UST030

66 Dossier | Ishikawa Energy Research ARE series hybrid the pump for cooling and cleaning the lubricant. The pressure and condition of the oil are checked via an oil level gauge and an oil sensor respectively. These are installed in the right half of the sump, and are key to determining when the oil needs to be topped up or replaced. Electricity generation Each generator is roughly 15.5 cm in diameter, and notably features coils wound from aluminium rather than copper. “Copper has a specific gravity of 8.89, while aluminium’s is roughly 2.7, so the latter is far lighter,” Ishikawa explains. “And while the electrical conductivity of aluminium is maybe 40% less than copper’s, the fact remains that aluminium has a much higher conductivity-to-weight ratio than copper, meaning a lighter and more power-dense hybrid engine for the same consistent power output than a copper-winding-based system.” In normal operation, each generator produces 4.5-5 kW (or 5.5 kW at maximum continuous output), so the ARE can output up to 11 kW of electricity altogether. “However, our system is also designed to integrate supercapacitors,” Ishikawa points out. “It may be that up to 13 kW are needed at certain points of operation, for example if hovering amid very strong gusts while operating a payload. So the capacitors can quickly supply up to 4 kW for 3 s, to help with sudden spikes in power being drawn.” The company notes that the capacitors do not serve principally as energy storage units – that is their secondary function – the bulk of onboard energy storage is already provided by the fuel tank. Primarily they are a buffer to respond to the constantly changing power demands typical of a heavy-lift multi-copter. The Aichi multi-rotor UAV’s motors will constantly be adjusting speed and power to maintain attitude, position, heading and other command functions, and the capacitors are critical for quickly changing the power supply to help them with that. Ishikawa and Fukuda anticipate that their system could be designed to use batteries if the charging system were engineered to provide enough safety and protection, and if the end-user strongly preferred a standard battery pack. However, the ability of the supercapacitors to absorb excess current from the engine when needed, or to instantaneously discharge power to satisfy the lift motors when they suddenly demand another 2-3 kW without warning – and without damaging the capacitor – makes them a default choice for regulating the power supply from the generators to the UAV. That is especially so since batteries can be severely damaged or even explode if they’re subjected to uncontrolled input or output current. In its standard configuration, the ARE integrates three 16 V (nominal) DC capacitor modules from Maxwell Technologies, which are rated to 500,000 charge-discharge cycles and a maximum output of 17 V each. Weight in particular was a key consideration behind this selection, and each module weighs 630 g. A power converter is installed between the capacitors and the engine to regulate the generators’ 70 V AC supply down to 50 V DC to go through the capacitors, which add or subtract power and current as needed before being fed into the UAV’s subsystems. “Although batteries store more energy, the capacitors store enough ignition power for three start-up cycles, with up to 0.35 Wh maximum per cell,” Ishikawa adds. Future and conclusions Testing of the ARE is ongoing. Three of the current prototype have been built, and it has been tested to 100 hours of continuous operation so far, compared with the more matured co-generation engine, which has been tested to over 10,000 hours of non-stop operation, twice. “Our next target for continuous operation of the UAV engine is 200 hours non-stop,” Ishikawa says. “We’ve experienced no issues over 100 hours so we’re comfortable stating that as our current rated TBO for the time being – but of course we’ll be testing it over longer and longer periods as development goes on.” The company expects to achieve 200 continuous hours easily, and if IER’s past projects and experiences are any indication, further milestones doubtlessly lie ahead for the ARE. February/March 2020 | Unmanned Systems Technology The engine generates a peak of 11 kW, with aluminium wire used for its higher conductivity-to-weight ratio than copper