Just when it looked like tightening emissions regulations finally would kill the notion of an automotive 2-stroke engine, the concept rises from the dead like the villain in a Hollywood sequel.

This time, Achates Power Inc. has joined upstart EcoMotors International Inc. in reviving talk of the 2-stroke.

Achates (pronounced a-KAY-tees), which like EcoMotors is experimenting with an opposed-piston concept, says the 2-stroke has new life thanks to more sophisticated modeling tools allowing researchers to perfect the combustion process and eliminate emissions concerns.

Achates' work amounts to more theory than hardware. The San Diego-based developer has built a 1-cyl. prototype that has gone through some 1,200 hours of testing and has modeled multi-cylinder versions of its concept, but no full-scale automotive engine has been produced.

That's the next step, says CEO David M. Johnson, if Achates can convince an OEM or engine builder to license the technology on a development project.

“We see ourselves as an R&D organization, not a producer,” Johnson tells Ward's in an interview. “So we want to find a partner to produce and test the engine — and we'll provide our expertise.”

The Achates 2-stroke features two horizontally opposed pistons per cylinder and two crankshafts, although the company says a single-crankshaft version is possible. Elaborate connecting arms tie the pistons to the crankshafts, only one of which links directly to the driveline.

The block is aluminum, but Johnson says a production engine could use cast iron, depending on the application. Crankshafts and connecting rods are steel, and the prototype employs a Denso Corp. high-pressure fuel pump and rail.

Delphi Corp. solenoid-based diesel injectors rated at 23,200 psi (1,600 bar) are incorporated into the direct-injection system. They are capable of five injections per combustion cycle, Johnson says.

The compression-ignition engine runs on diesel, but Johnson says it could be designed for flex-fuel use.

Calculated to meet Environmental Protection Agency 2010 and Euro 6 emissions standards on an engine-out basis, it would incorporate a diesel particulate filter, but use of a urea-based selective catalyst reduction system would depend on whether the objective is to maximize fuel economy or further lower oxides of nitrogen emissions.

Although the engine first was conceived in 1998, and Achates has been working on it since its formation in 2004, it was only this past year that key hurdles were cleared, paving the way for the 2-stroke's next step, Johnson says.

Achates sees the biggest potential for the multi-fueled engine in commercial vehicles near term, and Johnson says the design could be upgraded with new technology to meet more stringent heavy-duty emissions regulations to come. “We're (currently) applying off-the-shelf components to the engine,” he says.

Johnson says the Achates engine is 15%-20% smaller and lighter than a conventional diesel, and a 2-cyl. version using four pistons would produce about 10%-15% more torque and 15% better fuel economy than a 4-cyl. diesel.

The target was for 55% thermal efficiency, and the company now believes it can exceed that. A typical diesel runs at about 42% efficiency, while gasoline engines operate well below that level.

All in all, there are 40% fewer parts in an Achates in-line 4-cyl. vs. a conventional V-6 diesel, Johnson says, including the elimination of the cylinder head and valve train, while accounting for the extra piston in each cylinder.

Commercial vehicles have been the primary target, because the Achates cylinder has a 3.1-in. (80-mm) bore and displaces about 1.0L overall.

Scaling that up to a multi-cylinder configuration would produce an overall displacement that lines up best with truck applications, Johnson says, adding smaller versions easily could be developed for light vehicles.