Valve wear has been a small but serious problem to engine designers and manufacturers for many years. It has been described as ‘One of the most perplexing wear problems in internal combustion engines Although new valve materials and production techniques are constantly being
developed, these advances have been outpaced by demands for increased engine performance. These demands include:
higher horsepower-to-weight ratio;
lower specific fuel consumption;
environmental considerations such as emissions reduction;
extended durability (increased time between servicing).
The drive for reduced oil consumption and exhaust emissions has led to a reduction in the
amount of lubricant present in the air stream in automotive diesel engines, and the effort
to lengthen service intervals has resulted in an increasingly contaminated lubricant. These
changes have led to an increase in the wear of inlet valves and seat inserts.
Lead, originally added to petrol to increase the octane number, was found to form
compounds during combustion that proved to be excellent lubricants, significantly
reducing valve and seat wear. Leaded petrol, however, has now been phased out in the
UK (since the end of 1999). As an alternative, lead replacement petrol (LRP) has been
developed. This contains anti-wear additives based on alkali metals such as
phosphorus, sodium, and potassium. Results of tests run using LRP containing such
additives, however, have shown that, as yet, lead is unchallenged in providing the best
protection. In several countries where LRP has already been introduced, a high
Automotive Engine Valve Recession incidence of exhaust valve burn has been recorded. In Sweden the occurrence of valve burn problems has increased by 500 per cent since LRP was introduced in 1992 [2].
The suspected cause of the valve burn problems is incomplete valve-to-valve seat
sealing as a result of valve seat recession (VSR). The occurrence of VSR is blamed on
hot corrosion – an accelerated attack of protective oxide films that occurs in
combustion environments where low levels of alkali and/or other trace elements are
present. A wide range of high-temperature alloys are susceptible to hot corrosion,
including nickel- and cobalt-based alloys, which are used extensively as exhaust valve
materials or as wear-resistant coatings on valves or seats. Materials used for engine
components have always been designed to resist corrosive attack by lead salts. No such
development has taken place to form materials resistant to alkali metals or other
additive chemistries. It is clear that LRP will not provide an immediate solution to the
valve wear issue, which is likely to cause tension between car manufacturers and
owners for some time to come.
The impending reductions in the sulphur content of diesel fuel and the introduction of
alternative fuels, such as gas, will also have implications for valve and seat insert wear.
Dynamometer engine testing is often employed to investigate valve wear problems.
This is expensive and time consuming, and does not necessarily help in finding the
actual cause of the wear. Valve wear involves so many variables that it is impossible to
confirm precise, individual quantitative evaluations of all of them during such testing.
In addition, the understanding of wear mechanisms is complicated by inconsistent
patterns of valve failure. For example, failure may occur in only a single valve
operating in a multi-valve cylinder. Furthermore, the apparent mode of failure may
vary from one valve to another in the same cylinder or between cylinders in the same
engine. An example of such inconsistency is shown in Fig. 1.3. This illustrates exhaust
valve recession values for four cylinders in the same engine (measurements taken on the cylinder head).
The valve in cylinder 1 has recessed to the point where pressure is
being lost from the cylinder, while the other valves have hardly recessed at all.
No hard and fast rules have been established to arrive at a satisfactory valve life. Each
case, therefore, has to be painstakingly investigated, the cause or causes of the problem
isolated, and remedial action taken. In order to analyse the wear mechanisms in detail
and isolate the critical operating conditions, simulation of the valve wear process must
be used. This has the added benefit of being cost effective and saving time.
Based on the wear patterns observed, the fundamental mechanisms of valve wear can
be determined. Once the fundamental mechanisms are understood, a viable model of
valve wear can be developed that will speed up the solution of future valve wear
problems and assist in the design of new engines.
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