The Rating of Petrol Engines.
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The discussion which followed the presenting of the report of the Horse Power Formula Committee, at the Incorporated Institution of Automobile Engineers, does not encourage one to the belief that commercialvehicle-engine designers can place much reliance upon the formula suggested by the Committee. The R.A.C. formula is unreliable, but it has the merit of simplicity; the formula which it is now proposed shall supersede it is also unreliable, or at least many of the speakers during the discussion maintained that it is so, and it is certainly not simple. Mr. Dugald Clerk, F. IL S., in his notes on the report, states that the proposed formula gives the maximum b.h.p. rating which, in the opinion of the Committee, can be obtained from a petrol engine, having the specified dimensions, when running bolted down to a bench, and with every part working at its best. It by no means follows that this power will be given by the same engine on the road, and it still less follows that any engine of the specified dimensions, whether old or new, would give the calculated power. All that the formula means is that it is possible, by careful proportioning of the valves, by the cuttingdown of the weight of pistons, connecting rods and other parts, and by careful adjustments to the carburetter, etc., to produce the deduced power.
In the face of this opinion, why did the Committee, of which Mr. Dugald Clerk is chairman, recommend the adoption of such an utterly-impracticable formula 'I There is no denying that the report and the paper by Mr. Cr. A. Burls, which were simultaneously presented to the members of the Institution, contain much veryvaluable information, and Mr. Burls deserves the hearty thanks of all designers for the practical and scholarly manner in which he has laid down the results of his investigations into data supplied to him by representative makers, or obtained by personal observation of engines in various works. Valuable though the figures may be, this collection of data proves that petrol-engine designers have not yet arrived at any agreement on such points as mean pressure, cylinder proportion and piston speeds, and, therefore, no rule can be formed which will correctly express the power that can be given by any engine from consideration of bore and stroke alone.
The adjourned discussion, on the loth ult., was .opened by Mn. F. W. LANCHEiTER, the President of the Institution, who, however, surrendered the chair to Mr. L. A. Legros, in order that, as Mr. Lanchester said, he might be free to make remarks which he felt lie could not make from the chair. Evidently, Mr, Lanchester had gone to the meeting fully determined -to be undignified, for many of his interjectory remarks -during the course of the discussion could not be otherwise described. It appears that Mr. Lanchester was, at one time, a member of the Rating Committee, but that he resigned because he was not in agreement -with its other members. He at once took up an attitude of "What is the use of making any alteration in liorse-power formula ?" "When once a rule is established," he said, "it is almost futile to try and shift it, owing to the trouble entailed by making a change. To be able to avoid the disturbance and inconvenience of making a change seems almost to compensate for any advantage which may be achieved by making the change!' He then proceeded to show that some change is really necessary, and stated that a point to be considered was the Callender correction, which correction was suggested by Professor Callender in his paper before the Institution two or three years ago. The Callender correction was based on the assumption that, if a small engine be tested and corn'pared with similar tests in a large one, it will be found that cooling takes place more rapidlyin the smaller engine than in the larger one, and it was stated by Professor Callender that it was impossible to obtain as high a mean pressure in the smaller cylinder as in the larger one, owing to the increased cooling in the smaller cylinder. "As a matter of fact," said Mr. Lanchester, "it is perfectly true that, if one takes two engines built from the same drawings, but to two different scales, one would naturally find that the smaller engine gave a considerably-lower mean pressure than the large one, and this would appear to prove that the Callender correction is wanted." He did not think the sp correction now suggested by the Committee was of any value, unless it could be proved that the designers of the engines were men of considerable ability working independently of one another. Mr. Lanchester then criticised, in a manner which was described by subsequent speakers as unfair and groundless, a chart to which he referred as a " shot " diagram, and he occupied quite an appreciable amount of valuable time in disproving the value, for cylinders smaller than 2i in. in diameter, of the Committee's recommendation for the cooling correction, quite regardless of the fact that the Committee put forward the correction as only applying to cylinders between 211 in. diameter and 3 in. diameter.
