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

71 Strange Development REVolution | Dossier prone to knock) began detecting knocking, the REV and supercharger speeds could be slowed down to limit its occurrence. To achieve the optimal swirl of air for mixing with fuel in the injection stage, the cylinder liner has five scavenging air ports. Each is only half as tall as the exhaust port, so the intake stays cut off during exhaust. With the exhaust port, this gives a total of six ports. “The liner actually looks a lot like a traditional two-stroke sleeve, but with a single larger exhaust port and without reed valve intake ports. We’re controlling upper cylinder pressure purely with the REV’s sealing,” Krzeminski notes. “The most important scavenging air ports are the ones immediately next to the exhaust port. If we had only the crossflow-type ports that sit across from the exhaust, our CFD showed that the air would travel straight across the cylinder, with nowhere near enough scavenging of this air out of the chamber. “The air from those Schnuerle-like ports ‘catches’ the air from the crossflow ports in the middle of the chamber during the charging phase, so it mixes just how we want it to in the injection phase. “Balancing the flow from intake ports is something Detroit Diesel always had problems with in their uniflow-scavenging system. Turbulence can be a bad thing for uniflow systems, because you want almost linear movement of the air as a monolithic ‘slug’, but for us, turbulence is actually quite useful.” Fuel is sprayed at 50 psi and an angle of 15 º , after the REV is closed but before the intake port has shut, to avoid pushing fuel out with the exhaust. The compression stage starts once the intake port is fully shut off. Krzeminski notes again that in most two-strokes the exhaust port would still be slightly open at this point, resulting in less pressure build-up. “So this gets us about 87 cc of additional compression that you wouldn’t normally have, hence more cylinder pressure than an average two-stroke, not just from better supercharging control but from starting the compression stroke earlier.” Finally, the power phase starts as the piston nears TDC. The air inside the cylinder is compressed completely for ignition; outside the cylinder, although the exhaust port is now shut off by the piston, the REV has reopened almost fully. Once combustion occurs and the piston thrusts back down into the crankcase, the valve will again be wide open to enable the operating cycle to be repeated. “The rotational momentum of the valve runs on fairly loose roller bearings, and receives ample lubrication. These keep losses and friction down to a minimum,” Krzeminski says. Iterative development After successful 1D and 3D simulations of the new engine and valve concept, Strange Development began prototyping the system to begin testing and iterating its designs. Through its in-house cutting and casting machinery, the team was able to make quick adjustments to geometries and tolerances wherever it wished. “We probably went through 10 iterations of the cylinder head geometry alone, for optimising the airflow and mixing dynamics inside the top of the cylinders,” Krzeminski says. “We’ve been running it on the dynamometer for six or seven months, and have accumulated nearly 400 hours of testing. We built a 300 hp dyno and customised pretty much everything on it, other than the head itself, to suit the engine’s specialised design.” Most of its components have been machine-cut from 7075 aluminium alloy. This was selected for having the highest strength-to-weight ratio among the available metals, and for its track record in aircraft engineering. “And because it’s so strong, we’re confident we can reduce our weight below 110 lb,” Krzeminski says. “We’re engineering on the safe side for now but we also have side projects aimed at weight optimisation, since that’s crucial for UAVs. “Complexity and time between overhauls [TBOs] are big bottlenecks for two-stroke performance as well. Many two-strokes have lubrication problems that mean their top ends have to be torn down every 50 hours or so, replacing pistons and rings and so on, which of course is time spent not flying. “So we’re aiming for a 1000-hour TBO on the REVolution, with just a minor oil change every 50 hours, and the engine can be stripped and rebuilt from scratch in less than 4 hours.” While the engine is oil-injected in a similar manner to a four-stroke, the Unmanned Systems Technology | August/September 2020 The cylinder liner is designed with five scavenging air ports and one (larger) exhaust port