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 Automobile Engineer on poppet valves and developments in cooling methods.
| Identifier | ExFiles\Box 174\2\ img285 | |
| Date | 1st September 1931 | |
| SEPTEMBER, 1931. THE AUTOMOBILE ENGINEER. 357 1360 FROM KINA POPPET VALVES.* Some Recent Developments in Cooling Methods.* By A.{Mr Adams} T. Colwell.† EXHAUST valves of both aircraft and automobile engines are to-day subject to far more severe conditions than they were a few years ago. On automobiles, this is due to increases of approximately 21 per cent. in engine speed, 19 per cent. in brake mean effective pressure and 8 per cent. in piston displacement during the last few years. Power increase in aircraft engines has likewise caused more severe valve-operating conditions. Higher compression alone is favourable to valve temperature ; however, when this is coupled with higher speeds and improved volumetric efficiency, more heat passes the exhaust valve in a given time. The hottest part of an exhaust valve is the radius portion under the head. This is due to the heat of the impinging exhaust flame and its distance from the cooling of the block seat and the guide. Valves often fail at this point because of the lowered strength at heat and the tremendous bending moment which may be caused when the valve strikes one side of the block seat and then slaps into place. This action is permitted by too large guide-clearance or worn guides. The stress in the outer fibre of a heavy valve may be over 20,000 lb. per sq. in. ; it is therefore necessary to cool many valves in some way to maintain the strength of the steel. Cooling valves increases their life greatly and retards detonation. Ethyl gasoline allows higher compression-ratios, and cooled valves are an aid with these higher ratios in Airplane engines, particularly when supercharging is used. Valve materials and types have pro- the operation of present steels in various ways. Cooling by salt is an effective method, the salt used being a eutectic mixture of two-thirds potassium nitrate and one-third lithium nitrate. Sodium is now replacing salt as a coolant because of its physical properties, which are given in Table I. In addition, the vapour pressure of sodium is far below that of salt. This allows a valve to be heat-treated, after being filled with sodium, whereas this procedure is not satisfactory with salt, because of the high pressure developed. Sodium is more dangerous than salt to which the valve is immediately sealed. Considerable progress has been made in the process of sodium filling. In some cases it is desirable to block the sodium well up the stem to prevent heat from reaching the spring. The method of doing this with a capsule is illustrated in Fig. 1. A swaging operation seals the sodium, and the welded tip doubly seals. Fig. 2 shows a method of inserting a cold plug in the head and then pressing the valve nubbin in around it. This method seals the sodium effectively, but any sealing operation in the head or hot portion of the valve is not considered to be the best practice. A plug is sometimes screwed and welded into the head of the valve. An objection to this method is that the welding heat expands the entrapped air, causing blow-holes in the weld. Fig. 3 shows a satisfactory method of sealing the sodium without swaging. A plug is pressed against a shoulder in the stem; behind this two copper gaskets are placed and a threaded plug forced against them with considerable pressure. Hollow heads and shrouds. In Fig. 4 is a hollow-head one-piece valve. The process by which it was made holds considerable promise. No doubt cooled hollow-head valves will be widely used in the future. Raising a nabbin in the centre of the valve and bringing sodium up into the head aids considerably in cooling, and this effect will be greatly increased in a hollow sodium-cooled head. Guide clearance is important with cooled valves. It must be greater than with an FIG. 1. FIG. 2. TABLE I. PHYSICAL PROPERTIES OF SODIUM AND OF EUTECTIC MIXTURE OF POTASSIUM AND LITHIUM NITRATES. | ||