Wear of diesel engine inlet valves and seats.

Valve wear has been a serious problem to engine designers and manufacturers for many
years. Although new valve materials and production techniques are constantly being
developed, these advances have been outpaced by demands for increased engine
performance. The drive for reduced oil consumption and exhaust emissions, the phasing
out of leaded petrol, reductions in the sulphur content of diesel fuel and the introduction
of alternative fuels such as gas all have implications for valve and seat insert wear.
The aim of the project has been, through the use of a representative bench test and engine
testing, to diagnose the predominant wear mechanisms in diesel engine inlet valves and
seats. This information was then to be used with other test data to develop a model for
predicting valve recession and other tools to assist in solving valve failure problems.
Test apparatus has been developed that is capable of providing a simulation of the wear
of both inlet valves and seats used in automotive diesel engines. Investigations carried
out using the apparatus have shown that the valve and seat wear problem involves two
distinct mechanisms; impact of the valve on the seat insert on valve closure and sliding
of the valve on the seat under the action of the combustion pressure. Wear has been
shown to increase with valve closing velocity, combustion load and misalignment of the
valve relative to the seat. Lubrication of the valve/seat interface leads to a significant
reduction in valve recession. Valve rotation ensures even wear and promotes debris
removal from the valve/seat interface. During testing it was established that resistance to
impact was the key seat material property determining the amount of recession that
occurred.
A semi-empirical wear model for predicting valve recession has been developed based on
the fundamental mechanisms of wear determined during test work. Model predictions
were compared with engine tests and tests run on the bench test-rig. The model can be
used to give a quantitative prediction of the valve recession to be expected with a
particular material pair or a qualitative assessment of how parameters need to be altered
in order to reduce recession.
Flow charts have also been developed, based on the review of literature, failure analysis
and modelling carried out, to assist in diagnosing and rectifying valve/seat failures and to
help in reducing valve recession by design.
The test apparatus, valve recession model and design tools can be integrated into an
industrial environment in order to help reduce costs and timescales involved in solving
valve/seat wear problems using the current trial and error methods.