Unmanned Systems Technology 025 | iXblue DriX I Maintenance I UGVs I IDEX 2019 I Planck Aero Shearwater I Sky Power hybrid system I Delph Dynamics RH4 I GCSs I StreetDrone Twizy I Oceanology Americas 2019

66 Dossier | Sky Power hybrid system roughly the same cylindrical space, but in a Wankel engine they’re spread along the trochoid surface, giving rise to a non- uniform temperature distribution around the rotor housing.” That is a notable drawback of the traditional Wankel. Having zones of different temperatures around the trochoid surface means there is an inherent mechanical instability in the structure, owing to differing thermal expansion and contraction rates. Addressing this drawback was key to the design of Sky Power’s clean- sheet-of-paper rotary. It resulted in a comprehensive thermal management system incorporating closed-loop water cooling for the rotor housing plus closed- loop oil cooling for the rotor. The rotor housing cooling system sees water run sequentially over hot then cold areas, in turn heating then cooling again; this approach resolves the trochoid’s inherent temperature imbalances. The housing side plates are sandwiched by cooling plates, which direct the coolant as necessary to address the temperature disparities around the trochoid surface. Sealing issues Seidl does not wish to add further detail on this. He does note though, “If you have the metal of the housing expanding and contracting in different ways around the circumference of the trochoid, it becomes harder to precisely seal each ‘chamber’ from the next, causing further inherent fuel losses. If you can resolve the temperature imbalances of the traditional Wankel engine you can resolve the sealing problem. “Also, if you do have different temperature zones, you have mechanical stability issues that affect the lifetime of the engine and its components, which has also a direct input on the TBO. “Perhaps the biggest problem with rotaries – and a significant problem for piston engines as well – is the coating inside the chamber housing, which suffers the most stress from the temperature disparities and other issues over the hundreds of times the engine is used. “Rather than trying to innovate a coating to use, our team instead began exploring the idea of creating a Wankel trochoid surface that could work with no coating at all. If this engine could work without a coating, and with a stabilised temperature management system, it could work without the typical issues on the housing and seals that Wankel-types are inherently prone to. And omitting a coating from the design means less to maintain and repair,” Seidl says. In order to work without a coating, Sky Power has constructed the main housing and both end plates out of an undisclosed aluminium alloy that has thermal expansion characteristics similar to the steel alloy used for the rotor and the eccentric shaft (which runs in needle roller bearings). This April/May 2019 | Unmanned Systems Technology Sky Power SP-180 SRE Wankel type Single rotor Effective displacement, 180 cc Naturally aspirated Gasoline fuel, HF (optional with HPI injection, also for special applications) Steel eccentric shaft Steel rotor Twin spark plugs HPI injection or manifold injection Engine management system Water-cooled housing Oil-cooled rotor Weight, 6.8 kg Maximum rpm, 6000 Datasheet Sky Power’s hybrid system offers five different modes of use, which are separated by the manner and scale of electrical power drawn from the motor/generator