Issue 45 | Uncrewed Systems Technology Aug/Sept 2022 Tidewie USV Tupan | Performance monitoring | Bayonet 350 | UAVs insight | Xponential 2022 | ULPower UL350i and UL350iHPS | Elroy Air Chaparral | Gimbals | Clogworks Dark Matter

85 ULPower UL350i and UL350iHPS | Dossier To carry out these functions effectively, the ECU is embedded with a 3D fuel map containing the mixture and timing control orders for operating at up to 18,000 ft, including timing corrections to prevent detonations, and a built-in rev limiter set to 3300 rpm on most ULPower engines for safety and fuel efficiency. The mapping is based on data received from five sensors. The first is the sensor for crankshaft position and speed, data from which enables the ECU to infer the position and speed of each cylinder’s piston and valves. That then allows the ECU to calculate when to trigger the spark plugs, and when to advance or retard the ignition timing. Next is the throttle position sensor (TPS). Throttle position data allows the ECU to infer and track the power and acceleration rates that the engine is running at any given moment. As a rule, TPS and crank rpm data are two of the most important inputs, as they form the x and y axes of the ECU’s fuel map. Meanwhile, the oil temperature sensor provides an insight into whether the engine is starting hot or cold, so that the ECU knows how lean or rich the charge should be to ensure easy starting and smooth running. Lastly, the sensors for manifold air temperature and air pressure supply the data inputs for calculating ambient air density, which is then used to further inform the correct amount of fuel to use. “If one of these sensors fails or becomes disconnected, the engine will continue to run but not as efficiently as it would be using ‘default values’ in place of the real ones,” Defoer says. “The only sensor that is critical to continued operation of any ECU- controlled engine is the crankshaft position sensor.” Cranktrain Being a flat-four design, the UL350 can run using a flat-plane crankshaft, with its crank throws and pins disposed such that the first and fourth pins sit at 180 º to the second and third. That makes for a simple design and construction with minimal counterweighting. The company cuts the crankshaft from a single piece of steel alloy (hence the con rods are split at their big ends), including drilling of oil channels for the con rods, with final balancing tests or modifications performed on each shaft before it goes for assembly. “We’ve also done special frequency tests to test the crankshaft’s torsional and bending characteristics,” says Defoer. “We put a laser vibrometer onto the engine and ran it across its operating range to see if there were any vibrations or harmonics at key points, and found we’re well below the practical limits for all regulations and mission types, so we don’t need any damping systems or similar.” A total of five split bushings (or plain bearings) hold the crankshaft in place. “We also have an extra ball bearing on the top of the shaft to pick up the axial forces acting on it,” Denorme says. “That’s important, because when you’re flying an aircraft, compared with an automotive engine the propeller can impart some serious axial pulling or pushing forces on the crankshaft. So we’ll put one ball bearing on our four-cylinder engines and two in our six-cylinders to keep the shaft exactly in position. That means our engines can be used in pusher or traction propeller configurations in UAVs.” He adds that as well as the crankshaft being nitrided for hardness and its pins polished for smooth motion, the Uncrewed Systems Technology | August/September 2022 Power is transmitted from the combustion chamber to a flat-plane crankshaft via split con rods and pistons forged from aluminium The ECU pulses all four injectors at once, rather than timing each spray for each cylinder at individual, specific crank angles; tests show it works fine