Talk:Steam car
From Wikipedia, the free encyclopedia
Contents |
[edit] Ted Pritchard
Why not mention Ted Pritchard and his steam car?
- Why not indeed - who is brave enough to tackle it? (wish you people would sign and date your comments)--John of Paris 18:32, 26 November 2006 (UTC)
[edit] Cugnot's Fardier
I don't know if this can really be described as "the first step in independent transport". It was built for the French army for dragging cannons around. Not the sort of thing in/on which you would take your girl out for a joyride. --John of Paris 18:34, 7 February 2007 (UTC)
[edit] Removed link
Enginion AG link redirects to some other website.
[edit] (90 percent efficiency?
Whilst a boiler may have 90% efficiency, the amount of heat converted by the engine to mechanical work is nothing like this high. It is unlikely that the overall thermal efficiency will be any better than the 30% quoted for the Otto cycle. For the simple plant which could be accommodated within the space available in a road vehicle, even this appears hopeful.
Space limits preclude a condenser, so the minimum cycle temperature will be 100 degrees Centigrade. External combustion must have a lower cycle temperature than the flame temperature, and if long boiler life is required, would not be operated much above 800 degrees. The Carnot cycle efficiency is, therefore 35%. It is difficult to see how 90% efficiency could be achieved within the constraints of the Second Law of Thermodynamics. Gordon Vigurs 12:34, 17 February 2007 (UTC)
- Space limits certainly do not preclude a condenser! You really should verify your facts before making such affirmations. Many steam cars including Doble's and Ted Prichard's had a condenser taking the form of a fan-cooled radiator. I won't enter into this arcane "efficiency" debate, a time-honoured red herring that does not begin to explain why steam cars have always given a very good account of themselves when compared with internal combustion as regards fuel consumption, and emissions-wise have generally proved far superior. Surely this begs more searching questions as to why this should be in spite of all this "received theory".--John of Paris 17:39, 18 February 2007 (UTC)
In general, the condensers fitted served to recover water or avoid excessive emissions, rather than reduce minimum cycle temperature. I think you are confusing combustion efficiency (which for all forms of extermal combustion is high) with thermal efficiency.
Ok, let's drop the minimum cycle temperature to 30 degrees; that yields a Carnot cycle efficiency of 1-303/1073=72%, still a fair way from the 90% claimed. The only Rankine cycles that approached this were mercury/steam binary cycles. Steam alone cannot achieve it because the maximum temperature is well above the critical point, so the isothermal heat addition needed to even approach the Carnot efficiency is not possible. I am intrigued to hear that these vehicles are not constrained by the Second Law of Thermodynamics, what other fundamental physical laws do not apply to their operation? Gordon Vigurs 09:22, 19 February 2007 (UTC)
- First of all, I don't know who put that 90% figure in the article; I for one would not defend it and am for taking it out (but the whole paragraph will need rewriting). Overall efficiency claimed for steam cars is nowhere near that or even the 72% you suggest. It probably applies to boiler efficiency, which can reach almost 90% in some cases, but is usually somewhat lower. When you come to overall efficiency, it's a different kettle of fish altogether. Ted Pritchard in a paper circulated privately, claimed 10-13% for his steam car whilst adding that the figures given of 30-40% claimed for an Otto cycle car are "erroneous" and nearer to 10%. A diesel is better giving around 40% at constant full load; the problem is that full load does not apply to conditions in automotive service with varying load, idling etc. which Ted believes probably brings diesel overall efficiency down to around 16%. However the advantage of a modern steam unit is its intrinsic ability to store power. Doble and Pritchard cars had an automatic control box sensitive to pressure and temperature; it cut the burner or the water feed in and out (which notably meant lower, not higher consumption in urban conditions). No I am not confusing thermal efficiency with fuel efficiency although, the latter should logically derive from the former. Nor would I ever claim that steam vehicles are not constrained by any law of nature. What I do say is that theory should closely reflect practice. When it does not seem to do so, either the theory is wrong, or incomplete, or wrongly interpreted. The application of pure theory can be relevant when seeking a fresh appoach to a specific technology but is only part of the story when developing hardware or when it comes to comparing (and choosing) rival technologies where many more parameters are involved and the proof of the pudding is always in the eating. Reality is always more complicated - that's why theories get reassessed from time to time. Did Carnot have the whole truth? He believed that heat is a substance called caloric.--John of Paris 14:53, 19 February 2007 (UTC)
It was only the figure of 90% which caused me concern, the rest of the article is fine.Gordon Vigurs 09:15, 20 February 2007 (UTC)
The point about 'practical' efficiency and 'theoretical' efficiency is well made and is highly relevant to this article. Mis-match between the two usually arises from incomplete definition of the problem, the use of an inappropriate approximation to theory, or the current problem (e.g. deciding how long an engine should run before it requires overhaul) is not readily amenable to theoretical analysis, and is better approached empirically. Most engineering problems tend to be of this latter, theoretically messy nature.
The reduced transmission losses, insensitivity to the variable loading conditions inherent in road vehicles, particularly when the vehicle is stationary, all favour steam propulsion. They also serve as arguments against road transport in general - but that's a different issue. I'm not sure that Carnot was in fact responsible for the Carnot cycle, it may well have been named in honour of him as the first to recognise the fact that heat engines must reject a proportion of the supplied heat energy in order to work at all.Gordon Vigurs 14:18, 7 March 2007 (UTC)
I think we can say that it is more than an honorific title and that Carnot was responsible for defining the cycle in the first place along with laying out the First Law [1]. I think the mismatch you speak of stems from more than "inappropriate approximation". It comes from polarising on a single viewpoint, ill-adapted to the intricacies of the steam process, where heat transfer, fluid dynamics, boiler chemistry and tribology are of equal importance and crucial to getting the complete picture. For this holistic approach it is instructive to study the research of James Watt, D.K. Clark, Anatole Mallet, Abner Doble, André Chapelon, and more recently L. D. Porta, who had an extraordinary multi-faceted vision of the question. Ted Pritchard is also worthy of close study, if you can get hold of his writings which are now off the web since the "pretty flowers" rehash of his site --John of Paris 10:33, 20 March 2007 (UTC)
