Unmanned Systems Technology 001 | UAV Factory Penguin C | Real-time operating systems | Hirth S1218 two-stroke twin | Base stations | ASV C-Enduro | Composites | Datacomms

58 Moreover, no water-cooled engine met the C-Enduro’s weight budget; those at the required power level were at least three times as heavy as an air-cooled equivalent. In the end, Hyperdrive was able to re-engineer an off-the-shelf single-cylinder air-cooled diesel unit from an undisclosed manufacturer to meet its weight target. This simple, light yet robust engine is a naturally aspirated unit with a bore of 74 mm and a stroke of 65 mm for a displacement of 279.6 cc, and was designed to run at a fixed crankshaft speed. In the C-Enduro application it normally needs to run at its maximum efficiency speed for most of its operating schedule, and at other times at a higher power level, when the craft needs additional electrical energy in certain transit conditions. In view of this it has two operational conditions – high-efficiency mode and high-power mode. The diesel engine used by Hyperdrive features light-alloy construction and the use of a single camshaft to control its two valves and a mechanical injection pump. The valves are each operated via a rocker, pushrod and overhead finger follower while the pump is operated via a lever. The gear driving the camshaft is incorporated into the oil pump, helping to keep the entire package simple and hence light. Fuel is supplied by an electric 12 V lift pump from the tank to the injection pump, which in turn pulses fuel at high pressure to the single injector. The lift pump and the injection pump fuel rack are both under the command of the CAN ECU, and in trials the engine has run satisfactorily on automotive and marine diesel as well as biodiesel. “The really interesting thing is running an air-cooled engine in a sealed environment,” Irish says. It sits sealed within the left-hand hull and its standard cylinder and head cooling fins are retained; cooling air is supplied to them via a fan driven by the flywheel. The ducting not only takes it over the engine but also carries it beyond so that it slows and passes through a bespoke air-to-water heat exchanger designed by Hyperdrive. Chilled back down in this manner, it is then recirculated in a loop, back over the engine. As with the cooling air, the water in the heat exchanger circulates around a sealed system. This is a fresh-water circuit that passes through a fresh-to-sea water heat exchanger incorporated into the rear of the left-hand hull. In effect, the rear corner of the hull is a double- skinned aluminium pod, designed to allow the coolant to flow next to the surrounding seawater, lowering its temperature normally from about 85 C to about 30-35 C. The fresh-water coolant is circulated by a Stewart EMP electric pump, the speed of which is controlled by the CAN ECU (the pump being linked via the CAN system) according to cooling requirement. The fresh-water coolant system can be used to cool the power electronics as well as the engine cooling air supply, but that has not been found to be necessary. Having a compression ratio of 21.5:1 the engine runs to 3500 rpm and produces maximum power of 4 kW (5.3 bhp) at the crankshaft. Fuel consumption is 265 g/kWh at 2300 rpm, the engine running at 2300 rpm for maximum efficiency and at 3000 rpm when maximum output is required. The crankshaft is fitted with Hyperdrive’s own flywheel. This dispenses with the standard alternator and provides less flywheel effect since the associated electrical machine has its own flywheel effect. The electrical machine is a permanent-magnet device with a three- phase output, power electronics feeding its current to the dc supply or reversing its operation to act as a starter motor. Compared to applications for which it was originally designed, the engine thus needs no starter motor or alternator. The coupling between the flywheel and the electrical machine is cushioned: the engine has a counter-rotating balance shaft but still the electrical machine needs to be protected from its vibration. “We had to work hard to match the torsional modes of the engine and those of the e-machine,” remarks Shaw. “This torsional damper is a resilient coupling, and there was a lot of tuning of that to ensure we could dampen it, not only from a mechanical longevity point of view but also from load spikes. With it being a small single-cylinder diesel engine, that is the worst torsional vibration source you could imagine, so we had to be very careful to protect the e-machine.” Being a diesel, the engine has no spark requirement and no throttle, and it uses a straightforward mechanical injection system, the only adjustable parameter for which is the quantity of fuel November 2014 | Unmanned Systems Technology The C-Enduro carries a diesel engine range extender as standard