One of the main functions of the valve stem seal is to maintain lubrication on the valve bodies. Conventional valve stem seals are, however, a source of oil and/or blow-by leakage. In modern and turbo-charged diesel engines with high exhaust-gas counter pressure in particular, the sealing lip of the valve stem seal can lift, thereby causing leaks. With conventional valve stem seals, the amount of blow-by actually leaked at corresponding differential pressures is over 40 per cent higher than the actual target value. At the same time, combustion residue is deposited on the valve stem seal, thereby impairing its function.
An entirely new generation of valve stem seals has succeeded in reducing blow-by dramatically. To this end, modern valve stem seals are fitted with a special counter-pressure sealing lip which, in addition to a conventional sealing lip, also seals the valve stem from the exhaust gas stream.
Extensive texts have shown that standard valve stem seals lift at a differential pressure of between 1.5 and 4bar. With a secondary reinforcement lip, this lifting reaction can be delayed until a differential pressure of between 4.5 and 5bar has been attained. However, only the use of an additional counter-pressure lip can delay the undesirable lifting of the valve stem seals until a differential pressure of well over 6bar is attained an uncritical range even in turbo-charged engines.
For comparison purposes, the different valve stem seals were installed in the same engine and subjected to load tests at high engine speeds over a time-frame of 300 hours. There were major differences: the standard valve stem seals were noticeably contaminated with exhaust-gas residue at the end of the test, whereas the valve stem seals with the special counter-pressure sealing lip remained completely unaffected.
These valve stem seals, says Freudenberg Sealing Technologies, help to create ideal sealing conditions at this point in the engine, thus contributing in a long-lasting manner to minimising oil consumption and blow-by as well as preserving the functionality of the particulate filter.
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