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The basic counter-rotating mechanism of Duke Engine's compact axial piston power plant shown in this illustration.

New axial-piston engine aimed at range extenders, military applications

Anyone who attended an SAE Congress back in the day may have come upon a booth featuring an unconventional engine prototype that had been built by a doggedly determined Aussie or Kiwi inventor hoping to arrange a dynamometer trial at one of the big car companies. Few such “boutique” engine designs ever got much beyond a cursory look by major auto and engine makers which were generally too invested in standard power plant configurations to seriously consider something unfamiliar and unproven.

Things may have changed a bit with the recent rise of electric and other alternative propulsion technologies, but the prospects for any unorthodox engine project are not promising. One novel design that may face somewhat better odds is a compact axial piston engine developed by a small Auckland-based firm called Duke Engines. After years of effort, the unusual engine shows signs of practicality, enough that the New Zealand-based company claims it is ready to begin the commercialization process.

Axial engines are a class of compact engines with a set of pistons that rotate around a central output shaft as they reciprocate in line with the main axis. Despite their conceptual elegance and potential for good package and performance efficiencies, the spinning arrays of plunging pistons have often led to unacceptable vibrations and wear.

The Duke engine design seems to mostly avoid such traditional drawbacks. The company's Web site, for instance, contains a video of a coin balancing on its edge as it sits atop a running prototype (, backing up their assertion that the design produces only negligible first- and second-order vibrations.

Long road to the market

The unconventional axial power plant is the result of an extended R&D effort headed by chief engineer and company founder Noel Duke, who first invented his concepts in 1993. With support from local investors and governments, Duke formed his garage startup several years later, and with subsequent equity financing his small team steadily worked to perfect the technology. By 2011 the company had entered into a co-development relationship with Mahle's engineering consultancy in Farmington Hills, MI. To this day, the team has only a half-dozen full-time employees.

“Today, the engine is on the cusp of commercialization,” said Mike Fry, Chief Technology Officer at Duke Engines. “We’ve demoed proof-of-concept and are now looking for the right first application.”

With Mahle consultants, the company has determined that the engine seems best suited for 100-hp-class (75 kW) automotive range-extenders; light aircraft engines (including light sport planes, microlights and military drones); marine propulsion; auxiliary power units, as well as portable systems for soldiers or fire fighters. Another sweet spot might be 200- to 300-hp engines—“the lower end of gas turbine engine output range,” he said.

The latest iteration of the Duke axial engine, Fry explained, incorporates five conventional pistons and cylinders that are mounted in a cast and machined monoblock that resembles a revolver mechanism. Essentially, the design takes an in-line array of piston heads, connecting rods and cylinders, and forms them into a circle that revolves around a single center shaft.

Novel mechanism

The pistons, he continued, actuate a star-shaped nutating element (wobble plate) called a reciprocator that slowly precesses around, enabling the combustion components to operate in a near-sinusoidal fashion. The nutating body attaches the connecting rods to a ‘Z-crank’ with a single inclined journal bearing that drives the output shaft in the opposite direction. The rotating cylinders slide past intake and exhaust ports and spark plugs that are mounted in a stationary head ring. As each cylinder comes up to an intake port, air and fuel enter, and the charge is compressed before it is exposed to the spark plug.

Upon ignition, the fuel/air charge burns, sending the piston downward in the power stroke. When the piston rises again, it ejects the combustion gases out the exhaust port to complete the four-stroke cycle.

“The engine has five cylinders that are served by three sets of ports, spark plugs and fuel injectors to produce the same number of power strokes as a six-cylinder engine,” Fry explained. “So it has only three sets of plumbing, which saves on money and complexity; they’re just busier than in other engines.”

The Duke engine is indeed different, he acknowledged, joking that “we’re certainly not in Kansas anymore.” But the unusual configuration offers real benefits. “It’s just damn small,” he noted. Further, it can provide a 30% weight savings and a "significant" reduction in bill of material compared to engines of similar output, Fry claimed.

The air-cooled axial design features a compact combustion chamber that is “the size of a small can of tuna,” Fry said. This translates into “pretty efficient burning due to the short combustion path lengths.” The engine is claimed to have excellent resistance to pre-ignition and detonation because there are no hot exhaust valves and the cylinders have ample surface area for cooling as well as time to lose excess heat.

The engine can also handle high compression ratios, having run at compression ratios as high as 14:1 on 91-octane fuel, he noted.

Seals are a development focus

The Duke engine currently is fitted with port fuel injection, “but there’s no reason it couldn’t use direct injection” or other fueling systems. Emissions performance has yet to be characterized, but “it’s similar to a two-stroke.”

Hugh Blaxill, General Manager at Mahle, who has benchmarked several prototypes burning fuels including gasoline and JP-8 military fuel, said that the axial engines proved “fairly reliable right from the start.” He also cited their high specific power, good balance and knock characteristics, and relatively good—competitive—fuel economy.

Blaxill mentioned two potential weak points in the design. The Duke engine’s seal arrangement—sliding metallic seals operating on an oil film— evokes questions about durability. The company contends that the seals are analogous in function to those in a ported two-stroke or Wankel rotary engine, which means similar measures can be applied.

“It not as bad as a Wankel though,” he commented. The other design issue is the need for the single bearing to deal with crankshaft stresses, but “it’s nothing that good design and engineering can’t handle.”

Blaxill conceded that "a complicated engine like this is a hard-sell for automotive OEMs," but with the shift in the marketplace, it could find use as a range-extender. Probably its best chance, though, is in a military and aerospace application, he said.

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