two piston movements per cycle, instead of four, and each cylinder contains two opposing pistons, instead of one. A single crankshaft sits in between pairs of cylinders. The design relies on precise computerized control of all the components.

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A conventional car engine takes four piston movements, or strokes, to go through intake, compression, combustion, and exhaust. In a two-stroke cycle, these stages are completed with just two piston movements, delivering twice as many power strokes per revolution and requiring fewer parts. But two-stroke engines tend to spew out more unburned fuel in the exhaust, which is why the four-stroke design became more common.

Putting two pistons inside each cylinder also means that each piston only travels half as far as it normally would in a two-stroke engine, allowing the engine to run faster. Having half as many parts as a conventional engine (the OPOC does not have cylinder head or valve-train components, and it has fewer bearings) helps to reduce friction and heat loss. These factors, combined with “a long list of 1 and 2 percent improvements” in other areas, says Ecomotors’s CEO Don Runkle, account for a 15 percent efficiency improvement.

Runkle says several technologies have helped reduce the emissions of the two-stroke engine. First, an electric turbocharger allows for “variable compression ratio,” whereby the inlet pressure fed into the engine’s cylinders is varied to maximize efficiency. Second, an approach called asymmetrical port timing–opening the intake and exhaust ports at different times–helps improve the efficiency of gas exchange. And third, high pressure injection and computerized control of injection improve the overall efficiency of the cycle. While “not unusual in other engines today,” says Runkle, this technology is now “much better than what existed when many of us tried to make two-strokes work” in years past.

The basic OPOC engine consists of two cylinders on either side of a crankshaft. Multiple engines, or “modules,” can be used together to boost the fuel efficiency of a vehicle by as much as 45 percent. In addition to the efficiency gains of the engine itself, extra modules can be deactivated when they aren’t needed. Key to the design is an electrically controlled clutch, which disengages a module when necessary. Some modern V8 engines feature cylinder deactivation, but the pistons continue to turn due to their connection to the crankshaft, resulting in what’s known as “parasitic loss.”

Breakdown: Each engine module has a pair of opposing cylinders and a single crankshaft between them. Two pistons within each cylinder move in opposite directions.

Giorgio Rizzoni, director of the Center for Automotive Research at Ohio State University, says the OPOC engine “looks like a combination of different ideas, none of which are absolutely revolutionary.” Opposed-piston engines have been used in airplanes and motorcycles, he notes, while electric turbochargers have been around for years as a way to improve the performance of turbo engines. What strikes Rizzoni as different is the new engine’s “synthesis of old ideas” combined with a dual-piston arrangement.

EcoMotors, which secured an $18 million development deal with China-based automotive supplier Zhongding in May, is one of several young companies working to update the internal combustion engine ahead of stricter fuel economy standards. A number of them, including San Diego-based Achates Power, are focused on two-stroke architectures. “In the mid-’90s, people thought two-stroke engines really couldn’t meet emissions standards,” says Achates CEO David Johnson.

Dave Hurst, a senior analyst at Pike Research, which specializes in energy technologies, says the biggest challenge for any engine startup hoping to license innovations to major automakers is that these would-be customers consider engines to be their core technology, and are therefore more likely to design new engines themselves.

At this point, says Runkle, EcoMotors is working to refine the engine’s combustion process and determine emissions, efficiency, and power in all possible circumstances. Most car companies, he explained, will want to see that data before making the leap to implement EcoMotors’s technology.

Although EcoMotors aims to secure licensing and codevelopment deals, the company also hopes to do at least some manufacturing itself. EcoMotors has requested $208 million in loans from the U.S. Department of Energy to support that effort, and according to Runkle, the company expects to get into manufacturing over the next two or three years.

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