From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Article from 'The Autocar' magazine discussing brake horsepower tests, engine performance comparisons, and road surface improvements.
| Identifier | ExFiles\Box 113\2\ scan0005 | |
| Date | 28th September 1912 | |
| THE AUTOCAR, September 28th, 1912. 581 various sizes and makes. As previously mentioned, the b.h.p. figures are obtained from the formula B.H.P. = R x W x V ÷ 375 where R is the resistance to motion measured at the clutch in pounds per ton, W is the car weight in tons, and V is the speed in m.p.h. Gear Ratios and Wheel Diameters. On Dr. Dugald Clerk's suggestion, the Club has endeavoured to obtain from the owners of the cars particulars of gear ratios on top gear, diameters of rear road wheels, and information as to whether the top gear was a direct drive or not. The purpose for which these additional particulars were desired was to get figures for the engine r.p.m. at top speed, the piston speed at that point, and the brake mean effective pressure (ηP). Table II. gives the additional data so collected. Comparison with R.A.C. Rating and Effect of Age. A point of interest in the tests is that it enables a comparison to be made of b.h.p. with R.A.C. rating not only for new cars but for old ones. It will be seen that the b.h.p. is greater than the R.A.C. rating in the case of 1911 and 1912 cars. The great loss in relative power in passing from the 1909-10 group to the 1907-8 cars is marked, and indicates an advance in motor car design in the intervening period. It will be seen that cars four or five years old give on the average half the b.h.p. of new cars of the same R.A.C. rating. Engine and Piston Speeds and Mean Effective Pressures. In tables II., III., and IV. are given the engine and piston speeds and the mean effective pressures for new (1911-12), middle-aged (1908-10), and old (1903-7) cars. The engine speed (mean) is 1,689 for the new, 1,615 for the middle-aged, and 972 for the old. There seems, therefore, to have been little increase in engine speed in the last three or four years, but a material advance between the old and middle-aged. The low engine speed of the old cars is, however, due in part to the fear of damage to engine or car if the throttle were opened to its full extent. The mean piston speed for new cars is 1,439 feet per minute, compared with 1,358 for the middle-aged, and 817 for the old. The mean effective pressures are striking. The new cars give values almost identical with that assumed in the R.A.C. rating, viz., 67.2. Middle-aged and old cars have about the same mean pressures, viz., 50 and 54, suggesting that the loss of pressure due to piston and valve leakage and to lower mechanical efficiency gets to its maximum when the car is still only middle-aged, and that the further drop in h.p. is due to the lowness of piston speed of the very old cars. Accuracy of Measurements. When these tests were decided upon, it was expected to get readings to within five per cent., and it is interesting, in conclusion, to examine into the probable accuracy achieved. The figures for R (resistance in lbs. per ton) may be taken as ± 5 lbs. per ton, or on the average, some three per cent. above or below the correct figure. (When tests are made on a downward slope the accuracy is very much greater, as there is more time to make the observations, and the complicating effect of rotational momentum is almost wholly absent.) The momentum stored in the rotating parts of the car tends to keep up the speed of the car when coasting on a level road, and this effect may make the reading of R (resistance) some 2 or 3% low. The fact that the 'straight' at Brooklands approached from a downward slope tends to make the speed a little higher than it would be if due to the engine alone, and this tends to make the readings high, thus more or less balancing the effect of rotational momentum. The above considerations apply to the four-seater cars; in the case of the two-seaters the only place for the Wimperis accelerometer was on the floor-boards, and, as these were not, in general, screwed or fastened down, there was uncertainty as to whether they may not have shifted. Great care was taken to minimise the chances of error on this account, but all observers can testify to the difficulty of so doing. For this reason the measurements made on the two-seater cars were not so accurate as those made on the four-seater cars, but it is impossible to say precisely to what extent. TABLE II.—Engine R.P.M., Piston Speed, Mean Effective Pressure (ηP) and Stroke-bore Ratio for Ten New Cars (1911 and 1912). Car No. | Engine. (r.p.m.) | Piston Speed. (ft. per min.) | ηP. | Stroke-bore. 3 | 1,950 | 1,550 | 68 | 1.53 5 | 1,870 | 1,470 | 91 | 1.33 6 | 1,430 | 1,220 | 67 | 1.44 10 | 1,790 | 1,290 | 69 | 1.47 12 | 1,540 | 1,310 | 65 | 1.27 14 | 1,620 | 1,350 | 62 | 1.42 15 | 1,290 | 1,100 | 54 | 1.30 16 | 1,560 | 1,820 | 62 | 1.75 20 | 1,860 | 1,590 | 71 | 1.44 21 | 1,980 | 1,690 | 51 | 1.73 Average... | 1,689 | 1,439 | 66 | 1.47 TABLE III.—Engine R.P.M., Piston Speed, ηP and Stroke-bore Ratio for Four Middle-aged Cars (1908-1910 inclusive). Car No. | Engine. (r.p.m.) | Piston Speed. (ft. per min.) | ηP. | Stroke-bore. 8 | 1,870 | 1,400 | 50 | 1.44 9 | 1,330 | 960 | 46 | 1.08 19 | 1,430 | 1,030 | 56 | 1.29 24 | 1,830 | 2,040 | 49 | 1.60 Average ... | 1,615 | 1,358 | 50.2 | 1.35 TABLE IV.—Engine R.P.M., Piston Speed, ηP and Stroke-bore Ratio for Three Old Cars (1903-1907 inclusive). Car No. | Engine. (r.p.m.) | Piston Speed. (ft. per min.) | ηP. | Stroke-bore. 1 | 1,080 | 900 | 45 | 1.00 11 | 1,060 | 940 | 61 | 1.42 22 | 775 | 610 | 56 | 1.50 Average ... | 972 | 817 | 54 | 1.31 The members of the Council of the Roads Improvement Association the other day inspected, under the guidance of Col. R.{Sir Henry Royce} E.{Mr Elliott - Chief Engineer} Crompton, a new type of road surface now being laid in Addison Road, Kensington. The material consists of graded granite, cement, bitumen, and sand heated and mixed to a temperature of 600°. The cost of the work—comprising a sub-layer of material 1in. thick and a 'wearing carpet' of smaller material 2¾in. thick, making a total thickness of new surface 3¾in.—is about 4s. 6d. per super yard, and the contractors have given a five years' guarantee. After inspecting the work of laying the already prepared material on the road, the members travelled to the depot and there saw the various materials mixed and heated. Col. Crompton afterwards, in his laboratory, demonstrated the various processes in fuller detail. | ||