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| Military UAV fitted Rotary Engine
The demanding technical specification of the engine meant that it was not only necessary to design bespoke subsystems to ensure it would start quickly and run efficiently but also to build the engine from materials that could withstand high-temperature operation while offering the wear resistance required.
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Like other larger rotating Wankel engines, the Cubewano design employs a triangular rotor that incorporates a central ring gear driven around a fixed pinion within an oval-like housing, while apex seals at the corners of the rotor seal against the periphery of the housing, dividing it into separate moving combustion chambers.
But that’s where the similarities end. To ensure that the engine would start quickly from kerosene fuel, the designers had to develop a proprietary throttle mechanism that could preheat the kerosene-air mixture from a 150W electrical source before it was injected into the combustion chamber. The design enabled the engine to start burning fuel within a few seconds.
Not only that, but they also developed an electronic control system to control the ignition sequence of the two spark plugs used in the engine to ignite the air-fuel mixture. As a result, both spark plugs can be fired independently as a function of engine speed and load, resulting in a controlled burn that does not pre-ignite the kerosene fuel and cause knocking.
Aside from the mechanical and electronic design issues that needed to be overcome, the serendipitous use of materials was fundamental to the development of the new engine.
It was important to create an engine that would operate in a wide range of environments for extended periods
Owing to the high temperatures and stresses found within the mechanism of the engine, the company was faced with no alternative but to design and commission its own custom components, including all the gears, bearings, shafts and rotor.
These were manufactured by external suppliers using specialist experimental steels that not only have strength and ductility but that can retain both (rather than just the ductility) of these properties at high temperatures. Strength is directly related to hardness and this is essential for gear and bearing operation.
Aside from the specialist steels, the Cubewano team also took advantage of modern ceramic materials, manufacturing engine seals from a hard-wearing, low-density ceramic material, as well as using ceramics to coat the internal metal housing of the engine that the seals come into contact with as the rotor moves. The result also led to reduced heat loss and an increase in combustion efficiency.
According to Tim Shires, principal engineer at Cubewano, the miniaturisation of highly functional parts to make a sophisticated assembly was extremely challenging in order to create an engine that would start and operate in a wide range of environments and altitudes for extended periods of time.
Nevertheless, after a two-year development effort, the new rotary engine has now proved itself in the field. In April this year, the engine was installed into the military UAV by Honeywell, after which it completed a successful first test flight in Albuquerque, New Mexico, flying flawlessly for eight minutes.
Being so small and powerful, the engine also has many non-military applications. As such, the company is currently modifying it for use as a range extender in electric vehicles and developing a prototype of an ultra-lightweight portable generator that can be carried by just one person.
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