Temperature
Inlet valve temperatures are not normally high enough to cause significant corrosion or thermal fatigue failures. Such failures are far more likely to occur in exhaust valves.
However, a recent study of inlet valve failures [15] led to the conclusion that deposit build-up on the seating face of an inlet valve (formed from engine oil and fuel) had reduced heat transfer from the valve head (a valve transfers approximately 75 per cent of the heat input to the top-of-head through its seat insert into the cylinder head [11]), resulting in tempering and reduced hardness. As a result, some valves had suffered failures due to radial cracking of the seating face induced by thermal fatigue while others had failed due to valve guttering.
Cherrie [16] found that when the temperature of a 21.4N steel valve was increased from 704 to 732 °C, the stress that could be sustained to rupture (failure accompanied by significant plastic deformation) in 100 hours decreased by 35 per cent. De Wilde [17] found, however, that at the temperatures experienced in exhaust valves and seat inserts, there was no significant reduction in mechanical properties and thus discounted
temperature as a major influence on valve wear.
Matsushima [18] investigated the wear rate of valve and seat inserts at elevated temperatures. Several insert materials were tested with Stellite valves. Figures 3.8 and 3.9 show the wear rate of the exhaust valve and seat insert, respectively. Valve seating face wear increased as the temperature exceeded 200 °C and continued to rise as the temperature was increased to 500 °C. At temperatures below 200 °C the wear was
almost negligible. The wear rate of the seat inserts peaked at 300 °C, decreased at 400 °C, but rose again as the temperature was increased above 400 °C. The use of superalloy seat inserts reduced the seat insert wear below that of cast iron inserts, but increased the wear on the valve seating face. The powder metal insert containing materials to form lubricious oxides improved both seat insert and valve wear.
E N G I N E E R I N G R E S E A R C H S E R I E S
Automotive Engine Valve Recession
R Lewis and R S Dwyer-Joyce
Series Editor
Duncan Dowson
Professional Engineering Publishing Limited,
London and Bury St Edmunds, UK
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