Issue 55 Uncrewed Systems Technology Apr/May 2024 Sellafield’s UAV equipment l Applied EV Blanc Robot l Battery tech l Robotican’s Goshawk l UGVs l UAVHE RW1 rotary l Roboat UVD l Autopilots l Arkeocean UVD l UMEX 2024 l CycloTech UVD

76 air/fuel mixture from the intake channel starts to burn it creates a back-pressure wave, causing imbalances in the air/ fuel ratio along the intake cycle. Hence, we compensate for that with a second, calculated set of fuel sprays. It helps us to run the engine leaner overall than if we relied purely on that first, manifoldinjected stage.” Combustion and control The spark plugs are surface-discharge, platinum-coated devices, customised in-house for long life. While the engine could work perfectly well on COTS NGK- and Bosch-type plugs, Bogdanov notes the latter types would not last as long. “Much more work went into ignition control. We’ve a sensor and an internal back loop for gauging whether gas ionisation in the chamber postcombustion is above or below where we want it to be, as anything from a sparkplug error to an injector mishap to an air bubble in a pressure regulator can cause discrepancies,” he says. “So, maybe on one rotation the combustion is incomplete, meaning there are leftover fuel deposits in the chamber; based on what the ECU detects, it can fix that by skipping one of the next fuel-injection stages or targeting a leaner fuel/air mix. And we re-gauge every rotation to precisely re-optimise until gas ionisation and combustion conditions are normalised.” Mapping for this and other reoptimisations by the ECU took extensive simulation (including 2D mathematical modelling, 3D CFD analysis and structural FEA) in Ansys and Matlab, followed by bench-testing measurements. Power componentry The rotor is cut from the same forged aluminium as the supercharger impeller, and the side walls (including the combustion pocket) are also treated with a form of DLC, though of a different type to that in the apex seal grooves. Rather than producing the internal gear separately and installing it afterwards, it is integral to the rotor, being cut from the same aluminium billet before being thermally treated and surface coated. “That has been a controversial choice, having an aluminium internal gear that interfaces with a steel stationary gear, but we’ve run a lot of experiments showing that the geometry of our internal gear teeth distributes the contact from the stationary gear across a wide enough surface that there is no excessive wear,” Bogdanov says. “We cut the rotor as a monolithic part to get that precise geometry, whereas if we’d used a steel insert, it would be much harder – some find it impossible – to perfectly align the gear and teeth into the rotor such that the internal and stationary gears line up for optimal contact distribution and minimal wear. It’s expensive and complex to cut the rotor so finely, but modern CNC techniques make it possible. “Having an all-aluminium rotor makes it lightweight, reducing secondary [longitudinal] imbalances in the engine. Of course, the shaft has a counterweight, but there is always some secondary difference between those two counter-rotating masses, so making the rotor lighter reduces the engine’s vibration output.” The eccentric shaft is cut from forged steel, and polished and plasma-treated at its bearing mounts, with three standard ball roller bearings per shaft (two on the front end, one at the rear). The remaining gears are cut from the same steel. “We deliberately use slightly oversized bearings and mounting points, just to hold the shaft as rigidly as possible, and because we may produce dual- or April/May 2024 | Uncrewed Systems Technology The eccentric shaft is forged steel, and polished and plasma-treated at its bearing mounts