WELCOME to the web site of mRT - Regenerative Thermal Machines and of its alter ego Communicable Insight. The site is dedicated to advancing prospects for the Stirling engine. Emphasis will be on gas-path design and on promoting understanding of the core component  - the thermal regenerator.

Communication with other sites/pages is via underlined links and/or the navigation bar at the top of this - and each - page.

If your interest is in the hot-air engine variant, try: http://web.me.com/allan.j.o/The_Hot-Air_Engine/Welcome.html

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STOP PRESS!

1. *If you are visiting via Internet Explorer and experiencing overlay and/or distortion of diagrams and text, try Compatibility Options on your Tools menu: 23 November 2010
   

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3. *Constructive comment is welcomed: <allan.j.o(#)btinternet.com> (Symbol (#) is to throw web-trawlers off the scent: replace by conventional ‘@’). 02 April 2011
   

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5. * Design charts added. 06 April 2012
   

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7. *Ultimate Lagrange formulation? Added 23 March 2012
   

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9. *A question of adiabaticity added. 08 March 2012
   

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11. *‘Equivalent’ volume variations added. 08 March 2012
    

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13. *Dynamic Similarity added. 28 February 2012
    

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15. * Editorial 04 replaces Ed. 03 - which remains relevant, and becomes Window 02. 01 August 2011.
    

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17. *Window 03 added in pursuit of the Holy Grail. 02 Aug 2011
    

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19. *Site page 1818 and All That supplemented by drawing of the engine as it might have appeared modified to pump water from the Ayrshire quarry. 07 June 2011.
    

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21. * Editorial 03 replaces O2. It addresses the long-standing issue of the alleged laminar/turbulent transition in the tubular heat exchanger duct. In conjunction with SnapShot 06 and Window 01 it arrives at usable numbers.  The finding raises misgivings about all simulation and modelling based on traditional steady-flow St-Re and Cf-Re correlations - including those of this Site! Repercussions extend to Scaling, FastTrack alone remaining immune. 26 February 2010
    

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23. *Preliminary details of new mRT engine posted on sister site. See page mRT-1C at The Hot-Air Engine: 18 November 2010
    

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The Stirling engine promises quiet power in the range 5W - 100kW. When heated by hydrocarbon fuels exhaust contaminants can be held to ultra-low levels. To that extent, this is a consummately civilized prime mover, consistent with 21st Century priorities. So where can you buy, say, the 1kW model?

The answer, for practical purposes, is that you can’t! Getting a commercially competitive Stirling engine into volume production is, on the evidence, a greater challenge than putting a man on the moon. A paradox of this magnitude has to have an explanation. More importantly, the explanation, when identified, may lead to resolution of the paradox - and onwards to unrestricted commercial exploitation. 

The closed, regenerative Stirling cycle is a concept of spellbinding originality and elegance. If we can’t deliver the commercial goods after two hundred years of technological development, we should at least have qualified ourselves to offer a reason. A definition of design from the Oxford Dictionary may shed light: Design: The action or art of planning and creating in accordance with appropriate functional or aesthetic criteria. Designing to criteria means, above all, designing to a specification. Up-dating the dictionary definition to include environmental and economic/market criteria confirms the obvious: for any chance of market impact, the specification must be an appropriate specification: if you design a Rolls-Royce Silver Cloud you will not attract the customer base of the Volkswagen Beetle.

With this definition of design, an explanation emerges for the current commercial status (or non-status) of the Stirling Engine: something has been chronically wrong with our approach to design.

The raison d’etre of the mRT site is thus identified: to initiate the necessary radical re-think and to carry it forward. If a Stirling engine is appropriately designed to a market-relevant specification, then it will perform to specification. If it performs, then it will sell. ‘Specification’ includes sale price - and extends to the thermal characteristics of essential sub-systems - combustor (or other heating provision) and air pre-heater.

A high priority must be to de-couple research study from design.  The former is open-ended and, by nature, in a state of perpetual flux; the latter must, above all, be communicable. This imposes a degree of inflexibility. The ultimate goal - a commercial product - is probably best served by empirical design guidelines in the form of charts and graphs.

A strategy is called for - perhaps a return to square one and a fresh start. Seeing the challenge in this light focuses attention on the myths surrounding the original invention of 1816 (see Page 1818 and all that) and on the unrealistic expectations which those myths - rather than the concept per se - have spawned. Romantic notions have to go: there is no point in achieving internal design refinement and then negating it by heating with a natural-convection flame or dispensing with exhaust heat recuperator. A cool-headed look at the net contribution of basic research is overdue: what, for example, is the benefit/cost ratio of CFD modelling at ever-increasing resolution?

It is worth recalling that the internal combustion engine enjoyed a century of increasing technological refinement before the computer became a sine qua non for yet further performance enhancement. The Stirling engine, by contrast, was re-introduced to the world (by the Philips Company of Holland) in precisely the era when electronic computation - digital and analogue - was just becoming a practical reality. Finkelstein's visionary work inspired a compelling association between cycle thermodynamics and simulation. Hopes for realizing the elusive potential of the Stirling concept have resided with computer studies ever since.

For contrasting reasons in the two cases, computer simulation has not been the defining influence on the development of either engine - internal or external combustion! A Stirling cycle simulation requires data. Flow and heat transfer data currently available as ‘open source’ reflect 1950s thinking, 1950s requirements and 1950s laboratory technique. Flow and heat transfer correlations for wire screens as offered by the most widely-quoted source do not derive from tests on regenerators! (the 2007 text elaborates). Only drastic remedial measures will now do: re-acquisition of the correlations for candidate matrix materials using modern instrumentation and real-time data capture and processing. Testing must extend into and beyond flow conditions at the boundaries of the performance envelope of the best of current engines. Covering the continued use of air/N2 as working fluids means overlapping into the compressible regime.

The complementary medicine is design by scaling in one of its various forms (usually invoking Dynamic Similarity). This can be completely independent of specific flow and heat transfer data (try FastTrack pages). The complexity of the gas processes of the practical Stirling engine makes scaling a valuable resource. Extending the portfolio of these cost-effective tools promises to speed progress.

Prospective designers working from first principles are going to require all the know-how they can tap into. This should involve visiting the site of Dr Izzy Urieli of Ohio University.