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 'The Automobile Engineer' journal discussing cylinder bore wear, thermostats, and piston ring specifications.
| Identifier | ExFiles\Box 132\5\ scan0082 | |
| Date | 1st March 1939 | |
| MARCH, 1939 THE AUTOMOBILE ENGINEER 89 Cylinder Bore Wear—contd. which is equal to the operating depression when the crankcase ventilator is working properly. When these tests are complete, the engine is run under full power at all speeds, and the vacuum pump is adjusted to maintain the same depression in the crankcase as before. The difference in the number of cubic feet registered at each speed when the engine is motored and under power, gives the measure of the blow-by.{R.W. Bailey - Chief Engineer} All the curves reproduced were made by this method, which lends itself to general as well as to specialised use. The Use of Thermostats.—The use of thermostats to give the quickest possible warm-up is well known, but as users appear to believe that thermostats give more trouble than they are worth, it must be emphasised that thermostats are necessary. If ordinary care is used in manufacture and positioning they should last many thousands of miles. Figs. 17 and 19 illustrate the application of thermostats of various models. There are two types, (1) the by-pass type which allows a certain amount of water to be short-circuited around the system, but not through the radiator, and (2) the bleed type, which shuts off all circulation except a predetermined amount through a small bleed. This bleed need only be large enough to ventilate the system for filling. Both systems do very well and are very widely used. The position of the thermostats at Vauxhall is located with reference to the varying temperatures of water in different heights through the head. The desirable control should be at the mean temperature and thus must be located at a point where mixing of the different strata of water has already occurred. The omission of thermostats is an invitation to trouble. Time required for warming-up affects the time the strangler is in use and this, coupled with the cold walls, is a serious handicap to overcome. Small engines do not escape this evil; in fact, cold operation, due to many causes, is one of the principal reasons why small engines are credited by many with inherently bad cylinder bore life. Fig. 18 shows a design where everything possible has been done to offset any prolonged cold operation. Water is circulated through the cylinder head under pressure, cool water being directed at the necessary points. The water in the cylinder block does not circulate under pressure, but merely moves thermally when heated. This combination, coupled with a thermostat in the circulating part of the system, enables the metal to be warmed in the shortest time without danger from overheating. Piston Ring Practice.—It must be remembered that proportion alone does not make a ring, although the lack of proportion will give bad results. The selection of metals must be given consideration, although, according to the experience of the author in truck and motor car practice, grey iron rings give adequate results. It must be recognised, however, that the type of engine or usage may require metallurgical characteristics beyond the scope of grey iron. Tables 1 and 2 give details of the rings which were originally used and of the rings which replaced them. In American practice, by far the largest number of rings are individually cast to have a special radial pressure, their composition and properties being as follows :— Total carbon, 3·25-3·75. Combined carbon, 0·50-0·80. Silicon, 2·75-3·25. Manganese, 0·50-0·80. Phosphorus, 0·85 max. Sulphur, 0·10 max. Rockwell hardness No. ''B'' 98-106. Scleroscope hardness No. 47-55. Tensile strength, 20,000 lb. per sq. in. There is a tendency in England to deprecate the need for this type of ring. English rings made to give a correct radial pressure and tension are doing very well in service. Certainly rings that give 6,000 miles per gallon after 20,000 miles of service, with mediocre bores, are good rings, and the author believes that they work well because the tension is high and they are properly TABLE I. COMPARISON OF HIGH- AND LOW-PRESSURE COMPRESSION PISTON RINGS Bore, mm. | Width, inches. | Type | Diametral load to close gap, lb. | Type of ring. --- | --- | --- | --- | --- 57 | 3/32 | [Plain Ring] | 7-10 | High pressure 57 | 3/32 | [Plain Ring] | 2¾-4 | B.S.I. standard 61½ | 3/32 | [Plain Ring] | 7-10 | High pressure 61½ | 3/32 | [Plain Ring] | 2¾-4 | B.S.I. standard 63½ | 3/32 | [Plain Ring] | 7 (minimum) | High pressure 63½ | 3/32 | [Plain Ring] | 2.8-4.25 | B.S.I. standard 82 | 1/8 | [Plain Ring] | 9 (minimum) | High pressure 82 | 1/8 | [Plain Ring] | 4.9-6.95 | B.S.I. standard 3 1/8 inches | 1/8 | [Plain Ring] | 12 (minimum) | High pressure 3 1/8 inches | 1/8 | [Plain Ring] | 5.9-8.5 | B.S.I. standard TABLE 2. COMPARISON OF HIGH- AND LOW-PRESSURE SCRAPER PISTON RINGS Bore, mm. | Width, inches. | Type | Diametral load to close gap, lb. | Type of ring. --- | --- | --- | --- | --- 57 | 5/32 | [Scraper Ring] | 7 (minimum) | High pressure 57 | 5/32 | [Scraper Ring] | 3¼-5¼ | B.S.I. standard 61½ | 5/32 | [Scraper Ring] | 7 (minimum) | High pressure 61½ | 5/32 | [Scraper Ring] | 3¼-5¼ | B.S.I. standard 63½ | 5/32 | [Scraper Ring] | 7 (minimum) | High pressure 63½ | 5/32 | [Scraper Ring] | 3.8-5½ | B.S.I. standard 82 | 3/16, 5/32 | [Scraper Ring] | 10 (minimum) | High pressure 82 | 3/16, 5/32 | [Scraper Ring] | 5¼-7½ | B.S.I. standard 3 1/8 inches | 3/16, 5/32 | [Scraper Ring] | 10 (minimum) | High pressure 3 1/8 inches | 3/16, 5/32 | [Scraper Ring] | 5¼-7½ | B.S.I. standard Fig. 17. Blow-by type of thermostat. Fig. 18. Circulation with thermostat. | ||