Effects of Non-Newtonian lubricant rheology on elastohydrodynamic lubrication of piston skirts in engine initial start up condition

At global level, automobile engine manufacturers ascertain that piston skirts are sufficiently lubricated during normal engine operation to prevent adhesive wear of skirt and cylinder liner. This is possible as Elastohydrodynamic lubrication (EHL) between piston skirt and cylinder liner does not allow actual metal to metal contact. At the time of engine start up, absence of EHL film between piston skirt and cylinder liner results in dry contact followed by sequential formation of boundary, mixed and then EHL films, while the engine attains normal operating conditions after warm up. Advancements in engine design have necessitated use of synthetic, multigrade Non-Newtonian engine lubricants to reduce engine friction and wear and enhance its operational life. Multiple blends of Non-Newtonian engine lubricants cater to various aspects of engine lubrication requirements during normal operation. However, their peculiar rule to minimize adhesive wear of piston skirt and liner surfaces during engine start up conditions under ideal conditions has yet to be properly modeled and thoroughly investigated. This calls for proper investigative research to model rheological behavior of non-Newtonian engine lubricants with particular focus on initial engine start up conditions. This research will mathematically model and simulate transient EHL phenomenon of piston skirts during engine start up conditions while using three different type of Non-Newtonian engine lubricants. Comparative analysis and parametric studies of Newtonian and Non-Newtonian lubricants rheology and effects on EHL of piston skirt during engine initial start up conditions are also part of this research work. The major conclusions of the present study are: 1. EHL film gets established in the initial engine start up. 2. Viscoelasticity improves piston secondary eccentricities, whereas shear heating/ Shear Thinning increases chances of solid contact between skirt and liner surfaces. 3. Viscoelasticity improves friction coefficient and reduces shear stress. 4. Ree-Eyring model shows more chances of solid contact due to shear thinning effect, which implies more energy losses. 5. High initial engine start up speed is detrimental to contactless and smooth engine start up operation.