Since a significant percent of fuel in cars is consumed by friction, with powertrain friction being one of the chief culprits, the development of low friction powertrains is an important task [1-3]. The introduction of advanced surface finishing and coating methods, such as thermally sprayed coatings, self-lubricating hard coatings, mechanochemical surface finishing, helical slide honing etc. together with a shift towards low viscosity synthetic lubricants help further improve fuel economy and reduce GHG emissions. For passenger cars, a change from the legacy SAE 15W-40 grade to SAE 0W-20 brings on average 3-4% improvement in fuel economy under the NEDC or EPA conditions, and the subsequent migration to 0W-8 can bring an additional 2-3%.
The primary obstacle to continually lowering lubricant viscosity is increased engine wear. The hydrodynamic lubricant film thickness is directly proportional to lubricant viscosity. Therefore, to maintain hydrodynamic lubrication, substantial modifications in the engine hardware are often required including surface finish specifications, bearings, filtration systems, and oil pump, galleries and squirters.
In the present communication, a system approach to the tribological optimization of passenger car engines is demonstrated. Experimental data and simulation results are presented to demonstrate the role of surface specifications, ring pack, and lubricant on the piston/bore tribology. The importance of in-design “pairing” of low-viscosity motor oils with the ring pack and the cylinder bore characteristics in order to achieve maximum improvement in fuel economy without sacrificing the endurance is elucidated.
[1] P. Lee, B. Zhmud, Fuel Economy Engine Oils: The Formulator’s Dilemma, Lube, April 2021.
[2] B. Tormos, B. Plaa, et. al. Fuel economy optimization from the interaction between engine oil and driving conditions, Trib. Int. 138 (2019) 263.
[3] B. Zhmud, E. Tomanik, et al. Optimizing the Piston/Bore Tribology: The Role of Surface Specifications, Ring Pack, and Lubricant, SAE Tech. Paper 2020-01-2167.