Historical Notes: Thermodynamics



Even bearing in mind how easy it is to criticise other people’s work in retrospect, it is still quite extraordinary how long it took before the fundamental laws that govern the operations of the heat engine were known and appreciated, and a considerable ignorance prevailed amongst those who designed motor cars well past the turn of the century.

There were, of course, some very notable exceptions to this generalisation, amongst whom pride of place unquestionably must go to Frederick Lanchester, but, as the general impression these days seems to be that all the early manufacturers were highly gifted folk technically, it is perhaps worth while to probe the matter further.

Amongst the steam engineers of the period from 1860 onwards there were some exceptionally gifted men, but it would appear that the motor car and its attendant heat problems were considered beneath the dignity of the reigning kings of applied thermodynamics in 1896 or thereabouts. Probably many saw no future in it, and probably many more were put off by the fact that a large proportion of the early motor-car engineers sprang from the ranks of the cycling brigade, and the “sporting” element of the early motoring days was, to them, a poor proving ground compared with the precise laboratories and test-houses then common in steam-engineering circles. Poor Pennington has been singled out as the target of other people’s humour and it was the custom, and still is, to laugh at the “long-mingling” spark and the somewhat magical laws that he claimed came to his aid in removing heat from his cylinders. In actual fact Pennington was far from the exception that some people would have us believe. He merely failed to follow the lead that some of his contemporaries set, that of copying slavishly the French school of motor engineers, thus inheriting their thermodynamics without fully comprehending the meaning of what they did. This is not to whitewash Pennington, but, frankly, it is difficult to see anything more fundamentally wrong in his thin cylinders than in the retention past the turn of the century of the continuous-tube radiator by some other manufacturers, which was clearly a blatant error from the heat transfer point of view. Going back well before the motoring era, it is extraordinary to reflect that, for example, the calculus had been a well-known mathematical tool for many years when Sadi Carnot first presented the world with the real theory of the ideal heat engine in 1824. Quite why it should have been ordained by a just Providence that this young French officer, trained in military engineering (he died of cholera at the early age of 36), should have been chosen as the one through whom that light came, is one of the fascinating mysteries of history. Be that as it may, it was he who wrote the classic little 118-page book entitled “Reflexions sur la Puissance Metrice du Feu et sur les Machines Propres a Developer cette Puissance,” which failed signally to convince anybody until Clapeyron and then Lord Kelvin took his ideas up and developed them in conjunction with those of James Joule in the ’70s. Poor Joule, with the equivalence of heat and mechanical work as his doctrine, was equally received in stony silence in 1843 and 1845 when he faced his lecture audiences, but the seemingly irreconcilable theories of Carnot and Joule were finally joined and vindicated primarily by Lord Kelvin, and the science of thermodynamics was born. At once the steam engine of Watt, hitherto without a true heat balance, took a new lease of life, and the real meaning of “efficiency” was apparent.

In the simplest possible language, Carnot visualised a “cycle” that was “reversible,” the operations of which may best be demonstrated by fig. 1, and fig. 2 shows the same cycle plotted on a Temperature == Entropy basis, included only to demonstrate the fact that as the various amounts of heat received and rejected during the “cycle” are proportional to the absolute temperatures concerned, so the maximum possible efficiency of any heat engine is given by T1 — T2/T1. Joule’s contribution was to supply the constant that linked the areas under the two diagrams shown, that is, to say that 778 ft. lb. of work equals 1 British Thermal Unit. A moment’s consideration will show how fundamental these new concepts were.

Now de Rochas could advocate compression in the internal combustion engine (as yet for industrial purposes only) and the way was open for Daimler and Benz, themselves grounded in the gas-engine industry, to produce their first epoch-making motor cars. Incidentally, this is, perhaps, a convenient point to register an opinion that Benz, often criticised because his engines were then, and continued to be, slow-turning, actually showed considerably greater thermodynamic ability than did Daimler.

The first Benz engines had  “overlap” valve timing and electric ignition, with firing before t.d.c., and his radiators were tubes in “parallel,” to quote but two obvious examples.

The lack of practical thermodynamic application in the industry that sprang from Benz and Daimler’s work was the more surprising, in that Dugald Clerk, Rudolph Diesel and Brian Donkin, amongst other notables, had all published books, and the subject had been very adequately dealt with by the engineering institutions, at the time of the start of the motor industry.

The chief stumbling block, undoubtedly, was the fact that petrol was the fuel used and nobody knew a great deal about its application in those far-off days. Even today, when people should know better, all manner of gadgets and fuel additives are foisted upon the motoring public and technically unsupportable claims made about the complex subject of carburation. How much easier, then, before 1900, for the public, and often enough the motor-car designer, to be misled? To take but one example, cars were made with all manner of inlet piping arrangements. Some were long, thin and tortuous, some short and fat, but all had their advocates who produced their special reasons why the petrol-air mixture should oblige them by doing exactly what they thought it should do in their particular system. Carburetters were a particularly fruitful field for the designer with the proverbially-dangerous half-knowledge. A well-known example due to Beaumont was the 1899 Peugeot carburetter, which was so contrived that the air velocity was at its lowest as it passed the jet, and at its highest by a large well at the bottom of the carburetter, where it had to turn a sharp corner, which by sheer luck acted as a “surface.” Opposite the horizontal jet was a cone upon which the issuing petrol could not possibly impinge, unless it obliged by disobeying all the natural laws, but this was nevertheless supposed to be the “big” technical feature of the carburetter. However, thanks to the aforementioned entirely fortuitous “well” the car ran nicely, and no doubt the designer was a great apostle of the principle of slow speeds past the jet and of cones in jet streams.

