The Rating of Petrol Engines.
Page 8
Page 9
If you've noticed an error in this article please click here to report it so we can fix it.
Report of the Horse-Power Formula Committee of the Institution, consisting of Mr. Dtgald Clerk, F.R.S., M.Inst.C.E. (Chairman). Messrs. A. Craig, J. S. Critchley, C. R. Garrard, L. H. Hounsfield, Mar. R. Lawrence. Mervyn O'Gorman, L. H Pomeroy, D. J. Smith, representmq the Institution of Automobile Eniineers ; Col. H. C. L. Holden, Captain R. K. Bagnall Wild, Prof. Calender, Dr. W. Watson, Messrs. W. Worby Beaumont, E. Russell Clare, and Mervyn O'Gorman, representing the R)yal Automobile Club; and Mr. G. A. Burls, M.Inst.C.E., representing the Society of Motor Manufacturers and Traders.
The report of the Rating Committee of the Institution was read and discussed at a meeting held on 10th November, 1908. It, dealt with proposals put forward on behalf of the Society of Motor Manufacturers and Traders, by Mr. G. A. Burls, M.Inst.C.E. The Rating Committee agreed with the proposals in principle, but considered that the tests submitted did not support the modifications of the R.A.C. rating required by the formuhe. The report recommended the formation of a committee coinposed of members of the Institution, members of the R.A.C., and representatives of the Society of Motor Manufacturers and Traders. This committee was called " The Horse-Power Formula Committee." Several meetings were held, and a scheme was agreed upon, Mr. G. A. Burls, M.Inst.C.E., being good enough to undertake the collection of the necessary material from the leading automobile firms in this country and abroad ; he has now prepared the tables of particulars which accompany this report, and made the many calculations and deductions shown. Mr. Boris's letter to the chairman of the committee accompanies the report; and it describes the nature of his work.
At meetings held on the 30th June and the 13th October, 1909, the committee resolved to recommend for cousideration the following formula d (ad N
as a formula giving a rating with a stroke-bore correction. The constants K and a were purposely left without any specified value to enable the committee ultimately to arrive at values from experiment and observation. In this formula d is the bore, and s the stroke, in inches; N is the number of cylinders.
Before arriving at the above formula the committee considered the two main corrections proposed to be applied to the R.A.C. formula, viz., variation of mean pressure with dimensions of cylinder, and variation of piston speed with ratio of stroke to bore. The R.A.C. formula emTe. According to the R.A.C. formula, the rating 0.4 D2 x N.
This simple formula, notwithstanding frequent statements to the contrary, involves both 'specified mean pressure and piston speed. It assumed, however, that piston speeds did not vary materially from 1,000 ft. per min. Examination of the accompanying Tables of results proves conclusively that piston speed dues increase with the strokebore ratio.
Stroke-bore Ratio.
The fables give sonic particulars from tests made of 144 engines, but only 101 tests contain all the data required for comparing the effect of stroke-bore ratio on piston speed for values of r from 1 to 1.61, and they do not necessarily represent the maximum brake horse-power which can be obtained. The following Table shows broadly the increase of piston speed with stroke-bore ratio. Five groups have been taken with a variation in each of 0.1 in stroke-bore ratio as nearly as could" be obtained.
Each vaioe of piston speed is the mean of the values obtained in the number of tests given : to sonic extent, the taking of average results masks the variation between the different engines. Accordingly, Fig. 1 has been prepared, in which piston speeds are plotted against stroke-bore ratio for the 101 engines.
The results plotted in Fig. 1 show an undoubted increase of piston speed with stroke-bore ratio, but the results obtained are exceedingly irregular. For example, between r = 1 and r _= 1.1 the maximum piston speed in three cases lies between 1.600 ft. and 1,700 ft. per minute, but the very large number of tests show 1,400 ft. down to about 1,000 ft. per minute. Between r = 1.1
and r 1,2 the maximum values appear to be 1,400 tt. and 1,500 ft. pet minute in two separate tests, but the greater 'lumber lie below 1,300 ft. per minute. Between r =, 1.2 and r = 3.3 the maximum values are above 1,600 ft. per minute, but the greater number lie below 1,400 ft. per minute, The highest. value of all is found between r =. 1.3 arid = 1.4, namely, 2,200 ft. per minute-but it is understood that this figure is open to doubt. In the same way between s = 1.4
and r 1.5 the highest value is just over 2,000 ft. per minute, and the highest value at 1.6 is 2,100 ft. per minute. From this it will be seen that it is impossible to formulate accurately any law of variation between stroke-bore ratio and piston speed. Notwithstanding this, if 1,200 ft. per minute be taken as the speed to be expected with r = 1, and 2.100 ft. per minute that corresponding to r = 2.5, then, assuming a linear law between the two points, the piston speeds found for different stroke-bore ratios arc such as can be obtained {bench test) at maximum brake horse-power from a carefully•designed engine. These piston speeds, it will be evident, are higher than those given by about 50 per cent, of the engines esamined. About 50 per cent., however, give greater values, so that, for a maximum rating no hardship would be occasioned by assuming the law to be that given by the black line shown upon Fig. 1. Any formula required to express b.h.p. on this basis must assume a piston speed of 1,200 ft. per minute for r 1 and 2,100 ft. per minute for r = 2.5.
