A model for lubricant stress in modern medium speed diesel engines and its verification in a wartsila 4L20 laboratory engine

摘要

This paper was voted best diesel paper at the 22nd CIMAC Congress, May 1998. Engine manufactures promote medium speed trunk piston engine designs equipped with flame rings on the basis of less bore polish and controlled, low oil consumption as adirect consequence of the virtual elimination of piston crown land deposits.A model of the influence of oil consumption and oil charge on oil stress factors has been used to illustrate the increase in oil stress when such modern designs are introduced. The oil degradation as predicted by the model is compared with actualpractices using as an example parameter the BN depletion rate. Results from the full scale Wartsila 4L20 laboratory engine shed light on the actual BN degradation process and the effect different oil chemistries have. Also the operating environment foracceptable equilibrium oil conditions - at which no oil change would be required - is analysed in detail and it is calculated that a minimum oil consumption of 0.5 g/kWh is required, in line with current recommendations by major engine manufacturers.In addition, the test results reveal to what extent critical engine condition parameters like piston under crown cleanliness and fuel pump plunger sticking are affected not only by different levels of oil stress, but also by variations of oil chemistry.Finally, the data generated in the controlled laboratory environment are shown to be consistent with information from service experience and may thus be used with confidence to guide selection of commercial lubricants.