Abstract
The chief problem affecting the exhaust valve is cooling, and most causes of failure emerge from it. For best results, therefore, development must extend to associated components such as guide, insert, etc., and all are surveyed in this paper. Development can be divided into three main channels: (1) heat transfer, (2) maintenance of gas seal, and (3) corrosion resistance.
Improvements in heat transfer were obtained by the introduction of sodium cooling, firstly in the stem only and secondly in the stem and head. Further advantage was derived from improving the cooling of the seat insert of the exhaust port.
The gas seal was improved by modifications to the insert. This was changed from Silcrome steel to Silcrome faced with Stellite, and finally to Nimonic 80. The latter material has good corrosion resistance qualities and yet is sufficiently ductile to be reformed by the valve when temperature gradients in the cylinder head have forced the insert shape out of round.
The search for corrosion resistance led to the testing of a variety of hard-coating alloys and valves forged in Nimonic alloys. The latter were unsatisfactory when made up as solid-head valves but, more recently, a similar alloy has given good results with the hollow head and stem design. In this instance, however, a hard-coating alloy was used on the seat and combustion chamber face.
Cylinder block life between overhaul is largely controlled by the performance of the exhaust valves, and whereas in 1947 block life at 400 hours was half that of the engine carcase, service testing has now shown that a block life of 1,250 hours is possible.
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