Uncrewed Systems Technology 049 - April/May 2023

Rotron Power RT600-HC | Dossier is still technically possible, so in some applications the system does not fit the exact definition of a FADEC. However, considerable time and resources have been spent on optimising it for full authority control in all speed regimes, along with other capabilities beyond most COTS ECUs. The system has been co-developed with General Engine Management Systems to tailor its functions to UAV operations. It is a major departure from Rotron’s use of an aftermarket automotive ECU used in motorsport. “For instance, the ECU continually tests and checks the twin-pump system to ensure it’s functioning correctly, and if one were to fail it would trigger a check valve to ensure fallover to the next pump,” Head says, adding that it will also send a warning signal to alert the platform’s flight controller. “For rotorcraft and hybrid applications, it’s important that the engine can manage itself relative to load, and we have full control and access to tune and develop the rates and gains within that, to ensure the engine can maintain a given rpm if needed.” The ECU’s programming, diagnostics and commands take place over CAN. A lambda sensor is installed for closed-loop control of the air-fuel mixture, although if the sensor fails the ECU drops back to its base mapping for open-loop control. “We also use manifold air pressure [MAP] sensing for engine management,” Head adds. “We used to do a lot solely on throttle position sensing, but found MAP gave better repeatability and throttle response. “We’ll add a lot of standard engine health sensors for things like fuel pressure, and the higher the altitude, the more we’ll want to measure things like vapour lock, temperatures and resonances in fuel systems. Fortunately, we have an altitude testing chamber that has been critical in developing the ECU and ancillaries, and for running the engine on Avgas, which is important for higher altitude use cases.” Ignition The version of the RT600 featured in UST 4 was notable for using four spark plugs – a pair of twin plugs per rotor. Through research into multi-fuel and heavy fuel Wankels around that time, Rotron had also begun experimenting with designs with four spark plugs per rotor, and Head notes that some test prototypes used far higher numbers. “Those helped us experiment with how greater numbers and different arrangements of spark plugs could aid the efficiency of the burn, and we eventually settled on a compromise of four plugs per rotor in the standard RT600-HC,” Head explains. A 2 x 2 arrangement of plugs visible from outside the engine achieves the desired spread of ignition across the combustion chamber; these fire as pairs in a phased order. Rotron says this firing order has increased fuel efficiency significantly, equating to an additional 3-4 bhp across much of the operating range, as well as achieving a cleaner burn. Inside, the trailing spark plugs in each group sit inside what Rotron calls a ‘pre-combustion chamber’. This is not a chute- or pepper pot- type pre-chamber as might be seen in reciprocating diesel engines, but a hole drilled into the epitrochoid to carefully specified tolerances, which contains the spark plugs and is pressurised with the introduction of charge. “The charge is ignited in the pre- combustion chamber as well as in the combustion pocket, providing an extra bit of burn as the flame front moves across the combustion chamber,” Head says. “Because of the way we’ve shaped the pre-combustion chambers and the phased firing order around the flame front, we get a much cleaner, fuller burn than in previous versions of the engine.” The trailing spark plugs are standard electrode-type plugs, while two slightly more expensive surface-discharge spark plugs are used in the leading plugs, this type having originally been invented for use in Wankels to prevent impacts with the apex seals. “Rotary engines have a long, thin, moving and square-shaped combustion chamber, which naturally makes it hard to achieve the efficient squish of a reciprocating engine’s round combustion chamber, and the way the charge is compressed into the spark plug in the middle,” Head explains. “But we’ve changed the profiles 87 Uncrewed Systems Technology | April/May 2023 This twin-pump fuel system enables a redundant fuel supply to the injectors