Mr. L. H. PomEnoY thought that it was necessary to emphasize the point that two distinct formulse had been suggested. One of these is that contained in the report, the text of which we gave in our last week's issue ; the other is a formula suggested in the paper by Mr. Burls, to which we have already referred. The latter formula is intended to deal with the horse power developed by racing-car engines under conditions very different from those obtaining with touring cars and commercial vehicles. He thought Mr. Burls deserved the credit of being the first to put forward a separate formula for racing-car engines--a class which is entirely different to the average engine. This speaker stated that the tests of one maker showed a great increase on mean effective pressure with cylinder diameter, but he wished to point out that the smallbore engines with which that maker had experimented were made many years ago.
Mr. DUGALD CLERK, in his note on the Formula Committee's report, gave two or three reasons for the adoption of the formula, although he did not favour the adoption of the sp correction. He gave reasons why it might be different in the small-bore engines. The first of these reasons he stated as increase in mechanical efficiency with size, and he adduced as evidence tests made by the Institution of Civil Engineers with three gas engines of 51-in., 9-in., and 14-in, diameter cylinders respectively. The mechanical efficiency of the 14-in, cylinder was about two per cent. nigher than that of the smallest cylinder. He was of opinion that small-bore engines did not deserve any advantage with regard to mechanical efficiency, and expressed the opinion that it was quite as possible to get as highly efficient an engine with a small bore as with a large one. He suggested that no ep correction should be included in the rating formula, as the only effect of its addition would be to produce a small-bore engine with long piston stroke. The next thing Mr. Dugald Clerk dealt with was the sub-committee's inclusion of the piston speed in the formula, in spite of the strokebore ratio. He disagreed very strongly with the formula, which, he thought, bore a suspicious resemblance to the theory put forward by Mr. Lanchester some years ago that piston speed is a function of inertia stress.. "If we exclude the idea of piston speed and the physical relationship of stroke-bore ratio," adds Mr. Clerk, "we shall come to think that it would be .quite a feasible scheme to base horsepower on cylinder capacity only."
Mn. T. B. BROWNE suggested an improvement of the formula which had been compiled, his suggestion being that the first constant should be .45 instead of .465, and he further suggested that the constant 1.18 should be 1.2, thus simplifying the formula. He stated that he had made calculations for various sizes of engines with the formula as suggested by the Corn.. mittee, and with the modifications he now proposed, and in the case of two engines, one with a 5-in, and the other with a 10-in, cylinder, it made differences of .045 h.p. and .135 h.p. respectively.
Mn. BEAUMOST, JUNR., hoped the Committee would be dissuaded from proceeding with its proposed formula ; he thought that a good many men were weary of the subject as discussed in the papers, he particularly so, because he had been to some trouble actually to test the formula as compared with others.
[In his reply, Mr. Burls said he was sorry to disappoint Mr. Beaumont, but the Committee had no intention of withdrawing its report.—En.]
MR. L. A. TAGROS then wound up an interesting discussion, and called upon Mr. Burls, in the absence of Mr. Dugald Clerk, to reply.
Mn. Bunts first dealt with the points raised by Mr. Lanchester. As he had understood Mr. Lanchester's
remarks, the latter had no objection to increased . piston speed with increase of stroke-bore ratio, but, on the question of variation of size, Mr. Lanchester had slated it was altogether impossible to get a big tip. with a big engine ; on that point, said Mr. Buns, there is a great deal of evidence which tends in the other direction. Mr. Burls remarked that Professor Callender pointed out that, if one wished to obtain the advantages of increased compression, one must do 5,3 by increasing the stroke of the engine, and must leave the other conditions unchanged. Mr. Remington, of the Wolseley Co., held that, by careful cooling and attention to design of a large engine, one can &et as high or an even-higher mean pressure than with small engines. Continuing his reply, Mr. Buns said that Mr. Poppe, of White and Poppe, Ltd., stated that. the formula he always used for the engines distinctly involved an increase of mean pressure with the size. of cylinder, and, if one referred to Fig. 4 in the Horse Power Formula Committee's report (which figure was reproduced in our last week's issue), one might plainly see that the curve for tests of 1,700 White and Poppe engines approximates very closely to the recommendations of the Committee.