Just when it looked like tightening emissions regulations once and for all 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 it’s 6-year-old Achates Power Inc., which 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.

At this point, 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 in the process, says CEO David M. Johnson, if Achates can convince an auto maker or engine builder to license the technology and collaborate 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 in designing and developing the engine.”

The Achates 2-stroke is far from a conventional engine. It 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.

“In off-road applications, you probably wouldn’t want to have an SCR system,” he notes.

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.

“New design tools enabled us to solve (critical emissions issues), allowing the engine to come to life,” he says.

Achates sees the biggest potential for the multi-fueled engine in commercial vehicles near term, and Johnson says he is confident the design could be upgraded with new technology to meet more stringent heavy-duty emissions regulations to come.

“We’re applying off-the-shelf components to the engine,” he says. “So we’re achieving an advantage (in performance and fuel economy) with the same components other truck engines are using today. As we go to tougher standards, we expect new technology to give us the same sort of advantage (over 4-stroke engines).”

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 an 3.1-in. (80-mm) bore and displaces about 1L 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-vehicle applications.

Medium- and heavy-duty trucks also are attractive targets because big fleet operators stand to gain the most from incremental gains in fuel economy, the executive says. Application in a serial hybrid similar to the Chevrolet Volt, where the engine is used mainly as an electricity generator, could hold some promise, as well, Johnson adds.

Achates appears to be a well-supported and intellectually rich start-up.

Founded by developer Jim Lemke, who initially conceived the 2-stroke as a backup engine for small planes, Achates is named after Aeneas’ faithful friend in Virgil’s epic poem the “Aeneid.”

It was financed initially by John Walton (son of Wal-Mart founder Sam Walton), but the developer now has $50 million in paid-in capital from such venture capitalists as RockPort Capital Partners, Sequoia Capital and InterWest Partners.

Achates employs about 50 engineers and scientists, Johnson says, and is overseen by a technical advisory board that includes university professors and former automotive engineers with a combined 200 years of experience.

Its facility is well outfitted, with two dynamometers, laser Doppler anemometry and laser-induced fluorescence equipment, plus what Johnson says is a state-of-the-art fuel bench for mapping injectors. Tours conducted for the U.S. Department of Energy and potential customers have left visitors “with a strong impression of our capability,” the CEO says.

Achates has been granted nine patents, applied for another 15 or so, and “there is maybe 25 in the pipeline beyond that,” he adds.

The last wave of 2-strokes was led by Australian developer Orbital Engine Corp. Ltd. in the 1990s, but emissions issues helped quash interest. Orbital went on to focus on the fuel-injection technology it had developed for the engine.

EcoMotors’ efforts to sell the industry on its opposed-piston/opposed-cylinder design have been well publicized and appear to have gained more traction with auto companies, but no production programs appear imminent.

Johnson admits cards remain stacked against the Achates engine, including the industry’s aversion to risk and the huge investment auto makers have in more traditional engine technology and manufacturing.

Packaging the 2-stroke into a vehicle also would be a challenge because, though smaller, the Achates has a unique shape and operates differently.

In addition, development still needed to bring the engine to fruition “is real work,” the CEO says. “There still are enormous challenges to overcome to get this to production.”

One enticement to would-be partners is manufacturing costs. Johnson says investment in an all-new plant would be lower than for a conventional engine, because the facility would require 30% less floor space and there would be no need for a cylinder-head line.

Comparing the costs to retool a brownfield site is more complicated, he admits. “There are no exotic materials used and no barriers to manufacturing (the Achates engine), but this would be a significant switch (in tooling and processes),” he says.

Emerging markets, more likely to “skip ahead” to next-generation technology, might offer the best opportunities to getting the Achates engine into production, Johnson says. Even so, the best-case scenario would see the 2-stroke in vehicles in four to five years, he adds.

Still, Johnson is optimistic there’s an engine builder out there somewhere that will take an interest in the concept. “There are 150 potential customers for this worldwide,” he notes.

dzoia@wardsauto.com

tmurphy@wardsauto.com