Effect of Size on Thermal Efficiency of Motors.
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Discussion on Professor Callendar's Paper.
The research by Professor Callendar, which is outlined in this paper, is of an extremely interesting character, dealing as it does with one of the problems in crinection with the internal-combustion engine about which knowledge has hitherto been incomplete, and the transactions of the Institution of Automobile Engineers will be the richer for the inclusion of this contribution. At the same time, the reading of two scientific papers in succession, Prof. Callendar's immediately succeeding the paper by Mr. Lanchester, was a little unfortunate, although the connection between the papers made the discussion upon them at subsequent meetings convenient. Another fact which rather militated against the success of the meeting was that the date clashed with the Royal Society's Conversazione and Col. Crompton, the Author and many of the members of the institution being anxious to attend that function, the debate on Professor Callendar's paper was adjourned at an early hour.
Mr. Dugald Clerk, in opening the discussion, said the subject dealt with in the paper was of the utmost importance, calling as it did for a great deal of physical and chemical knowledge in addition to ordinary engineering ability, and it was a: happy augury that a leading physicist should have consented to read a paper before them on the subject. Professor Callendar made out a strong case for some variation in the rating formula, and personally he was exceedingly pleased to not that Professor Callendar's results approximated so closely to his own experiments. By different results the conclusion was arrived at that about So per cent. of the total heat which would be given in mechanical work as shown by the air standard was all that it was possible to obtain by reason of the properties of the working fluid. Many physicists and engineers had made attempts to determine what were the real properties of the working fluid in an internal-combustion engine. Earlier experiments had not yielded altogether satisfactory results, and during the last three or four months experiments had been conducted on three engines at the National Gas Engine Company's works, and by his method a great many of the points mentioned by Professor Callendar had been determined. In the case of the large engine referred to by Professor Caliendar, the statement • as to the small variation of pressure with speed had 'been well borne out. That engine running at &w.p.m. gave a mean pressure of iI2lb, but raising the speed to izor.p.m. .only increased the mean pressure to 116lb. with the same
charge, and on increasing the revolutions to 18o there was no further raising of the mean pressure. That showed that the loss of efficiency due to increase of speed must be very slight indeed. That engine had cylinders of 14 inches diameter. Similar experiments were made on the 5,i-inch cylinder engine, which at 260r.p.m. gave 93.31b., at 300 revolutions 111.71b., and with 380 revolutions 11+81b,, clearly showing that the small size engine varied much more as to its efficiency with speed than the large one. He hoped, in the course of the present year, that he might be able from further experiments to arrive at accurate conclusions on many of the points mentioned by Professor Callendar. Professor Callendar made the possible efficiency 93 per cent.; he made it 88 per cent. On the question of heat loss, he read a paper before the Automobile Club dealing with many of the points discussed in the present paper and pointing out that in a rating formula account must be taken of certain points in particular, variation of mean pressure with size, and variation of piston speed with ratio of stroke to bore, and he was glad to find that Professor Callendar had gone carefully into those points and made out a good case for an alteration in the formula, it was for those interested in the automobile industry to say if they would agree to accept a formula a l!ttle less simple than the present one. In connection with the Automobile Club formula, the variation due to size was not great, and although there was variation of piston speed with stroke to bore that variation did not seem to be sufficient to justify it being definitely formulated.
Mr. Mervyn O'Gorman drew attention to the importance of haying a formula which would not lead to distortion in design. He did not think the Automobile Club formula led to that, and in many respects Mr. Lanchester's formula worked out like the club formula. He would like to ask whether Professor Callendar had included the loss to the cylinder walls which occurred previous to ignition and expansion, which might bear closely on the question of efficiency. It would be interesting to enquire the reason for the unpopularity of the pocketless cylinder head. He believed the reason was that such a type of engine was not flexible, and he endeavoured to find out haw much the pocket added to the surface. He found out from investigation that there was a very appreciable difference, and that the amount of gas in contact with the cooling surface must have a very considerable influence.
Mr. Henry Lea said he had been much interested in the paper, and particularly in the table dealing with power rating by bore and stroke. Reference was.made in the-paper to Bourne's rule where, to obtain the nominal power of a steam engine, one multiplied the square of the diameter by the cube root of the stroke and divided by a constant, and it was remarkable that that rule should give so remarkably close an approximation to the variation of power with size and piston speed of the modern petrol motor. That rule had been drawn up fifty years ago, referring to an entirely different kind of engine. He personally had taken the trouble to compare the results attained in the paper referred to with those arrived at by Bourne's rule, and the close approximation with the results stated in the paper was really extraordinary.
The discussion was adjourned until the j2th June.