Unmanned Systems Technology 033 l SubSeaSail Gen6 USSV l Servo actuators focus l UAVs insight l Farnborough 2020 update l Transforma XDBOT l Strange Development REVolution l Radio telemetry focus

74 has five air ports and one exhaust port designed to optimise the scavenging dynamics. The fuel injectors sit inside the intake port, very close to the piston clearances. “We’re using two extended-tip injectors per cylinder,” Krzeminski says, “There’s actually very little space between the fuel injectors and the piston skirt, and immediately after cylinder flush the injectors spray right after the piston thrusts down past them, almost directly into the cylinder. “While this is technically port injection, because the injectors sit in the intake port and spray from inside the port, it actually works almost like a low-cost version of direct injection. There’s effectively no fuel in our intake airstream.” Four injectors are installed because earlier CFD studies showed that using two points of injection per cylinder with smaller injectors provide more balanced atomisation and fuel-air mixing at wide- open throttle (WOT) than a single larger injector. This arrangement also allows one injector to be switched off when idling, to reduce emissions. “At idle, we’ll use just one injector at a higher flow rate, as using two injectors at half that rate can lead to inconsistent flow control,” Krzeminski notes. “At the lower ends of their power curves, injectors often experience sharply upward-curving rates of flow, so there’s the potential for wasted fuel and unnecessary emissions due to very slight changes in power. “Having trialled this approach for a while, we’re increasingly limiting the use of the second injector to WOT, to increase the accuracy of fuel usage.” Strange Development has also trialled direct injection systems in the earliest versions of the REVolution, in which the fuel can be injected any time before a spark and after the cylinder flush stage. Krzeminski explains, “We went to port injection for the standard version to reduce the cost and complexity of the prototype. We can easily add more complex subsystems once customers are used to running and maintaining the basic REVolution, and they’re confident in our ability to provide quick servicing and replacement parts when needed.” The cylinder head is made from two aluminium plates, with 12, M8 fasteners – six around each cylinder – bolting the upper plate atop the lower (and both onto the engine block). Once fastened together, the head encloses a figure-eight-shaped cooling jacket designed for balancing water flow and thus temperature as needed across the tops of the two combustion chambers; a pair of inlet holes allows the water to enter upwards from the cylinder jackets. The upper plate also has an exit port for the water to leave the engine through an external hose. Transmission The assembly of gears and belts on the engine’s front is removable and modular, in order to accommodate any need for a UAV to drive different alternators or package the ancillary systems differently. “Modularity for hybridisation is key for reducing operating costs, heat signature and noise, without relying on the 30 kg of batteries you’d need to achieve the kind of endurance a fuelled range extender can give,” Krzeminski says. “I worked for a time for two major US automotive OEMs, and I noticed that their intake manifolds, exhaust arrangements and accessory drives were always different depending on the engine packaging they were paired with. We took that kind of approach, to enable quick modifications of everything around our engine block, especially when seeing how many UAVs were using engines built specifically to fit within their airframes.” He adds that his automotive experience proved very useful when designing the engine’s transmission, resulting in something akin to a serpentine belt August/September 2020 | Unmanned Systems Technology The transmission system uses two belts for driving the supercharged induction, exhaust valvetrain and oil pump from the crank gear