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 and measuring air leakage in petrol engines.
| Identifier | ExFiles\Box 36\1\ scan 001 | |
| Date | 8th May 1915 | |
| THE AUTOCAR, May 8th, 1915. Air Leakage in Petrol Engines. By J.{Mr Johnson W.M.} S. V.{VIENNA} Bickford. IN almost every book or paper one reads on the design of petrol engines one finds references to the leakage of air into the combustion space by way of the valve stems, but I have never yet seen any statement as to the amount of this leakage. I have only seen one reference to the total amount of leakage of air into a petrol engine, and that was made by Prof. Morgan at a meeting of the Institution of Automobile Engineers some time since. He gave the figure for a quite new 40 h.p. Daimler engine as 2.5 cubic feet per minute. Now I know that this is not nearly as much as the leakage into a 14-20 h.p. Wolseley engine in good working order but not new; what the actual figure is I am not certain, but estimates, based on the measured engine vacuum running down hill at different speeds and the amount of fuel required to run such an engine unloaded, lead me to the conclusion that the leakage is between 3½ and 4½ cubic feet per minute. At any rate, I can say that a carburetter designed on the assumption that the leakage in such an engine is 4 cubic feet per minute gives entirely satisfactory results, and the engine when fitted with such a carburetter runs smoothly when unloaded, and there is no noticeable amount of soot from the exhaust. If, then, we assume that into a four-cylinder 90 mm. bore engine air leaks at the rate of 4 cubic feet per minute, it is interesting to enquire the source or sources of this leakage. By actual measurement I know that a 1/16in. bore round hole under the influence of a vacuum equal to that noticed in such an engine running idle will pass only 10 cubic feet of air per minute. It follows that into a 90 mm. bore engine as much air leaks as would pass through a round hole nearly 3/8in. in diameter under the influence of a vacuum of 20in. of mercury. Or, put in other words, if we assume that we have 1/8in. valve stems, such a leakage would require that all four stems should be very nearly 3/64in. slack in their guides, and since such wear as this would, of course, be uneven it would probably mean that the stems would be something like 1/8in. slack at the worst point; that is to say, they would be in such a condition that no one would consider the use of them for an instant. As a matter of fact, I have actually measured the air leakage past a well-worn valve stem 5/16in. diameter in a cast iron bush 2¼in. long, the valve stem being 2½ thousandths of an inch slack, which is about the slackness of the valve stems of my Wolseley engine after a year's use. I first of all tried to obtain a measurement direct on my carburetter testing plant, putting the valve stem on the carburetter flange, and attempting to measure the air delivered into a gasometer. The amount was, however, so small that leakage of the plant rendered the results hopelessly variable. I therefore adopted another method. The intake branch A (fig. 1) of my testing plant is provided with a stop cock C, below which is a bend B carrying the flange D to which carburetters to be tested are fitted. I noticed that when the cock C was closed, the vacuum measured by the mercury gauge E fell comparatively slowly. It occurred to me that if I noted the time taken by this column to drop from one point to another, I should have fairly accurate data from which to calculate the leakage. I therefore measured the capacity of the apparatus below the cock C by filling it with water. This was exactly 1 lb., i.e., 27.7 cubic inches. I then closed the valve stem bush F with a soaped rubber pad and noted the time of fall of the mercury column, thus measuring the rate of leakage of the joints less the leakage through the cock C. As this leakage would be the same whether there was valve stem leakage in addition or not, all I had to do was to deduce this leakage from that recorded with the valve stem in position to get the nett leakage past the stem. The stem was washed in petrol to remove grease. The curves (fig. 2) show the total leakage, the plant leakage, and, between these curves, their difference; that is to say, the nett leakage. The amounts were calculated by assuming that the volume of free air in the bend B at any moment is directly proportional to the absolute pressure in the bend, temperature being neglected. It will be seen that the maximum nett leakage of this valve stem is only .02 cubic foot per minute, that is to say, as compared with the total leakage into the engine it is negligible. The question then arises as to where the rest of the leakage into an engine takes place, and the most obvious answer is past the pistons. But the reader will probably say that, if the pistons fit so badly that they will pass 4 cubic feet of air per minute under the influence of a pressure from the outside of less than 15 lb. per sq. in., they would be useless against the explosion pressure of 300 lb., and the compression would be negligible. Fig. 1.—Diagram showing the method adopted of gauging the leakage of air past a valve stem. Fig. 2.—Chart showing the leakage of air past a dry 5/16in. valve stem .0025in. slack in a cast iron bush 2¼in. long. Fig. 3.—Diagram of a piston showing at A and B the surfaces against which a piston ring bears. 603 B19 | ||