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).
Page from an automotive journal featuring articles on engine air leakage and leaf spring characteristics.
| Identifier | ExFiles\Box 36\1\ scan 002 | |
| Date | 8th May 1915 | |
| 804 THE AUTOCAR, May 8th, 1915. Air Leakage in Petrol Engines. Last spring when I was overhauling my own engine, I found, I believe, the solution of this puzzle. Next time the reader has a piston out of an engine which has run more than 10,000 miles, let him carefully remove the upper ring and rinse the ring groove out with petrol, being, however, careful not to scrape it in any way, and then let him examine the groove under a magnifying glass. If his experience is like mine, he will find that whilst the lower face (A fig. 3) of the groove is bright and clean, the upper face B is coated with carbon. Here, then, is the explanation. On the explosion stroke the pressure is on the upper part of the ring which is driven down on to the lower face of its groove, where, the surfaces being good, a good tight joint results; on the suction stroke the pressure is from below, and the ring seats on a scale of carbon providing ample room for leakage. I suggest that, if it be desired to restore the slow running of an old engine, replacing the upper rings of the pistons is a much more promising expedient than packing the valve stems, and I also suggest to those who have noted improvement in running from the latter expedient to recall whether, in fact, they did not at the same time make some alteration to the pistons and rings. In the nature of the case, a man is very likely to overhaul his pistons when he sets to work to pack or replace valve guides, and he is apt to forget this and to attribute the subsequent improvement in the running to the part of the job on which his attention was specially concentrated, i.e., the valve guides. Even if no alteration has been made to the pistons it is possible that he has adjusted the tappet clearances, and, as we all know, this alone has a marked effect on slow running. The valve stem I have described, and which I tested, was not in really bad order, and I am hoping to procure a really bad one from someone's scrap heap; if I do I shall measure the leakage. I am prepared to find that it will be nothing serious. In conclusion, I offer a few remarks on the nature of the curves in fig. 2. It will be noted that all three of them are horizontal towards the right, that is to say, the recorded leakage of air at all vacua above 17in. mercury gauge was absolutely constant, no increase of vacuum making any increase in the rate of flow of the air. This is a peculiarity of air flowing into a vacuum through every form of hole I have tried, the only difference being that the point where the curve begins to drop varies with the shape of the hole. For instance, with a round hole in a thin plate it is at 15in., whilst with a vena contracta the rate of delivery remains absolutely unchanged till the vacuum has fallen to 6in. of mercury, and then falls with extraordinary rapidity. In fig. 4 is a curve for a ¼in. vena. This peculiarity of the influx of air into a vacuum will be found recorded in the text books, and, unlike some text book "facts," it is true, it has an important bearing on the delivery of gas through a partly closed throttle of a carburetter, for if the throttle be closed enough to have any noticeable effect on the engine power the depression in the engine rises to several inches of mercury, and it follows that there is very little change in the rate of flow of gas due to changes in engine speed provided the throttle is unchanged. As a matter of fact, there is either no change whatever, or, if there is, the flow varies only as the cube root of the engine speed. The foregoing is not only the result of calculation, but also a deduction from the depressions measured on the road with water and mercury gauges, the mercury gauge measuring induction pipe vacuum and the water gauge the vacuum at the jet. It was found that as long as the mercury gauge showed any noticeable reading, anything above 2in. say, then, as long as the throttle was not moved, the water gauge did not show any noticeable change even when the speed of the car rose, running down hill, from ten to thirty miles per hour, and the mercury gauge rose from, say, 3in. to 15in. As then the flow of gas was known by experiment to vary as the square root of the vacuum at low depressions (26in. water gauge and below), it could be deduced that there was practically no change in the amount of gas passing through the carburetter. Fig. 4.—Chart showing the flow of air through a vena contracta of ¼in. diameter at the neck. The curve is the result of 52 observations of which 38 were made between 2in. and 8in. vacuum. The Characteristics of Leaf Springs. The literature on motor car springs is certainly scanty: with the exception of certain articles we have published in The Autocar and in The Automobile Engineer, and a paper or two before the British Automobile Engineers and the American Automobile Engineers, it may be said that there is nothing. Useful as these papers have been, they have, necessarily, been restricted to certain branches of the subject. Although one is always apt to look at literature issued by a manufacturer of any commodity as possibly having bias, yet, even assuming this bias to exist, the volume of information may be so useful that the bias can be ignored. This is certainly the case with a handbook entitled "Leaf Springs—Their Characteristics and Methods of Specification," edited by Mr. David Landau, the consulting engineer of the Spring Department of the Sheldon Axle Company, Wilkes-Barre, Pa.{Mr Paterson}, the firm for which Messrs. Critchley, Evans, and Co., of Carlton House, Regent Street, S.W., are the British agents. The book opens with an historic chapter which reminds us that the inventor of springs is not known, but that springs only began to come into use about 1750. Various kinds of leaf springs are then specified and dealt with critically. The steels used then come in for attention, and it is pointed out that the advent of the automobile demanded from leaf springs more severe and exacting duties than they had ever been called upon to perform previously. Nothing is more practical than the methods given for securing the right springs for a car; in fact, the book should be invaluable to any designer, as it really shows him what the spring maker ought to know before endeavouring to supply springs to suit a car of any particular design or weight. See also X.2907 G/o B20 | ||