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

67 ATI weighed up redesigning the engines with stronger, specialised materials, coatings and heat treatments, but eventually determined that it was better to replace the pepper pot pre-chamber with the chute-type on the Gen 2 engine. Previously, the WAM architecture had a piston that was free to rotate around the cylinder axis, using a spherical joint to the con rod, but the chute pre-chamber required a stable receptacle, and rotation was no longer permissible. As Franklin explains, “The surface [bearing] area of a spherical joint can be appreciably higher than that of a gudgeon pin, and is hence suggestive of a more robust joint – which in practice actually turned out to be incorrect. Several other engines and air compressors have used the layout, however. “So we went from the spherical connection to a more conventional gudgeon pin con rod, which is made from EN40B nitriding steel and has a split-cap big end,” Newton comments. In the interest of adding a further 40 hp, a design was generated that added a fourth cylinder to the rear of the standard engine. The architecture was designed from the start to be modular, to enable a series of longer and more powerful versions to be produced. “We could easily explore a six- or eight-cylinder version with 300-400 hp, maybe inline or even a flat-six or flat- eight configuration if the right opportunity arises,” Franklin says. “The standard, fully inverted piston arrangement is more challenging to model and engineer than a horizontal piston pack. We’ve done plenty of simulations, and it clearly works.” The first updated version of the engine to be put in metal was the three-cylinder WAM-125BB. It proved far quieter, more reliable and with less vibration than its predecessor as a result of the bore, pre-chamber and piston joint changes. Although that engine has not gone into full- scale production, it underwent thousands of hours of dyno testing and test stripping, with the first run in March 2008. Once the WAM-125BB was tested and proven sound, further development work followed, with the four-cylinder WAM- 167BB project starting in 2014. “It was about a nine-to-12 month exercise to create a four-cylinder proof-of-concept prototype, with the proof-of-concept testing on that first prototype being completed by the end of 2014,” Newton recounts. Enough parts and components have been machined for three WAM-167BB engines, with typically one or two at a time running on the company’s test rigs. More than 2000 hours of testing have been conducted to date. “We consider 160 hp or above to be more viable commercially for a full production engine than 120-125 hp, particularly for the UAVs that would use it. The 167 hp on the WAM-167BB is a conservative figure; we may well revise that to 180 hp in the future,” Newton notes. “The underlying technology has also long been proven on the Gen 1 engine. We’ve changed the bore, pre-chamber and piston joint, and added a cylinder, but tests have shown that the Gen 2 hits all the targets for power output, SFC and so on that we’d hoped for. “At the time, it meant a 30% increase in manufacturing cost – and that wasn’t from simply adding an extra cylinder. Time had passed, the 2007 financial crash had been and gone, and so many parts were different and not being bought in similar volumes, but it gave us a 40% increase in power output.” Engine performance The power band of the engine encompasses an ‘economy cruise’ mode and a maximum continuous power output rating. The former produces 82 kW of power (110 bhp), and 325 Nm of torque, at an engine speed of 2400 rpm. The latter works at a speed of 2560 rpm while outputting 100 kW (135 bhp) and 375 Nm of torque. The naming difference between the two is explained by comparing their SFCs – 240 g/kWh at economy cruise speed; 250 g/kWh at maximum output. As mentioned, peak output is achieved at 2750 rpm, which gives 124 kW (167 bhp) and 433 Nm of torque for up to five minutes, for all-up weight take-offs and potentially for use as an escape velocity or pursuit speed. Unmanned Systems Technology | October/November 2019 The first-generation WAM design used a spherical joint between the con rod and piston, which has been phased out in favour of a standard gudgeon pin joint