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

75 Strange Development REVolution | Dossier system for distributing the crank force to other onboard processes. The transmission assembly has two timing belts. An upper belt looks after the timing and control of the valves and oil pump, while a lower belt drives the supercharger. In the upper belt, the crank force is delivered via an idler wheel that sits above the crankcase on a bearing mounted on the transmission assembly’s inner front plate, and is directly actuated by a crank gear near the front of the crankshaft. The belt runs over the top of the idler, and is kept taut by running down beneath the valve gear and the oil pump gear. The forwardmost of the two REVs is machine-cut, and has a shaft extending out of the valve seat chamber, with a gear fastened on its end. A larger crank gear sits on the very front of the crankshaft, driving a sprocket on the front of the supercharger via the second timing belt. A tensioner wheel presses down on the upper part of this belt to secure it. “We’ve had to take care of a lot of different belt dynamics, and we’ve put a huge spread of design choices into the transmission to ensure the right tension and support during long-run operations,” Krzeminski says. Forced induction Although the engine previously used a centrifugal-style supercharger, it was originally designed with – and has returned to using – a Roots-style supercharger (with a screw-type Roots charger from Eaton expected to be used in full production). It supplies 500 cc of air with each revolution of the crankshaft. “When we used a centrifugal charger, we had a lot of idle-stability problems, because we simply weren’t moving enough air at idle,” Krzeminski recalls. “We looked at several types of supercharger, and simulated and dyno- tested a range of them. The centrifugal charger works with no problems at higher rpm, but the idle stability was so poor that we switched back to Roots-type positive displacement.” Strange Development plans to develop a twin-charged REVolution. This will use a high-efficiency Roots supercharger to help with idle and low-rpm stability without incurring parasitic losses, and a turbocharger at the higher rpm – most likely a BorgWarner model or one from Garrett Motion – to better drive the airflow through the engine. It also intends to use an intercooler between them to cool the charge air, resulting in what the company envisions as an even more power-dense version of the engine. The future As mentioned, the company is also working on a heavy-fuel version of the engine, which will feature moderate changes to the cylinder head and injection system, although the block and other components are expected to remain largely unchanged. At the time of writing, Krzeminski and his team were finishing calibration work on the standard REVolution, studying how the engine responds at different throttle inputs. The next planned step is to run UAV-specific durability cycles before planning flight tests with an appropriate partner. Unmanned Systems Technology | August/September 2020 Inline two-cylinder Two-stroke Gasoline Supercharged Liquid-cooled Weight: 49.89 kg Dimensions: 400 x 350 x 300 mm Maximum power output: 220 hp (164 kW) BSFC at maximum power: 221 g/kWh Maximum torque: 130 lb-ft (176.26 Nm) Maximum rpm: 7000 tested, 8000 expected Bore: 74.8 mm Stroke: 65.8 mm Displacement: 578 cc Static compression ratio: 10:1 BMEP: 25 bar TBO (expected): 1000 hours Some key suppliers Crankshaft: in-house Cylinders: in-house Con rods: in-house ECU: Cosworth Throttle: Bosch Fuel injectors: Bosch Supercharger: Eaton Coatings: Line2Line Coatings Bearings: NTN Bearings Reed valves: Boyesen Cylinder sleeves: LA Sleeve Specifications The rotary valve interconnect is machine-cut to fit the timing between them