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.
It is seen that the exhaust valve ran cooler when a long
valve guide was used. When the valve guied is carried closer to the valve head.
The guide has the effect of protecting the valve stem from the hot gas passing through
the valve immediately after opening. One objection to such a long valve guide
is that it is difficult to lubricate and as a result wear on both valve stem
and guide is rapid.
If an exhaust valve is leaky as for instance through dishing
of it’s head by reason of loss of strength at high temperature, through
improper adjustment, the head will be destroyed very quickly, as burning gases
will then blow by it.
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