Experimental and Theoretical Investigations of Lube Oil Performance and Engine Friction

The feasibility of using a motored small, single-cylinder 517 cc Hatz 1D50 diesel engine to evaluate lube oil performance and engine friction at conditions typical for a fired engine is investigated in the present study. In addition a commercial engine modeling software, AVL Excite Power Unit, is used to predict the effects of lube oil formulations on the engine friction of the same engine. The motored engine can be operated with and without compression and with and without the engine oil pump. Lube oil performance is evaluated for 19 different lube oils by using either instantaneous motoring torque (motoring torque over an engine cycle) or friction mean effective pressure (fmep). For the latter, lube oil performance is evaluated by plotting fmep as a function of lube oil dynamic viscosity calculated using the Vogel’s equation with the mid-stroke cylinder liner temperature. Furthermore, the contribution of engine components such as piston/rings/liner assembly, connecting rod, journal bearings, valve train, and oil pump to the total engine friction is determined by removing components from the engine. For the model, the engine friction is estimated only for two different lube oil formulations via a friction coefficients measured with a line contact friction rig.

Lube oil performance is examined for several different base oils, commercial oils with additives, and commercial oils without additives by comparing the motoring torque over a range of viscosities. Engine friction represented by either peak instantaneous torque or fmep is found to decrease with decreasing viscosity; whereas the effect of additives is to increase friction observed as an increase in peak instantaneous torque or fmep. The contribution of several engine components to the total engine friction is also examined by comparing the fmep obtained for different engine configurations. The piston, piston rings, and journal bearings are found to contribute the most to total engine friction, followed by the valve train, and finally the oil pump. The fmeps for two different oils, a 15W40 base oil and a commercial 15W40 oil, predicted by the simulation are generally lower than those for the motored Hatz engine and highly dependent on the modified Stribeck curve.