Application of DLC Coating in Cylinder Liners for Friction Reduction

In the past decades, continuous improvement to the finishing process (honing) of cylinder liner has led to a considerable reduction in engine friction, wear and oil consumption via optimization of the running surface topography. However, in order to meet the constantly restrictive environmental regulations, the automotive industry has invested in reducing fuel consumption and exhaust emission to expressive low levels mainly by minimizing friction losses. The properties of Diamond-like carbon (DLC) coatings are known to offer superior wear resistance and low friction making it suitable for many different tribological applications. This paper discusses the application of DLC in Heavy-Duty Diesel (HDD) cylinder liners as an alternative to reduce friction on the power cell as well as its tribological performance when submitted against coated top piston rings with CrN. Reciprocating rig and fired engine tests are presented. Evaluation criteria for rig test included friction under lubricated condition at constant oil temperature of 120°C. Test results show that DLC coating improved tribological system decreasing friction losses by 19%. Finally, results from a short durability test on HDD engine platform have confirmed a significant reduction of 2.5% on specific fuel consumption as well as superior wear resistance of DLC-coated cylinder liners when compared with base-uncoated ones.