The reader is asked to understand that this is not written in a facetious spirit, but merely to stress the peculiarity observed at the start of this article that all this happened at a time when the fundamental laws of air, if not of mixture flow, were, or ought to have been, common knowledge.

In reality, however, it took a long time before the study of flows of mixed fluids was completely mastered and it must be admitted that even today there are puzzling phenomena associated therewith, especially in the longer inlet pipework systems.

Once, however, there was obviously a trade to be done in the despised motor cars and they ceased to be the playthings of the few, then the real work commenced and the foundation of the Institute of Automobile Engineers in 1906 was probably the turning point that brought together the best of the new school of motor-car engineers and many of the old school of engineers and scientists who had the real traditions of applied thermodynamics behind them. From this happy Union sprang a new generation of automobile engineers, Pomeroy and Coatalen being outstanding examples, who combined the best of the old with the best of the new, and who found the happy balance between practical trial and error and the laboratory. They did not, for example, consider Brooklands “infra dig,” but neither did they despise the costly and laborious test-house procedures if needed. As your Editor well knows, Brooklands served many useful purposes, but none so useful as when it allowed the “load factor” on motor car engines, hitherto considered well-nigh perfect, to be raised sky-high and kept there. Then indeed did the heat problems rise into serious prominence. With that realisation, the fuel technologist assumed a new importance, and the rules of thermodynamics could no longer be ignored, nor the “heat rejected” of the Carnot concept forgotten by those who sought to design motor cars. A simple mathematical transformation of the efficiency of Carnot yields the “Air Standard Efficiency” 1 — (1/R) n-1, a more practical yardstick as far as the motor-car engine is concerned, and this leads to a real appreciation of the effect of the term “R” on the “possible” efficiency. In this connection, it is interesting that “R.” or what we should now call “compression ratio,” was at one time frowned upon by those who should have known better. As early as 1895. Professor Hartmann made experiments in Germany using alcohol fuel, showing efficiencies of 12.2 per cent. as compared with 13.6 per cent. on petrol, but it would appear to have been Professor Dr. Meyer, of Charlottenberg, who first published the results of using really high compression ratios, when he tested the entries of gas engines operating on alcohol for a competition organised by the German Agricultural Society. Efficiencies as high as 32.7 per cent. were recorded, and this in 1902!

But, in spite of this, high-compression ratios were still considered a disadvantage for a time, as the engine scantlings had to be heavier to cope with the additional loadings. It was even suggested in 1907 that for fuel consumption races, such as the T.T., high-compression ratios should be penalised! To summarise briefly what is meant by thermodynamics in the petrol engine, it may be said that it is the acquiring of the basic knowledge of the properties of fuels, and the application thereof in terms of compression ratio, valve-timing, carburation and all those basic things normally associated with the word “tuning.”

Space does not allow for dealing with all that those engineers like Pomeroy and Coatalen learnt and placed on record in this connection, nor does it permit a lengthy dissertation on the influence of the war, and the aero-engine in particular, on the sum total of man’s knowledge of this exceedingly complex subject, but it is necessary to move rapidly to one of the events that in the writer’s humble opinion, stands alone in importance in the history of the thermodynamics of the petrol engine. Amongst the galaxy of talent that combined to produce the report of the Empire Motor Fuels Committee in 1923, must be mentioned Mr. H. T. Tizard, Mr. D. R. Pye and Mr. (now Sir) Harry Ricardo, and Mr. A. H. Midgely. The report, whose title is so unpretentious, was, as has been said, a milestone on the long road to perfection in the automobile engine, and for many years to come it proved to be the cornerstone upon which the house of progress was built. Even now, it still embraces fundamental truths that time cannot alter. For the first time, the whole subject of the nature and application of fuels of differing types was made clear and the processes of combustion and the phenomena of detonation probed. Thanks to the generosity of the Asiatic Petroleum Co., the R.A.C., the Institute of Petroleum Technologists, the Commercial Motor Users’ Association, the Distillers Co. Ltd. and the Institute of Automobile Engineers, and the skill of the aforementioned technical folk, four years of work, of value quite inestimable, were completed. It would be pointless here to attempt to describe this work in detail or to summarise the conclusions reached. These conclusions are woven into the pattern of every worthwhile book published on the motor car engine since 1923. They have left their mark in the racing-car world, as well as in the performance of the ordinary road vehicle which abounds today.

To conclude, one of the “hidden” difficulties in the way of progress down the years was undoubtedly that there were few people who had the happy knack of combining practice and theory in the correct proportions—those two mental outlooks being fundamentally poles apart—but that is all changed now and, since 1924, real strides have been made based upon the Motor Fuel Committee’s report and other similar research programmes.

Much as we vintage types may feel apathetic towards American motor cars in particular and modern cars in general, there is no denying that the great commercial research establishments run by organisations like General Motors Ltd., have produced quite staggering results, in essence by having “tame” scientists, research workers, test-drivers and business men, all working under one roof for a common end, and frankly it is doubtful whether they would have done as well if they had gone motor racing instead! Modern motorists demand that their cars shall start from stone cold at the press of a button and continue to work smoothly and economically as long as there is petrol in the tank; but nevertheless it is fascinating to remember that there were times when they perspired freely at the handle and joggled with the controls for several minutes (or sometimes hours) and were very content if they progressed at all, no matter what the fuel consumption. The foregoing is indeed an inadequate summary of the link between those two states, but space and time are the eternal enemies. Who, however, shall say that even now, somewhere quite close at hand, Sadi Carnot and James Prescott Joule are not shaking hands and smiling happily together?

“A. B. C.”