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

of the cylinders. Keeping them airtight is key for preventing losses of charge air throughout the crankcase. The crankshaft interfaces with the engine block via six steel ball roller bearings – one at the front, one at the back, and two on either side of each counterweight. Four more such bearings are installed on the two REVs, and four needle bearings are installed across the two con rods, for a total of 14 internal bearings. The ECU controls the engine using a speed density strategy, mapping intake charge air pressure and temperature against engine rpm to determine fuel injection and ignition adjustments across varying speeds and altitudes. Also, since airflow can be controlled via the exhaust valve end and the throttle, there is potential to layer a throttle control strategy atop the speed density to increase the accuracy and speed of adjustments in the REVolution’s performance. Self-vacuuming crankcase To further assist in isolating the cylinder gases, six reed valves (arranged in two rows of three) sit under the crankshaft. Unlike with most two-stroke reed valves though, these six are not used for air intake. Instead, as the piston comes down, the valves provide an evacuation route for pushing the air beneath the piston out of the crankcase. The force of this air also helps push oil collected in the oil pan back out to the external dry sump. “We don’t use a scavenging component on the pump, as you might in a traditional dry-sump arrangement where you mechanically pump out the oil; we’re just using the engine as a pump,” Krzeminski says. “When the piston goes back up, the reed valves shut. That effectively creates a vacuum under that piston, limiting the rotational losses, because you’re not forcing the con rods and crank parts to move and spin through air anymore. “If you have a diesel engine running at 1000-3000 rpm, that’s not really an issue, but with a two-stroke gasoline engine spinning at 7000-8000 rpm, there are actually a lot of losses owing to air-induced drag inside the crankcase. That’s why our engine is designed the way it is, with the reed valves and shaft- mounted seals. “It’s not a totally new idea. For example, there have been a few studies in four- stroke designs that used reed valves in this way to reduce drag losses, but our system was designed from a blank sheet to suit our requirements for vacuuming the air and pumping out the oil.” Cylinder design Each cylinder has a factory-set injector bore angle as well as a perpendicular cylindrical valve seat. The cylinder liner is iron alloy (with carbon chrome and molybdenum) and as discussed it