Suppose you could take an old engine concept discarded in the 1970s, add modern fuel metering and injection control hardware and software, and offer a compact powerplant that cuts the fuel consumption of today's diesel truck engines by a third..
That's the promise of Achates Power, which has been working for a decade on its horizontally opposed two-stroke diesel truck engine.
Now, it says, the company expects to announce its first pilot program with a global engine or vehicle maker within 18 months--almost surely outside the U.S.
And it hopes to have a large-scale production contract with a global maker by 2020 or so.
Achates opposed-piston two-stroke diesel engine vs Cummins Atlas advanced diesel - test results
But why promote the engine now?
Largely because recent dynamometer tests of an Achates two-stroke engine in a full-size pickup truck have produced remarkable reductions in fuel consumption.
Achates says that against the most advanced four-stroke diesel engines, its design can cut fuel use by 30 percent--a significant amount when even an improvement of 0.1 mpg is considered worthwhile on CAFE testing.
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Started in 2004, Achates (pronounced "Ah-KAY-dees") has had test engines running for nine years now and has raised more than $100 million in venture capital.
By 2010, its prototypes had met current and future limits on emissions and fuel consumption--and the company published the results of its tests widely in target markets, including China and India as well as the U.S. and Europe.
It sees three sets of potential vehicle types that could use its technology: commercial, off-road, and passenger.
The opposed-piston two-stroke engine has a history that's more than a century old, mostly in very large industrial applications.
Achates CEO David Johnson notes that the company's goal is to "bring a 1920s engine into the 2020s" using modern technology.
2014 Ram 1500 EcoDiesel, Bear Mountain, May 2014
From the early 1930s, for instance, the Fairbanks-Morse company in Beloit, Wisconsin, sold opposed-piston designs 12 to 15 feet long and 10 feet high.
The 3.28-megawatt (4,400-horsepower) engines were used to power electric generators, offering the advantage of high power density for their size.
But two-stroke engines in general fell out of favor starting in the 1970s, for a variety of reasons.
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Those included not only the noise and vibration characteristics of two-stroke engines, but also their characteristic blue exhaust with a mix of combustion and engine-oil emissions.
Those emissions, in fact, are what led two-stroke engines to vanish from passenger cars by 1970.
Modern modeling, emission controls
Now, however, Achates says it has completely rethought the basic design--with 26 U.S. and 27 overseas patents granted, and another 90 or so in the pipeline.
Modern computer-aided design and combustion modeling, plus the latest high-pressure diesel fuel injection systems, cured many of the problems that had led the design to be viewed as a backwater.
Much of the work went into modern lubrication systems--no more mixing oil and fuel--and extensive, time-consuming combustion and structural modeling to reduce the noise and vibration from each combustion stroke.
Achates opposed-piston two-stroke diesel engine: Surface-to-Volume Ratio vs Displacement
The company also says it benefits greatly from fast-growing production of modern diesel aftertreatment systems, including selective catalytic reduction and particulate filters.
Those are now required to meet new and tougher European emission standards going into effect this year for mass-market diesels, meaning they're no longer restricted to small volumes of North American vehicles.
Along with the high-pressure fuel injectors and pumps, plus the turbochargers used on all of today's diesels, the cost of diesel hardware is falling quickly for third-party makers like Achates as global volume rises.
One cylinder, two pistons
The Achates design can scale from outputs of 55 kW (74 hp) to 3,800 kW (5100 hp) by adding cylinders and enlarging them.
It can be as small as a single cylinder, in which a single pair of opposed pistons travels back and forth--cancelling out the majority of combustion vibrations.
Achates opposed-piston two-stroke diesel engine, as it would be packaged for truck installation
But there's little interest in the smallest version, Johnson said, with the sweet spot being the commercial trucks that represent 5 to 10 percent of vehicles on the road but consume 20 to 30 percent of the world's fuel.
Achates believes that in volume, its engines will cost no more than the ones they would replace, while drastically reducing fuel consumption.
Designed to drop in
It will use similar engine mounts to the current ones, and its twin crankshafts will connect to a single geared output that feeds into existing transmissions.
And Johnson claims that while the company's goal is to replicate the torque curves of existing engines it would supplant, the long-stroke and high-compression Achates design inherently produces higher torque at low engine speeds with the same output than incumbent engines.
Which drivers tend to like (as in electric cars).
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The engine also, Johnson notes, is designed to be built in existing engine plants--with a similar sequence of manufacturing steps, requiring only tooling changes rather than entirely new production processes.
The engine and vehicle makers who've spoken in depth to the company and dug into its technology, he says, have validated all those similarities.
Pickups that meet 2025 rules
More recently, Achates says it has tested a smaller prototype engine in a light-duty pickup truck--the size of a Ford F-150 or Ram 1500--on a dynamometer in San Diego.
Against the most advanced diesel engines expected to arrive in the market, Achates said, the improvement is 30 percent--giving a combined rating of 28 mpg.
That level counts as 37 mpg against the 2025 overall average of 54.5 mpg.
At that level, "the truck would actually beat the CAFE targets for this specific vehicle size," said Fabien Redon, vice president of technology at Achates, "and would provide a credit to the automaker to offest a less efficient vehicle."
Across a blend of vehicles from a full-line manufacturer, it's likely enough to let the maker achieve the overall average--which translates to about 42 mpg on the window sticker.
54.5 MPG CAFE standard for 2025
The company issued a technology briefing paper earlier this year on its results, and Johnson says Achates is working with a variety of third-party engine suppliers to have them review and validate the test data it published.
Creating test vehicles is the next step, and Johnson says engine providers have "strong buy-in" into future test programs.
Against gasoline engines in lighter trucks, the improvements are even greater: in one case, an astounding 86-percent increase in gas mileage with identical power output to the engine removed.
That vehicle was for dyno testing only, Johnson says; the company doesn't currently have a running pickup truck to test the engine in real-world usage.
The company's biggest competition, he says, is not other startups with similarly unusual engine designs but the existing, incumbent, in-house engine programs at the large vehicle makers.
Achates will have to prove its aggressive fuel-reduction claims in testing across a variety of different vehicles--which it's now in the process of doing.
If it succeeds, look for that test program to be announced next year or in 2016.