AIN Exclusive: The Higgs Diesel

 - November 8, 2016, 7:41 AM
The Higgs step-piston two-stroke engine applies new technology to an old idea.

Andy Higgs, a Brit who has lived in Japan for the last 32 years, is on a mission to bring a new engine to aviation, in the form of a jet-A-burning “step-piston two-stroke.” The three-year FAA certification testing program will begin late next year.

“We’ve built several engines to validate the technology, and they have all worked outstandingly well. There are no technical show-stoppers, and I signed off on the final design a couple of months ago.”

President, CEO and sole owner of Tokyo-based AC Corp., Higgs says the engine will be offered in various configurations, among them a 1,000-hp V12 weighing 665 pounds/302 kg (including reduction gearbox); a 350-hp four-cylinder weighing 302 pounds/137 kg (again including a gearbox to reduce the crankshaft’s 5300 rpm down to 2300 rpm for the prop); and a 1.5-liter V4 weighing just 103 pounds/47 kg and producing 120 hp. The commercial business model for the V12 centers on “competing with the low-end PT6 market, 580 to 1,200 hp, as a replacement engine for Cessna Caravans. The killer for Caravan operators is the cost of fuel. Like for like, power for power, our engine versus the PT6 will give the operator 40 percent more range.” For grins and some measure of technology demonstration, the 1,000-hp V12 is also destined to power at least one NXT in the Sport class at Reno at some point.

Higgs describes the technology: “It differs from a conventional two-stroke in that we’re able to isolate the crankcases, which have typical four-stroke lubrication. We can use plain bearings, and we do not have to mix oil into the fuel. We can separate the two-stroke concept from the crankshaft and lube mechanism.

“The cylinder is slightly longer than a typical two-stroke’s but much shorter than a four-stroke’s. We’ve divided the cylinder into two separate bore diameters: smaller for the upper bore, for what we’re calling the working piston; and below it we have the pumping piston. The piston looks a bit like a top hat. As it travels down the bore, it sucks air (and nothing else) into the pumping chamber. As it moves up, it pumps that air out of the pumping chamber and across to the next piston’s ‘working piston’ phase. As the piston travels farther up and closes the exhaust port, we fire the injectors directly into the cylinder, for a clean burn. The large volume of air being drawn in keeps the engine cool. We’re not relying on the pressure in the crankcases to move the air and fuel up into the transfer ports. This is why, on a conventional two-stroke, you’re limited to about 10,000 feet. Above that, the pressure reduces so much (and the delta becomes so great) that you can no longer transfer fuel.

“This engine is designed to have a critical altitude of 38,000 feet. With turbos it will go as high as anyone wants to take it. We’ve had people ask how they would deal with fuel congealing, which suggests they’re looking at exceedingly high altitudes. We have a way of heating the fuel, so there are no altitude limitations.”

The concept of the step piston engine dates back to about 1904, says Higgs, adding that “If you want to take a step forward into the future, you have to look back and see the ideas that were discarded because technology then (anti-wear coatings, manufacturing processes) weren’t clearly understood.”