Unmanned Systems Technology 036

68 Dossier | Hirth 3507-01M gasolines and heavy fuels using the compression wave from the piston. For the 3507, however, Hirth has instead opted for a direct injection approach more akin to that of the FlexDI system developed by Orbital UAV for its N20 engine (as featured in UST 8, June/July 2016). Similarly to FlexDI, Hirth has designed a compressed air-assisted direct injection system to optimise the 3507’s power efficiency and fuel consumption via fine atomisation of fuel droplets, maximising the fuel’s ratio of surface area to volume. A common fuel-air rail assembly mixes and injects the heavy fuel, with the pressure of the aforementioned compressed air acting as a method of transporting and distributing the charge spray. Fuel first runs through a preliminary fuel filter before passing through an electric pump, then a fine filter, and into the fuel-air rail at 7.2 bar via a fuel inlet nozzle on one end. It then goes to two top-feed solenoid fuel injectors that deliver the fuel (metered by the ECU) to the rail’s internal pre-chambers. Pressurised air is supplied by a compressor mounted on an adapter plate on the rear of the engine, and driven by a belt running off a gear on the back end of the crankshaft. Ambient air is first drawn in through an air filter before running into the compressor assembly. The compressor itself is a reciprocating assembly, with its own piston, con rod, cylinder head and monobloc made from aluminium silicon alloy, and a crankshaft made from hardened steel alloy and supported by two ball bearings for transmitting the motion of the belt drive to the piston. The piston compresses the air feed and sends it through a hose to an air inlet nozzle on the opposite end of the fuel-air rail from the fuel inlet. From there, the air runs to two air injectors, each of which sits in the rail beneath the solenoid fuel injectors and connects to the pre- chambers, which then inject the metered and atomised pre-mixture of fuel and air into the combustion chambers at 8.0 bar. “We wanted to make sure the fuel injected is vaporised before it goes into the cylinders, using the huge amount of air passing through the rail,” Bitter says. “Once the atomised pre-mixed fuel and air go into the cylinders, they combine with the fresh intake air coming from the crankcase for an ideal mixture of fuel and air for combustion. “The injectors are off-the-shelf, while other key components such as the pressure regulators and fuel pumps have been heavily customised to ensure the rail handles its fuel and air precisely as we need.” Any surplus fuel flows out of the rail through a fuel regulator and return port, with unused air exiting the rail through a surplus air port. Starter/generator Although the 3507 was originally designed with a 600 W electric starter system, installed on the opposite side of the crankcase from the throttle, Hirth has designed a starter/generator to replace it. The starter/generator is a February/March 2021 | Unmanned Systems Technology Graph showing the 3507’s performance over its rpm range The 3507 has incorporated extensive CFD work as well as direct injection to optimise its fuel consumption