Unmanned Systems Technology 028 | ecoSUB Robotics AUVs I ECUs focus I Space vehicles insight I AMZ Driverless gotthard I InterDrone 2019 report I ATI WAM 167-BB I Video systems focus I Aerdron HL4 Herculift

66 Dossier | Apple Tree Innovation WAM-167BB componentry designed into the engine,” Newton adds. “Comparing SFCs [specific fuel consumptions] is only part of the story, though – it has to be something like a fuel consumption-to-weight ratio.” As Franklin explains, “A direct injection version would almost certainly require heavier steel (rather than aluminium) pistons to survive the thermal and pressure loading on such a highly rated engine. These have a knock-on effect on the internal balance masses, together with the mass of electrically actuated injectors, sensors, high-current wiring harness, dual [redundant] ECUs and higher capacity dual [redundant] batteries and brackets, and so on.” IDI also means it can run on different fuels. As well as diesel, Avtur and JP-1, the company has successfully tested heating oil, kerosenes and a few biofuels. “It’s particularly challenging for a direct injection fuel pump handling kerosenes to survive anything less than the most perfect Jet A-1; battlefield environments aren’t exactly rich with that,” Newton notes. “But whatever you’re using for tanks, helicopters and jets, you can use in the WAM too.” Second-generation WAM engine The original engine was a 120 hp (89 kW) inline series dual-charged triple- cylinder two-stroke, aptly titled the WAM-120, and commonly referred to by ATI staff as the first-generation (Gen 1) engine. That engine had been widely used in light recreational aircraft for many years since its inception in 1997 and first flight in 1999, being issued as a ‘kit-built’ aircraft diesel engine. “While it worked well and met all the performance criteria of that application, our goal was to get it to a 2000 h TBO, taking it from a recreational system to a more commercial-grade one,” says Newton. “With proper maintenance, that engine was capable of up to 1000 h between overhauls, but the bigger aim of doubling the TBO is what spurred the development of the Gen 2 design qualities – what we call our ‘big bore’ engines.” The core aspect of the bigger bore involved widening the Gen 1’s cylinder by 7 mm, in order to increase the ratio of power generation to cylinder pressure within the existing bore spacing. Another key design change was spurred by a peculiar defect in the original pre-combustion chamber, which has been subject to an in-service replacement programme but never solved, despite hundreds of hours of investigation and considerable assistance from Engine Developments Ltd and Cranfield University, in the UK. The original ‘pepper pot’ type pre- chamber would sit in the cylinder head. At its end, a series of holes would spray burning fuel up into the firing end of the combustion chamber, onto the face of the piston as it closed its compression stroke (with a chamber formed in the piston crown, the floor of which was formed by the head flat). The IDI pre-chamber which ATI has since switched to has a single, tangential chute-like hole, which imparts a tangential velocity (or swirling motion) on the ignited fuel-air mixture, entailing a complete redesign of the pre-chamber body and injector. The pre-chamber is installed just to the side of the cylinder, rather than under it. The piston crown now has a different chamber, and the head has also been revised to fit the redesigned pre-chamber body. October/November 2019 | Unmanned Systems Technology The company’s original engine was the WAM-120, a three- cylinder unit produced mainly for light sport aircraft, with many of the same design points as the WAM-167BB The original engine used a ‘pepper pot’ type of pre-combustion chamber; this has since been replaced with a chute-type pre-chamber on the Gen 2 WAM engines