Unmanned Systems Technology 010 | nuTonomy driverless taxi | Embedded computing | HFE International marine powertrain | Space vehicles | Performance monitoring | Commercial UAV Show Asia report

55 tests was to establish how many times the engine could be started, stopped and then restarted consecutively without the starter motor overheating. The buoy’s battery allows 69 such cycles, and that was proven to be within the thermal capability of the starter motor. However, in practice the control system allows no more than 20 such attempts, to have a margin for error, in case of a problem such as seaweed clogging the impeller. In that case the H70 will go into a fault mode, meaning the user has to retrieve the craft. Anti-syphon system The H70’s water pump is in the bottom of the engine, and is driven off the crankshaft. It has a flexible rubber impeller so that it can handle seawater with whatever impurities come with it. The pump has a bypass since it is creating pressure the whole time. The bypass is built into the oil pan section of the bespoke lower crankcase, and its flow also goes around the starter motor/ generator to cool it before feeding out of the back of the buoy. Thus seawater cools the engine and the starter motor/generator. “We had to devise special cooling and exhaust systems because normally the engine sits above the water level, whereas in our case it sits below,” explains West. “We had to do thermal analysis and flow simulation to get all the temperatures right,” West says. “Our power unit is enclosed within the buoy’s sealed engine bay; the electronics [bay] and the fuel tank are all in the same sealed area. We have to keep that below about 100 F to prevent vapour lock in the fuel system. “That was a big challenge. The standard exhaust manifold goes down from the cylinder head, but that is above the waterline in the original engine design. “It can easily exit from that point in the new design, but since the waterline in our application is above the engine, we have an anti-syphon system whereby from the standard manifold we HFE H70 marine powertrain | Dossier Unmanned Systems Technology | October/November 2016 assembly. The spider maintains the stock bolt arrangement tying the lower to the upper crankcase. The lower crankcase is formed of three segments – upper, middle and lower. Immediately beneath the top segment is the helical mitre gear set to power the water jet impeller, which has its driveshaft at 90 º relative to the crankshaft axis, supported by two bearings and with an oil seal beyond them. Those are SKF roller bearings, supported by the adjacent segments of the bespoke lower crankcase. Beneath the power take-off is HFE’s own vertical shaft, extending down from the end of the stock crankshaft. An additional main bearing supports the top of the extended shaft. Immediately below that is a high-pressure oil seal, the upper of a pair that sandwich the camshaft’s straight-cut gear drive. The straight- cut gear on the extended shaft meshes directly with the gear driving the camshaft. The camshaft drive gear also powers the oil pump below, via a short axle. The pump is within the oil pan section of the lower crankcase. Beneath the lower of the pair of high-pressure oil seals is the water pump impeller, with its housing formed by the lowest segment of the bespoke lower crankcase. That bespoke assembly has oil and water passages that connect with the stock oil and water passages in the retained base engine section above. The engine coolant system sees seawater enter the coolant pump through a side inlet from the water jet drive (on the high-pressure side of the water jet drive impeller). The inflow passes through a soft vane impeller pump into a channel that incorporates a pressure relief valve for bypass and a thermostat. When the mechanical thermostat allows passage (once the cylinder head is up to working temperature), coolant is fed around the cylinder barrel and head with a single exit that takes the flow into a tube that runs coaxially with the exhaust pipe, which it surrounds.  The oil system is wet sump with lubricant circulated, including through to the cylinder head by the stock mechanical trochoid pump, which is fitted into the bespoke sump (the oil system is vented to the atmosphere). The bespoke lower crankcase incorporates the mounts that secure the engine to the buoy’s hull; isolators provide vibration damping. The H70 is 279 mm long, 254 mm wide and 209 mm high, and weighs 18 lb complete with intake manifold, throttle body and injector, flywheel, exhaust and starter motor/generator but excluding the water jet impeller and the engine management system control box. Some key suppliers to the H70 Fluid line quick connects: Battlefield International Waterproof servos: Hitec Servos Electrical connectors: Omnetics Connector Corp Electrical connectors: PEI-Genesis Starter/generator: Maxon Motor Fuel lines and fittings: Festo Spark plugs: NGK Fuel injectors: Honda Air filter: K&N Machining service: Central Components Dyno: Currawong Engineering Rapid prototyping: Stratasys Direct Manufacturing