Forty of the engines, Nos. 1 to 50, of the appended Tables were tested at approximately 0.9 of the highest b.h.p. i ecorded, and in all cases the tests show lower piston speed.
Mean Effective Pressure Corresponding to the Brake Horse-power.
This value has been called lip in the previous reports. The results of the various tests have been tabulated and plotted against cylinder bore, but no increase can be deduced in this way as due to increasing cylinder bore. Fig. 3 shows 88 tests in which up at maximum brake horsepower is plotted against cylinder diameter. The values considered are those given by tests from 124 engines. Out of those, 88 gave mean effective pressures of 65 lb. per square inch and above. The pressures rise as high as 95-1 lb. Expressed in percentages, approximately 5.6 per cent. of the 124 engines give values above 90 lb.; 15-3 per cent. between 80 and 90 lb.; 28.2 per cent.. between 70 and 80 lb. ; and 21.2 per cents between 65 and 70 lb. About 50 per cent, of the engines thus show mean pressures above 70 lb. The highest pressure was obtained in a cylinder of 3.35 in. diameter. It is interesting now to consider the highest values of qp given in these tests by cylinders of different diameters. Compare the two highest obtained at the highest brake ht.,' se-power recorded with cylinders of about 3 in., 4 in. and 5 in, respectively as follows: Test No. d =p
122 3.15 in. 90.5 lb. per sq. in, 83 3.13 in. 88. o
Mean 92.4 lb, per sq. in. Here an increase is shown of nearly 4 per cent. The value of np for about 3 in. diameter is 89 lb. per sq. in.. and for 5 in. diameter 92.5 lb, per sq. in.
Tests, by Messrs. White and Poppc cited by Mr. Burls show a large increase-over 25 per cent. ; 3.15 in. diameter giving = 80.6 lb. per sq. in., and 5 in. diameter yp = 102 lb. per sq. M.
The experience of many makers also proves that in testing under similar conditions increase of cylinder dimensions increases tu. In discussion many members of the, committee give this increase as part of their ordinary experience. It is known also that in closed cylinrier experiments the cooling loss in a given time is greater in a small vessel than in a large one; further, that in gas engine practice higher mean pressures are obtained under similar conditions of compression in large cylinders than in small : also that the mechanical efficiency of a large engine is visually greater than that. of a, small one.
Taking into consideration all these matters and 1 results of many tests of petrol engines for ears, the committee is of opinion that, it is necessary to allow for an increase nf the value of up with the. cylinder diameter, and the accept 681 lb. per sq. in. for ,,,t) when if = 2 in., and 99 when d .= 5 in. The best current practice as to tip is therefore taken 'is
p 130 (1-1.18,4) lb_ sq. in. ... la)
After consideration of the evidence contained in the Data of 143 Petrol Engines collected by Mr. G. A. Burls, M.Inst.C•.E., the committee is of opinion that the rate of increase of maximum practicable piston speed with strokedsore ratio can be adequately represented by the equation s= 600 (r + 1) ft. per min. ... (I)) where r is the ratio of stroke to bore. This implies a maximum practicable piston speed of 1.200 ft. per min. for r = 1, rising to 2,100 ft. per min. for r 2.6.
Proposed Formula.
The committee therefore propose a formula which includes On increase of up with cylinder diameter and increase of piston speed with stroke-bore ratio. It is:
Max b.h.p. rating per cylinder = 0.464 (d + s) (d 1.18). This formula may be considered to give the maximum practicable b.h.p. as determined by a. bench test under onerous but still sate conditions for carefully-designed and soundlyconstructed engines of from 2i in. to 5 in. cylinder diameter and stroke-bare ratio up to 2.5.