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

68 Dossier | Strange Development REVolution because it wasn’t getting any energy back from compressing the gas. “Once the compressed gas was pushed into the engine, the piston would come down and the extra charge would just escape out through the exhaust. We drew all of this out on a whiteboard and laid it out for the client, and I remember pointing at their exhaust manifold and saying, ‘You need a valve, right here, to stop or slow the airflow from leaving the engine’.” From these three observations – the need for high power-to-weight ratio, the excessive complexity of poppet valves, and the countervailing need for a valve to improve the scavenging and exhaust control to fully exploit the benefits of supercharging – the idea of the rotary exhaust valve (REV) was born. The rotary exhaust valve In 2015, Strange Development started running 1D simulations of a two-stroke with an axial-flow cylindrical rotary valve similar in principle to that developed by Bishop Innovation and Mercedes-Ilmor for the exhaust port in Formula One V10 engines. The company used the bottom half of its previous poppet-valve engine design but redesigned the top half to use a REV instead. The team quickly noticed that not only could the exhaust valve increase the two-stroke’s power density, and yield more from supercharged induction, but depending on how the valve was timed, it could also enable better control of greenhouse gas emissions. Considerable design work has gone into the timing of the rotary valve system and optimising it for the airflow in and out of each cylinder. As a result, the typical REV unit has a few key specifications. The valve is machine-cut from stainless steel (to prevent damage or thermal expansion from the exhaust gases, which can reach temperatures of up to 1200 F (649 C), and its port measures about 68 mm in diameter. “We might make it from titanium to save weight in the future, or from aluminium with a thermally resistive coating,” Krzeminski adds. “The way we cool the valve – with water circuits and oil injection – is absolutely critical to ensuring that it operates smoothly, as it fits with very close tolerances inside a chamber that runs across the two exhaust ports, and we can’t have it or its bearings expanding and sticking from the heat. “As for sealing, we don’t look at the valve like a poppet valve, which has to provide 100% sealing; the valve is there to control flow dynamics, not to completely seal. Even so, whenever the piston crosses up past the exhaust port, sealing by the valve is no longer needed because the piston provides all the sealing necessary.” August/September 2020 | Unmanned Systems Technology The rotary exhaust valves are machine-cut from stainless steel, and a seal is installed on each end of its cylindrical section The changing movements and relative positions of the piston and exhaust valve effectively make for six stages in the REVolution’s two-stroke cycle