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).
Description of the Wimperis Accelerometer, its theory, and charts showing its use on a 15 HP touring car.
| Identifier | ExFiles\Box 113\2\ scan0022 | |
| Date | 1st July 1912 guessed | |
| Page 10 WIMPERIS ACCELEROMETER [Chart showing two graphs of ACCELERATION IN FT. PER SEC.² over time in seconds] Fig. 8. The above charts of acceleration were taken with a Recording Accelerometer on a three-speed 15 h.p. touring car. The former curve was taken on Brooklands track when the driver was instructed to “accelerate as rapidly as possible,” and the latter on the road on the way to Brooklands without the driver knowing that any test was in progress. In both cases the road was pretty nearly level, rising about 1 in 80 in the case of the former and falling about 1 in 100 in the case of the latter. There is, as will be seen, a striking difference between the “test” and “non-test” starts. The gear change points are clearly seen. [Image of the Wimperis Accelerometer] External view of Indicating Accelerometer. ELLIOTT BROTHERS, Established 1800. CENTRAL BUILDINGS, WESTMINSTER, LONDON, S.W., AND CENTURY WORKS, LEWISHAM, LONDON, S.E. Page 7 WIMPERIS ACCELEROMETER This principle applies also to the retardation due to the frictional forces of the brakes; in each case the instrument acts selectively and shows the acceleration due to those forces alone and does not also add or subtract whatever gravitational forces may be induced by the measurements being made up or down hill. N.B.—It is useful to remember that the resistance in pounds per ton of total moving weight is equal to 70 × coasting retardation in feet per second per second. (The constant 70 is got by dividing the number of pounds in a ton (of 2,240 lb.) by the gravitational constant in feet per second per second.) Example: To measure the tractive effort of steam and electric locomotives at different speeds, add the measured road resistance in pounds per ton for the particular speed to the figure got by multiplying the acceleration reading at that speed by 70. Thus, if in an electric train proceeding at 20 miles per hour, the acceleration reading were 1’2 and the road resistance at 20 m.p.h. had previously been found to be 12 pounds per ton, then the corresponding tractive effort must be (1’2 × 70) + 12 = 96 lb. P ton. Then knowing the total effective weight of the train, the tractive effort in pounds is at once found. In the present illustration, if the effective weight of the train were 200 tons, the tractive effort would be 19,200 lb. It may be that this train was ascending a gradient all the while, but, as already explained, this in no way affects the above calculation, as the instrument itself allows for this. (The effective weight of a train is usually about 5 per cent. greater than the dead weight. The extra allowance is due to the fact* that the wheels have a motion of rotation as well as one of translation.) THEORY It has already been stated that when the road is not level the instrument reading shows the algebraic sum of the accelerations (or retardations) due to the engine, to the brakes and to the resistance, but is not affected by grade, so that in all cases where such measurements are made a knowledge of the gradients is unnecessary. To prove this let the downward slope of the road be S (so that for a grade of 1 in 100, S=1/100), the tractive resistance be R (lb. per ton), and the acceleration equivalent to the gravitational pull be A (feet per second per second). Then the needle of the instrument tends, by reason of the slope, to rest at a reading equal to 2,240S lb. per ton; and by reason of the gravitational acceleration to move in the opposite direction by an amount of 70A lb. per ton, so that the net reading will, so far as these factors are concerned, be (2,240S – 70A) lb. per ton. We have now to take account of the tractive resistance retarding motion—its effect will be in the opposite direction to the acceleration, and will in amount be R lb. per ton. Taking this into account we have the net result (2,240S – 70A + R) lb. per ton. Now the gravitational acceleration will be (g × S) feet per second per second equivalent to (70S) lb. per ton, or 2,240S lb. per ton, so that the expression (2,240S – 70A) becomes zero and the instrument reading is R lb. per ton, showing that we obtain in this way the value of the tractive resistance irrespective of the grade on which the measurement is made. We will now assume that the engine is working, or the brakes are on, or both, so that the net effect is to produce a certain tractive effort—positive or negative—in excess of the tractive resistance at the moment on the car, and show that the instrument will indicate this excess whatever the grade may be. In that case we will call E the excessive tractive effort lb. per ton, then the net reading = (E + R) – (2,240S – 70A + R) lb. per ton = E + R – R = E lb. per ton. And this surplus divided by 2,240 gives the gradient up which this engine effort could just maintain constant speed. * See “The Application of Power to Road Transport” (Constable & Co.) ELLIOTT BROTHERS, Established 1800. CENTRAL BUILDINGS, WESTMINSTER, LONDON, S.W., AND CENTURY WORKS, LEWISHAM, LONDON, S.E. | ||