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A valve stem seal is designed to reduce oil flow from the valve train area of the cylinder head into the intake and exhaust port of the engine. However, these seals have to allow adequate lubrication of the valve stem / valve guide contact area and therefore need to provide a minimal amount of oil flow. Even such small quantities of oil passing the seals into the ports and the combustion chamber will effect engine emissions. Exhaust emission measurements on a modern 16 valve gasoline engine illustrate the seals contribution on total engine emissions and allow prediction of the effect of adjustments of the oil metering rate to actual engine requirements.

Due to the current and future emission regulations, nearly every new internal combustion engine has to be equipped with valve stem seals. The optimal seal is designed to meet the increasingly severe emission regulations by constantly metering the oil flow to allow adequate lubrication of the valve stem/ guide while at the same time minimizing oil flow to the combustion chamber and hence minimizing emissions. The seal has to compensate for manufacturing tolerances of the cylinder head and to provide stability of the oil metering rate for all different running conditions over the life of the engine. To increase the constancy of the oil flow metered by the seals, a new stepped lip design was developed using finite element analysis.