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' magazine discussing copper-filled valves, valve cooling, and valve seat materials.
| Identifier | ExFiles\Box 174\2\ img287 | |
| Date | 1st September 1931 | |
| 358 THE AUTOMOBILE ENGINEER SEPTEMBER, 1931 rolled into a groove in the valve-stem. This device is very useful in some engines. Copper filling aids valve cooling. Copper-cooled valves, such as those shown in Fig. 7, are the latest development. The copper is forged into the stem, and the type of steel used determines whether or not there is a bond between the steel and the copper; an absolute bond is shown in Fig. 8. In cases where there is no bond, the fit is so tight that excellent heat conductivity is obtained. The thermal conductivities of copper, sodium, and several valve steels are listed in Table II. Alloying steel reduces its heat conductivity seriously; copper is more than 20 times as good in this respect as CNS, according to experimental determination with the apparatus shown in Fig. 9. The conductivity of aluminium increases as the temperature increases, but that of copper holds constant, being much the better of the two at low temperatures. Fig. 10 shows the theoretical increase in conductivity of a valve with copper insert over that of a solid steel stem. The conductivity by test is two-thirds the theoretical. Running tests show that the copper-cooled valve runs at considerably lower temperature than a solid valve. The section of copper in the head can be varied and the cooling effect of the valve is controlled by this section. The temperature of the stem is far more uniform with the copper inlay, the heat being distributed over the entire section. The guide temperature is higher, but valve-stems fitted with conventional clearance have given no trouble in the smaller sizes. Copper-cooled valves are much quieter in operation than solid valves, as they do not ring when struck. The results of a solid steel stem. The neck temperature has not exceeded 1,325 deg. Fahr. under the most severe conditions with copper cooling, while solid-stem valves in the same engine run at 1,450 deg. The top of the valve-stem guide on an air-cooled engine ran 140 deg. hotter than the bottom of the guide, which was at cylinder-head temperature. In a water-cooled engine, the top of the guide ran at 420 deg. Fahr.; which was also the temperature at the seat in the cylinder-block. The bottom of the guide was at 190 deg. Considerably more heat was dissipated through the guide than with a steel stem. The insert type of valve provides a practical and comparatively inexpensive method of making a hollow-head valve, by using aluminium as the insert and melting it out after forging. Fig. 11. the valve gradually working down into the block through successive layers like this. The pick-up also unseats the valve, causing valve burning and drawing of the block, which is quickly followed by block pounding. There are several remedies for this condition, the one to be used depending upon temperature. In many cases the use of ethyl gasoline alone will stop the trouble. This is probably because the products of combustion of ethyl gasoline form a thin coating on the valve-seat that acts as an insulator between valve and block. Better cooling of the block-seats helps greatly. The use of chromium-nickel or nickel-chromium alloy in the block helps in some cases. Substituting an angle of 30 deg. at the valve-seat helps, probably because the quicker opening of the valve [...] the heating as the valve begins [...] use of a high combined-carbon [...] the block has been useful in some cases. The one sure cure is to use an insert in the block. Several materials are available. One of the best is Elkonite, a copper-tungsten mixture having the same coefficient of expansion as cast-iron so that it can be installed with a press fit. An insert of fine-grained molybdenum-iron, high-chromium, aluminium-bronze or Stellite No. 1 or steel can be used, but these must be smaller than the block on account of expansion variations. A good method is to bevel the top of the outside diameter of the seat and roll the iron of the block over the seat. The seats can also be screwed in the block and locked by vertical pins. Excessive oil in the combustion-chamber helps to prevent pick-up. [...] helps to provide this, and adding oil to the gasoline in the tank also helps. A rich mixture also tends to eliminate it. The severe usage now imposed on aircraft and motor coach engines has caused considerable annoyance from pick-up; and reducing the temperature of the TABLE II. THERMAL CONDUCTIVITIES OF VALVE MATERIALS. | ||