Draft Text Only – Readers’ suggestions and inputs are welcome
Oliver Bullied, disciple of Gresley and famously progressive CME of the Southern Railway, is often quoted as saying “Thermodynamics never sold a single locomotive” (or words to that effect) when commenting on Chapelon’s contemporary locomotive developments in France. Whether true or apocryphal, the remark exemplifies the lack of understanding of Thermodynamics that was widely prevalent within the locomotive engineering fraternity of his day. Indeed it remains poorly understood by many engineers today, and is a complete mystery to most laymen.
In fact, the study of Thermodynamics is still evolving to the extent that it now spreads far beyond the mysteries of steam power that inspired its early development. The author of the web page http://thermodynamicstudy.net/history.html offers a broad view of its modern day scope:
The history of thermodynamics [is] not only one of the most interesting but one of the most dramatic episodes to be found in the story of the intellectual progress of the human mind. Starting in an investigation of a purely practical problem of engineering economics, it has grown into a body of doctrine of profound philosophical significance, with consequences which permeate the thinking of men on many subjects, from those with the most practical use to the problems of cosmology.
This page and the sub-pages under it, attempt to explain the rather esoteric and abstract concepts that underlie the subject of thermodynamics as it applies to steam traction, using terms that it is hoped will be more readily understood than those found in most texts on the subject.
History of Thermodynamics:
Many scientists of past ages could claim to be the “father of Thermodynamics” but it is probably most useful to give the credit to Nicolas Léonard Sadi Carnot, a young French military engineer who in 1824 set out to determine how the greatest amount of mechanical work could be obtained from a given amount of heat. In so doing, he invented the idea of the Carnot Cycle, being an idealized concept from which the maximum theoretical efficiency of any heat engine can be determined through the simple equation:
Carnot efficiency = (1 – T2/T1) where T1 and T2 are the temperatures
of the heat source and heat sink respectively, measured in oK.
It was not until the 1920s that André Chapelon began to apply the theories of Thermodynamics to the design of steam locomotives, with immediate and dramatic results. Unfortunately his work remained poorly understood in most steam locomotive design offices around the world and it was only in the 1950s that Livio Dante Porta took up the mantle and continued the work that Chapelon had started.
Important amongst the many lessons that Chapelon (and Porta) learned from Thermodynamics comes from Carnot’s simple equation which explains the importance of high temperature superheat since it shows that an engine’s efficiency is critically affected by its temperature. In the case of a typical “first generation” steam locomotive operating at a superheat temperature of (say) 350oC and with an exhaust steam temperature of (say) 180oC, its theortetical (maximum) Carnot efficiency would be 27%, whereas the 5AT operating at a superheat temperature of 450oC and exhausting at 183oC, its theortetical (maximum) Carnot efficiency is 37%.
[Note: Carnot’s theorem derives efficiency values that are purely theoretical. His equation does not apply specifically to steam engines but to all types of heat engine – steam, diesel, Stirling or any other. The purpose of comparing Thermodynamics dictates through Carnot’s Theorem that any heat engine’s efficiency is limited by the temperature differences between which it operates. See also Superheating page]
Laws of Thermodynamics
The study of thermodynamics is defined by three relatively simple laws:
First Law – energy can be neither created nor destroyed, or “energy is conserved”;
Second Law – energy will tend to dissipate from a hot or high energy body to a cold or low energy sink – or “heat cannot spontaneously flow from a cold body to a hot one”;
Third Law – defines absolute zero as equalling -273oC, being the hypothetical point at which energy becomes zero.
In fact the Second Law can be written in many different ways. Here is another one:
Alternative Second Law: It is impossible to extract an amount of heat from a hot source and use it all to do work. Some amount of heat must be exhausted to a cold sink. This precludes a “perfect” heat engine.
Whilst the laws themselves are simple enough, the interpretation of them leads to complications. Most notably, the Second Law involves understanding the concept of Entropy. The concepts of Entropy and Enthalpy are described in separate sub-pages, together with a brief explanation of what “Steam Tables” are about.
There are a wide range of interpretations covering the concepts of Thermodynamics that can be found on several websites including the following: