MOTOR Magazine

A MOTOR Magazine Newsletter
March 20, 2018

Contributed by Bob Chabot
Will Opposed Pistons Drive the Future?

It’s a no-excuses way to meet future efficiency and emissions standards

At the January 2018 North American International Auto Show (NAIAS), Achates Power showcased one of its Ford F-150 demonstration pickup trucks, which features an Opposed-Piston Gasoline Compression Ignition (OP GCI) engine. Compared with current production engines, the OP GCI design aims to deliver fuel economy levels beyond regulatory requirements, with lower emissions at less cost.

Watch this animation to see some of the Opposed-Piston Gasoline Compression Ignition engine’s features in action. (Video and images — Achates Power)

The Opposed Piston Formula
Throughout the 20th century, when the utmost fuel, weight and volume efficiencies were required to propel aircraft or ships, an unconventional type of compression ignition engine was used that featured:

  • Opposed-piston architecture — With two pistons per cylinder working in opposite reciprocating action, these engines do not need cylinder heads, which are a major contributor to heat losses in conventional engines. Ports in the cylinder walls replace the complex poppet valves and friction-creating valve trains of conventional engines. The intake ports at one end of the cylinder and exhaust ports at the other are opened by the piston motion and enable efficient uniflow air scavenging.
  • Two-stroke combustion cycle — A two-stroke engine produces twice as many power strokes per revolution as its four-stroke equivalent. This advantage leads to smaller displacement engines for similar performance and lower in-cylinder pressure to lower emissions compared to four-stroke conventional engines.

In the past, these advantages were offset by two difficult-to-overcome shortcomings of two-stroke engines, which limited their widespread scope of use. These included high hydrocarbon emissions (due to carburetion and over-scavenging) and excessive oil consumption (due to oil-fuel mixing in spark-ignition engines and port oil ejection in compression ignition, direct fuel injection engines) and are difficult issues to tackle in these types of engines.

The engineers and scientists who started Achates Power in 2004 believed that innovation and modern technology could transform the proven and record-setting two-stroke opposed-piston engines of the past into the clean and efficient engines of the future.

The Achates Power 2.7L 3-cylinder Opposed-Piston engine is rated at 270 hp and 480 ft.-lb. of torque. The Ford F-150 pickup demonstrator, equipped with the OP GCI engine, achieves a combined 37 mpg, exceeding the proposed CAFE 2025 regulation.

Putting the Pieces in Place
Achates Power received a $9 million award from the U.S. Dept. of Energy’s ARPA-E program in 2016 to develop the engine. Since then, the company has partnered with several industry powertrain leaders. These include Argonne National Laboratory, which provided initial testing and other services, Delphi Automotive, which helped Achates refine the engine’s combustion chamber to increase the compression ratio from 16:1 to 18.5:1 and improve indicated thermal efficiencies (ITE) above 50 percent and a financial injection by Aramco Services Company, which will speed the development and market introduction of the new efficient, low-emissions powertrain.

“Aramco has significant experience within the global fuel technology related to fuels and engines,” said David Cleary, director of the Aramco Research Center in Detroit. “We’re impressed by the work that Achates Power has done to bring their efficient, low-emissions OP GCI engine forward.”

“The OP GCI powerplant is engineered to achieve superior thermal efficiency by virtue of its lower heat losses, improved combustion and reduced pumping losses,” added David Johnson, president and CEO, Achates Power. “The engine eliminates the cylinder head, which improves surface-area-to-volume ratio of the combustion chamber for reduced heat transfer and rejection. In addition, conventional engine valvetrain and related components are eliminated.”

Johnson cited an example. “The architecture of the OP Engine enables a reduction in the aftertreatment system size and cost. A comparison between the 2.7L OP GCI engine and a comparable V6 with supercharger shows a part reduction of more than 60 percent, enabling an approximate 10 percent cost reduction. In addition, the OP GCI engine is 30 to 50 percent more fuel efficient than comparable diesel and gasoline engines.”

The Achates Power Opposed-Piston Engine is an alternative, cost-effective powerplant ideally suited to light duty truck applications. It requires fewer parts, yet offers increased fuel efficiency and lower emissions.

OP GCI Technology Could Revolutionize Light Duty Truck Engines
“The joint development agreement with Aramco will help Achates Power respond to our growing list of customers, as well as continue research and development work,” Johnson noted. “We chose to demonstrate our ultra-clean, ultra-efficient OP GCI engine in a full-size light-duty pickup truck at NAIAS because of the significant need for improvement in this segment. These trucks are driven more miles, sold in higher volume, consume more fuel and emit more CO2 than other light duty vehicles.”

“Light duty trucks currently account for 41 percent of light duty vehicle sales, but consume 55 percent of the fuel used by the light duty segment,” he continued. “In addition, light duty trucks are driven more miles per year than passenger cars. Using OP GCI engines in light duty trucks alone would reduce CO2 and fuel usage in volumes similar to completely eliminating half of all light duty passenger cars sold each year.”

Johnson’s bottom line? “The OP GCI engine is a no-excuses way to meet future efficiency and emissions standards. The technology and infrastructure to meet these future standards exists today. We expect the OP GCI engine to be fully integrated into a number of vehicles by late 2018 and available for consumers shortly thereafter.”

Time will tell.

[Editor's note: Visit for the latest diagnostic and service insights.]

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