Two stroke engine makes a comeback?
 

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The Two-Stroke Engine, Reconsidered

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

• Thursday, September 16, 2010
• By Josie Garthwaite

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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.
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.
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."