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

70 from about 90 º to 120 º , to begin cylinder flush. By the latter point, the piston crown is nearly at the bottom of the exhaust port, and the intake ports are partially opened, but the REV is continuing to revolve backwards, closing the exhaust port. “Again, if intake air escapes out through the exhaust, it’s a waste of the supercharger, and reducing the oxygen content in the exhaust allows for add-on technologies such as catalytic converters that you don’t usually get to use with a two-stroke,” he says. “With the REV changing the exhaust port ‘shape’ by closing upwards helps draw the incoming scavenged air in an arc towards the cylinder head, which helps a lot with fuel mixing later.” As mentioned, the engine’s unconventional scavenging design does not feed air into the crankcase. The intake air path is completely separate from the lower crankcase and the oiling system, with air fed directly into the cylinder via an intake piston port, and the piston behaving as the valve mechanism. As the piston uncovers the port, air flows in and is stopped when the port is covered again. So long as the intake port is uncovered, the low position of the piston ensures in-cylinder pressure is lower than the pressure of the supercharger air. The latter is typically around 1.2 bar, although this changes depending on where the engine is performing in its map and operating range, ensuring air flows undisrupted into the cylinder. In-cylinder pressure will not rise above intake pressure until after the intake port is already covered. When the REV has completely closed off the exhaust port, and the pistons have completely uncovered the intake ports, cylinder charging takes place. This brings the biggest benefit of Strange Development’s rotary valve – typically, when a piston is at bottom dead centre, a direct channel is opened between the intake and exhaust ports. With the company’s custom valve shutting this off to control the scavenging, internal cylinder pressure can be raised above atmospheric. “This is where we get our power density from,” Krzeminski says. “A traditional two- stroke cannot effectively supercharge without something stopping exhaust flow. Companies can try altitude compensation, pipe tuning or other approaches, but our method will actually significantly raise cylinder pressure above atmosphere in a reliable and consistent way, to an extent we’ve not seen in any comparable engine. Our long-term target for pre- combustion pressure is about 15 psi above atmosphere.” Also, by having a supercharger and a valve for controlling it, the combustion pressure can be changed dynamically. This means that if any future version running on heavy fuel (or other fuel type August/September 2020 | Unmanned Systems Technology The way in which the valve opening’s shape changes also improves the fluid-mixing dynamics before combustion