Helical lobe rotors reduce leaks in pumps

Tom Shelley reports on a novel lobed rotor principle that reduces leaks while achieving hig levels of compression and expansion in seal-less pumps, expanders and engines

. By adopting three dimensional design for helical rotor lobes, and having two lobes in the lobed rotor for every three apertures in the meshing rotor, it is possible to come up with a design that achieves high degrees of compression and expansion with very low leakage. The idea has been under development since the 1970s, but has only now reached the point where it is both properly patent protected, and has started to arouse interest from major commercial companies. It was original conceived as a rotary engine, but commercial interest seems to be focussing on its potential use as an efficient super charger, because of its high efficiency at all rotation speeds, and as a potential 30:1 ratio single stage expander to be run on steam from geothermal wells. A unique feature is the ability to fully vary its compression ratio by sliding a section of the containment wall in the direction of the axes of rotation. The new rotary machine has been developed by Tony Dye of Epicam based in Linton, just outside Cambridge. Mr Dye first came to our attention as a finalist in the 1989 Eureka Archimedes competition. His invention there was a mushroom valve with vanes on its underside to improve the combustion efficiency of lean burn engines. Unfortunately, lean burn was abandoned in the face of the three way catalytic converter as a means of achieving clean combustion and the vaned valve went the same way as all the other innovations developed to get the technology to work well. Tony Dye has had several successful careers in other science based fields, but in recent years, has decided to concentrate on the idea that he says he first geometrically modelled more than 30 years ago. Lobed pumps with lobes on rotors that engage in apertures in meshing rotors are far from new. We have written about more than a few in Eureka. They are quite commonly used in fuel and hydraulic pumps. In the new design, the rotors are linked by gears and run within a loose fitting plenum. A lobe contained by a close fitting containment walls starts at top dead centre. As the rotors rotate, the available space decreases, reaching full compression after only 90 deg rotation. The time available for leakage is thus very small, and so no seals are required. Leakage is further reduced by the helical forms of the rotors and by the fact that the pressure rise due to compression is exponential, and so the opportunity for leakage at the higher pressures only occurs during the very last part of the 90 deg rotation. The helical form additionally enables smoother running. Sliding the containment wall axially ensures variable geometry, by deciding the point at which the compression is started. Dye told Eureka that when he first conceived the device, it had only a single rotor, "But I had some good advice from Farnborough" to go to two rotors. He then started looking for geometries. He tried 1:1 ratios, but the shapes possible by turning to 2:3, "Looked a whole lot more interesting." First design work was on paper, followed by modelling on the University of Cambridge mainframe computer in the 1970s. Current modelling work is undertaken using Pro/Engineer. For the compressor/expander application, there is one compression for each quarter revolution using four lobes engaging in six cavities. Interest in using the device as a supercharger or turbocharger replacement stems arises because the present generation of turbochargers is far from efficient. The new design has almost no leakage without needing seals and works at a very much lower rotation speed. Tony Dye is of the opinion that conventional turbochargers can at best use only 50% of the energy in the exhaust flow because of the pulses in the stream. They also give zero boost at low engine rpm, and need a high compression ratio for cold starting. This results in an excessive cylinder pressure at full power with high NOx emissions as a result. He argues that by switching to his machine, it recovers the entire exhaust energy under all operating conditions. Furthermore, because of the variable geometry, it uses only the exhaust pressure energy necessary to deliver up to 4.5 bar boost from the compressor. The surplus pressure energy gives a power bonus to the engine output shaft. He claims that it should then be possible to improve fuel economy by between 6% and 50%, reduce emissions of NOx, hydrocarbons and carbon dioxide (because less fuel is burned) and greatly reduce peak cylinder pressure giving longer engine life. The test machine in the Epicam workshop has a 400cc maximum swept volume, and is coupled to a 55kW motor able to run at up to 6000 rpm. It was, however, demonstrated to Eureka in expander mode. It ran completely smoothly at 100 rpm from an air supply pressured to less than 5psi (One third bar). Despite the low pressure and low rotation speed, it produced more torque than could be overcome by hand force, demonstrating its efficient running under worst case conditions. Tests as an expander-compressor are to be undertaken on a 1.9litre 130 HP Volkswagon diesel engine. Epicam has a complete set of patents to protect its intellectual property and is looking for further investment and development partners. He has produced lots of figures and graphs to support his arguments, which we are sure he is more than happy to supply to readers should they wish to see them. Email Tony Dye Eureka says: There are multi-lobed pumps at infinitum but this one looks to be the breakthrough everyone has been looking for Pointers * Device has an unusual 2:3 lobe to cavity ratio and unique helical design * No seals are required since opportunity for leaks is greatly reduced by the geometry, the compression in a short rotation distance, and the fact that most of the compression occurs during only the last part of the rotation * The most applications arousing most interest are as an expander-compressor to replace conventional turbochargers, particularly in large diesel engines and in motor sport. There is also significant interest in using the device as a steam expander for geothermal energy systems