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Air Engines This page intentionally left blank Air Engines The History, Science, and Reality of the Perfect Engine Theodor Finkelstein and Allan J Organ The American Society of Mechanical Engineers, New York First Published 2001 Published in the United Kingdom by Professional Engineering Publishing Limited This edition published in the United States by ASME Press, Three Park Avenue, New York, NY10016, USA This publication is copyright under the Berne Convention and the International Copyright Convention. All rights reserved. Apart from any fair dealing for the purpose of private study, research, criticism, or review, as permitted under the Copyright Designs and Patents Act 1988, no part may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, electrical, chemical, mechanical, photocopying, recording or otherwise, without the prior permission of the copyright owners. Unlicensed multiple copying of this publication is illegal. Inquiries should be addressed to: The Publishing Editor, Professional Engineering Publishing Limited, Northgate Avenue, Bury St Edmunds, Suffolk, IP32 6BW, UK. ISBN 0-7918-0171-3 ASME order number 801713 Copyright © 2001 Theodor Finkelstein and Allan J Organ ACIPcatalogue record for this book is available from the Library of Congress. The publishers are not responsible for any statement made in this publication. Data, discussion, and conclusions developed by the authors are for information only and are not intended for use without independent substantiating investigation on the part of the potential users. Opinions expressed are those of the authors and are not necessarily those of the Institution of Mechanical Engineers or its publishers. Printed and bound in Great Britain by Antony Rowe Limited, Chippenham, Wiltshire, UK. Cover illustration The work of Dipl.-Ing Peter Feulner has much in common with an iceberg: of a unique and substantial contribution to Stirling technology, only a fraction is visible in terms of literary exposure. The fact allows this book to take the unusual step of introducing one of its most important items of material via the cover illustrations. The six-cylinder, radial Stirling engine is a computer-generated, solid model created by Peter Feulner and now under engineering evaluation by him. In explanation of the considerations leading to this, the latest of his many impressive designs, he notes: To attain target power density of 1kW/kg pressurization is necessary. The critical bottleneck in a pressurized Stirling engine is heat transfer from the combustion products to the cycle heater. So the Stirling engine – like any heat engine – is designed and built around the combustion chamber. The degrees of freedom open to the designer remain narrower than most authorities are willing to admit. Thus it is worth noting that certain areas of different functionality may be, to some extent, de-coupled. This allows optimization to be focussed on sub-systems of crucial importance, like the gas circuit. Some simple guidelines emerge, examples of which are listed below. The engineering challenge is to reconcile all of these. Multiple iterations are generally called for. • Attaining a compact engine calls for a crank mechanism that delivers the stroke with minimal space requirement while offering the opportunity of minimizing piston side thrust. The rod seal has also to be accommodated. • Single row tubular heaters are preferable to double row tubular heaters (i.e. hairpins). (Bent heater tubes are tolerance critical, i.e. suppliers state they cannot be manufactured to the tolerance necessitated by the requirement of uniform thermal loading.) • The general layout must harmonize with the combustion chamber shape as determined by flame stability over range of operation, pollutant (NO ) x formation, etc. The opposed, double acting design has been identified as a suitable starting point, and has been pursued in a number of variants. ‘Double acting’ in this respect denotes a coaxial arrangement of two opposed cylinders and pistons so that the two pistons can be rigidly coupled so as to move in unison. The gas circuits are disposed in the usual way with the hot (expansion) space above and the cold (compression) space of a neighbouring gas circuit below the piston. vi Air Engines The scotch yoke and the eccentric mechanism invented by Sir Charles Algernon Parsons have been identified as viable solutions. The latter is based on the hypocycloid that degenerates into a straight line for a radius ratio of 1:2. Both scotch yoke and Parsons’mechanisms produce simple harmonic motion, thereby making possible perfect balancing through use of simple bobweights. (The rocking couples due to the pairs of cylinders being situated in different planes remain unbalanced.) The rigid coupling between two opposing pistons gives freedom of choice of piston rod diameter, thereby enabling considerations of thermodynamic swept volume ratio to prevail over those of bearing loads etc. Thermodynamic phase angle leads to two overall layouts: the four-cylinder (thermodynamic phase angle, α= 90°) and the six-cylinder (α= 120°). The evolving engine concept lends itself to a consolidated heater design for all gas circuits (single row tubular) which, for viable power ratings, leads to a combustion chamber with the aspect ratio that has shown characteristics favouring low NO formation. x Cycle simulation studies based on V Eppert´s work at Kassel University suggest that the engine concept is appropriate to power ratings from 15 kW to 100 kW. Apower to weight ratio of 1 kW/kg can be expected above 50 kW rated output. The radial six has been conceived along with a ‘narrow V’, five-cylinder ver- sion of equal power. Neither design is yet fully engineered, but it is worth noting that they draw on comprehensive experience of an earlier engine designed by Peter which was built, comprehensively instrumented, and extensively test- ed at Kassel University. Amassive accumulation of experimental data remains to be interpreted and published. Interested readers will find further reference to the Kassel programme in student dissertations from the University. Peter holds the post of engineering project leader with Ingenieurgesellschaft für Fahrzeugtechnik mbH, Leinzell, Germany, for whom he carries out design and development consultancy for the automotive industry. Over and above his full-time profession and his involvement in Stirling engines he is a serious student of philosophy with a particular interest in the approach of A N Whitehead – possibly explaining a command of English surpassing that of many native speakers. Acontact e-mail address is [email protected] Related titles For a full range of titles available from ASME International, please visit the ASME Press website at http://www.asme.org This page intentionally left blank About the authors Theodor Finkelstein studied mechanical engineering at Imperial College, London, obtaining his BSc with first-class honours. A Diploma of Imperial College followed. Working under Professor H Heywood he was awarded his PhD in December 1952 for a thesis entitled Theory of Air Cycles with Special Reference to the Stirling Cycle. He is a registered Professional Engineer. He has worked mainly in the research departments of major corporations, but has also held academic appointments at the Universities of Wisconsin, San Fernando Valley State College, University of Calgary, and University of California. He pioneered the development of mathematical models of the Stirling cycle and the use of electronic computers, both analogue and digital, for obtaining solutions. His analytical optimization of the Schmidt cycle model has never been superseded. Inventions, some patented, have included the internal heat supply and transmission (1958) in the free piston machine (1960), balanced compounding (1975), and the introduction of the Delta class of Stirling engines (1998). He has contributed an average of one scientific paper per year over a period of 50 years to the Stirling literature. In 1978, he and Stig Carlqvist founded Stirling Associates International which continues to carry out consulting work in this field. His later activities have concentrated exclusively on industrial Stirling engine development. He lives with his wife, Hanna, in Beverly Hills, California. Contact details: PO Box 643, Beverly Hills, California, 90213, USA Phone: (USA) 310 472-2176 Fax: (USA) 310 476-2021 e-mail: [email protected] Allan J Organ studied for his BSc in the Department of Mechanical Engineering at Birmingham University, where a final year project provided an irresistible introduction to the Stirling engine. Study of the regenerator began at Toronto under Professor F C Hooper and led to the degree of MASc in 1964. He returned to Birmingham to take up a research position studying metal- forming at high rates of strain under Professor S ATobias, for which he gained a PhD in 1968. Metal-forming provided the bread and butter until 1970, when a British Council Visiting Professorship in Brazil gave the opportunity to divide the research interest between plasticity and Stirling engines.