Machine Parts

  Test have that under continued full-load conditions, exhaust valves may reach a temperature of 1475 F a cherry red. Valves of large diameter run hotter than smaller ones and the valve temperature increases with engine speed. An increase in the compression ratio as a rule lowers the valve temperature but if the compression is carried too high and detonation sets in, the effects is reversed. It is usually assumed that exhaust valve temperature are highest with retarded ignition and weak mixtures, probably because the exhaust pipe hottest under these conditions, but a larger of number of tests carried out on a particular engine showed that the reverse holds true, the exhaust valve temperature being lower with a weak mixture and retarded ignition. The explanation is that the temperature of the valve depends not only on that of the exhaust gases, but also on the temperature of combustion, which latter is lowered by weaking the mixture and retarding the spark.