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' discussing engine redesign with a focus on piston design, materials, and associated components.
| Identifier | ExFiles\Box 160\5\ scan0055 | |
| Date | 6th December 1940 | |
| 580 The Autocar December 6th, 1940. Half the Cylinder is Only a Guide A Redesign of Engines on More Scientific Lines is Long Overdue By The Editor of “The Automobile Engineer” ADMIRERS of “push” and “go” should undoubtedly revere the piston, for in it their ideal is well realised. At 3,000 r.p.m. each piston makes one hundred start and stop journeys every second, no mean activity. As exponents of liveliness, valves run a close second and get a good deal of knocking about into the bargain, but for the moment let us ponder on the piston. Of the engine's main working parts the piston has probably undergone most change, and it is still in metamorphosis. There was a decided trend at one time towards pistons of a composite design, that is, an aluminium head in conjunction with a cast-iron or steel skirt. Originally, all pistons were, of course, of cast iron, which is a very suitable material for any part of which the working entails high rubbing velocities without excessive pressures. Early aluminium pistons were sand cast in very simple material, but later came the die casting in highly developed alloys of greater strength and durability combined with more suitable expansion ratios, such as are in use to-day. This question of expansion ratio is important, as the new alloys more nearly approximate in their size variation under heat to the cast iron of the cylinders in which they operate. This makes for quieter running. Highly important developments have also taken place in methods of machining, not only in the direction of accuracy and economy, but also towards surfaces of more suitable finish and of better form. Many modern pistons are not round, but change from a round section at the top to oval on the skirt. Diamond turning has generally replaced other finishing methods as being quicker, more accurate, and furnishing a better working surface. As regards the ovality in shape, this is introduced in order that closer fitting pistons may be used and so reduce slap. The longer axis of the ellipse or oval is at right angles to the gudgeon pin, the pin itself being on the short axis. There is, therefore, clearance down the sides of the piston at the gudgeon pin ends, while there is closer fitting contact with the cylinder bore at right angles to the relieved portions, that is, where the working pressures occur. If, under expansion due to heat, these fitting faces come too closely into contact with the cylinder and set up pressures, then the skirt yields by distorting to a round, in place of an oval, taking up the clearance that is provided at the sides for this purpose. Seizure is thus avoided. idea of combating wear, although by running closer skirt clearances than is possible with aluminium alloy they make for quieter running also. The probability is that there is some reduction in friction as well, owing to the fact that such materials as iron and steel, and more particularly cast iron, are inherently more frictionless and probably take on a better working surface than aluminium alloy. A considerable amount of experiment and investigation has taken place during recent years in connection with scoring, silence and reduced oil consumption. The oil consumption problem is fairly well in hand now, but at one time was a very serious matter with many elusive features. For one thing, it is only comparatively recently that it was realised how vitally important the truly machined ring groove is. No matter how good a fit the piston rings are in their grooves, if the grooves are ever so slightly untrue in the horizontal plane of the engine, scraping with alternate edges of the ring takes place, and oil is scraped up instead of down. Further, there is rapid wear, giving the ring face a rounded form. This effect is one that occurs apart from wear of the ring in the groove itself. Ring design, more particularly as it affects cylinder wall pressures and contacts, has been much to the fore. Increased engine speed and gas pressures have set up a number of new problems in connection with rings. Further, the all-important matter of oil economy, yet with adequate wall lubrication, is much affected by the character and evenness of the pressure of the ring on the cylinder wall. A remarkable amount of research has been carried out on piston rings recently in solving the various problems. The net result, however, is that both design and manufacturing methods, after being standardised for some time, have lately undergone drastic change. Rings have been evolved well capable of dealing with the new conditions, and eliminating most of the shortcomings of the earlier types. Air Filtration and Oil Cleaning Whether or no composite designs would show any economies in weight remains to be seen. They should certainly reduce wear of the piston itself and also the cylinder bore, because, strangely enough, a comparatively soft piston will not only wear itself but will lap away the cylinder wall, because by its very softness it becomes charged with minute hard particles. Here, incidentally, is one very good reason for really effective air filtration and also for proper oil cleaning. Another factor in piston and cylinder wear that will presumably in time receive attention is that of providing more accurately prepared fuel mixtures. Over-richness, cold running, or extended use of stranglers and starting carburettors are prime causes of engine deterioration. What is necessary, particularly when starting from cold, is to give the cylinder at once a properly prepared homogeneous mixture such as will not damage the engine interior. At present there is no means of doing this. The next logical step in piston development would A 22 | ||