Unmanned Systems Technology 003 | UAV Solutions Talon 120 | Cable harnesses | Austro Engine AE50R and AE300 | Autonomous mining | AUVSI 2015 show report | Transponders | Space systems

48 In addition, no power is spent on driving a valvetrain. A peripheral port feeding the chambers never closes, the rotor’s apex (tip) seals closing off the gas flow to one chamber while simultaneously opening it to the next. Thus the charge column never has to come to a complete halt from which it has to be restarted. There is also no loss to crankcase windage, and without reciprocating components the engine’s balance is inherently good. There is not the same challenge as in a reciprocating engine in terms of stress at the big end, which smooths the path to high engine speed. A further advantage is that the speed of the rotor relative to the output shaft is such that the main moving parts are moving more slowly than their counterparts in a reciprocating engine. The way the charge is whipped around by the rotor and the fact that it is always moving arguably aids combustion. Moreover, the shape of the combustion chamber and the turbulence induced by the rotor together prevent the formation of localised hot spots, keeping the engine free from the danger of pre- ignition and detonation. On the other hand though, the inherent form of the rotary engine’s long, straight chamber is not in the best interest of flame propagation, and combustion can be incomplete, releasing unburned hydrocarbons into the exhaust. As a consequence the rotary engine, like the two-stroke engine, has fallen out of favour in the automotive world. There, engineers faced with increasingly strict emissions regulations have taken the easier option of ‘cleaning’ a four-stroke reciprocating engine rather than finding a means to overcome the drawbacks of inherently more efficient alternatives. The AE50R weighs only 27.8 kg, including fluids, yet produces a dependable 55 bhp from its 294 cc. It has been developed to run on avgas but will also happily use regular unleaded gasoline. Future developments might include direct injection and/or turbocharging, but neither are currently high on the list of the company’s priorities. The former implies a significant increase in fuel pressure, along with the complexity of a mechanical pump, while the extra complexity and weight of turbocharging would only be justified if a customer wanted to use the engine at very high altitude. Further up the list of priorities is the development of a version of the AE50R to run on kerosene-based fuel. Summer 2015 | Unmanned Systems Technology Dossier | Austro Engine AE50R rotary and AE300 I4 In 1924, mechanical engineer Dr Felix Wankel observed that by rotating an equilateral triangle in a certain manner relative to a containing member of specific shape, three variable volume chambers could be formed between them. The containing member needed to have an internal cross-section resembling an oval pinched at the waist – this shape was identified as an epitrochoid. The pattern of chamber shape variations as the triangular member rotated within it then satisfied the requirements of the Otto four-stroke cycle. Imagine an externally toothed spur gear sitting inside an internally toothed gear that has one-and-a-half times its diameter. The inner gear has 24 teeth and is fixed, whereas the annular gear has 36 internal teeth and is free to roll around it. As it rotates, given the 2:3 ratio, a point outrigged from it will trace an epitrochoid. Now imagine three outrigged points equally spaced (thus at 120 º ) tracing the same epitrochoid. Straight lines linking the three points form an equilateral triangle, which can rotate within the epitrochoid. In fact, our three points represent the points of contact between the epitrochoid surface and three seals projecting from a triangular form rotor. Thus the rotary engine’s rotor has three apex seals and a rolling annular gear, while the externally toothed fixed gear is fitted to one side plate of the stationary rotor housing. So the rotor does not merely spin about its axis but has a planetary motion that keeps its apex seals in constant contact with the epitrochoid surface formed by the housing. The volume of the three chambers formed between rotor face, apex seal and epitrochoid surface each increase and decrease twice for each revolution of the rotor. This enables the four strokes of the Otto cycle to be reproduced in each chamber on each complete revolution of the rotor. The stationary 24-tooth gear is mounted concentrically in relation to the central axis of the output shaft. The rotor is mounted eccentrically on a shaft boss and is left free to spin in relation to the shaft. However, since it is mounted eccentrically it rotates the shaft as it spins. In view of the rotor’s complex planetary motion it makes three revolutions for each revolution of the eccentric output shaft. Therefore, since the rotor forms three chambers, each of which is fired in succession, there is one firing stroke for each revolution of the output shaft. The displacement of one chamber – the measure of the difference between its minimum and maximum capacities – is multiplied by the number of rotors to give the nominal displacement of the engine. However, since each cylinder of a four- stroke piston engine fires only once for every two revolutions of the crankshaft, for a more realistic comparison the nominal displacement needs to be doubled. The rotary engine’s output is akin to that of a two-stroke engine of the same displacement, and likewise is inherently higher than that of a four-stroke piston engine of comparable physical dimensions and weight. Rotary engine basics