Internal Combustion Engine Poppet Valves: A Study of Mechanical and Metallurgical Requirements

Abstract

Some factors affecting the performance of exhaust valves are considered and the importance of improving engine design to give minimum valve operating temperature is stressed. Strength requirements are reviewed and it is shown that the advantages of higher mechanical strength and better corrosion resistance of austenitic compared with ferritic steels may be lost due to the valves operating at higher temperatures and hoop stresses. The corrosion conditions inside the cylinder head are considered and a new test is described in which lead monoxide coated specimens are exposed to atmospheres which produce valve corrosion similar to that which occurs in an engine. The order of merit established by this test agreed fairly well with the known behaviour of valve alloys when the effect of operating temperature was taken into account. Valve head corrosion has been shown to be due to surface carburization-oxidation and internal oxidation; by applying a suitable surface coating, valve life can be appreciably increased.