Unmanned Systems Technology 036

72 Dossier | Hirth 3507-01M from the bottom of the compressor to the throttle body. From there, the oil is scavenged to the crankcase. The recommended choice of lubricant is Aero Hirth Oil, developed in cooperation with Liqui Moly. It is a fully synthetic oil that can be used in pre- mixed as well as oil injection systems, and burns up effectively in both gasoline and heavy fuel without leaving any excess carbon deposits. Porting As discussed, the air-injected fuel mixes with fresh air entering the cylinders from the crankcase. The crankcase divides in half, with a ball bearing and sealing rings at the middle for airtight separation of either side. That ensures thorough compression of fuel and fresh air from the crankcase through to the combustion chamber. Air is drawn in through two filters on the side of the engine block – one for each crankcase half and each cylinder – via the throttle body assembly. The throttle assembly consists of two bodies cast from aluminium alloy and connected by a pair of carrier brackets. A throttle position sensor (TPS) is installed on the side of the forwardmost body, while the manifold air temperature sensor is installed in the front of the inlet duct of the rearmost body. Fresh air is then drawn up through the transfer ports by the piston’s downstroke to replace the exhaust gas as the latter escapes through the exhaust port. At the time of writing, the 3507’s cylinders were being constructed with four transfer ports disposed as per Schnuerle porting, with four ports running up from each crankcase half to each cylinder, evenly disposed either side of the exhaust port. Hirth has also recently confirmed via simulations and testing that adding a fifth transfer port will allow it to perform the scavenging in an even shorter time. This port will run up the opposite side of the cylinder from the exhaust, its outlet pointed in the direction of the combustion chamber. CFD simulations indicate this still induces sufficient ‘tumble’ with the air from the other transfer ports for effective mixing of fuel and air during compression. Bitter notes, “We have found a good supplier who can cast a high-quality cylinder to the complex and precise shape we want, and we will have implemented the fifth transfer port by the end of this year or thereabouts.” The exhaust manifold is secured with four fasteners on each outlet, and combines the gas flows into a single ball joint output for customers’ muffler attachments, with the exhaust gas temperature sensors installed between the ball joint and the cylinder outlets. “Although much of our design is still based on core knowledge built up over the years, recently we’ve redesigned the 3507’s cylinders to improve fuel efficiency via improved flow of intake and exhaust gases, since UMS Skeldar and other users are always requesting the ability to fly for longer,” Bitter says. “Part of that redesign was adding two outlet side channels to the exhaust – in addition to the bigger main exhaust outlet – for an overall increased exhaust area.” Another key part of the new cylinder design has been to lower the exhaust port by around 6-7 mm, while keeping the amounts of interchanged gases the same. The primary objective of that was to increase the power derived from combustion: as the ignited gases are kept in the combustion chamber (and kept from leaving via the exhaust port) for longer, more downward force is imparted to the pistons throughout the power stroke. Although the reduced height of the exhaust port has left a shorter window in which to evacuate exhaust gases to make way for fresh air, adding side exhaust ports has compensated precisely by accelerating the rate of exhaust gas evacuation and thus of air movement between crankcase and cylinder. “The degree of compression is about the same as before, owing to February/March 2021 | Unmanned Systems Technology Fuel is pumped into the 3507’s direct injection rail at (1), with compressed air entering at (5). The rail integrates many components, including fuel solenoids (2), air-assisted fuel injectors (6) and regulators for the fuel (4) and air (7) The air compressor is driven by a belt from the crankshaft to provide the pressure for the direct fuel injection system