Effect of Diamond-Like Carbon Coating on Anti-Scuffing Characteristics of Piston Pins 2019-01-0184

It is considered that downspeeding concept with high boosting is effective for further fuel consumption reduction of heavy duty diesel engines for commercial vehicle application. Under such low speed and high boost engine operating conditions that in-cylinder gas pressure force acting on the piston crown is greater than the inertia force acting on the piston system (no force reversal) over entire crank angles, however, it is concerned that the piston pin scuffing occurs. For the no force reversal conditions, the piston pin cannot have any chance to lift from the bottom section of the conrod small end bushing where contact pressure against the piston pin is highest and it is difficult to expect that sufficient lubricant exchange occurs. By carefully conducted engine testing, the authors clarified that the conditions lead to starved lubrication at the interface of the piston pin and the bushing and to the piston pin scuffing for the worst case. Furthermore, in order to prevent this piston pin scuffing under the no force reversal condition, the authors applied ta-C:H type DLC coating to the piston pins and evaluated its anti-scuffing characteristics in a multi cylinder engine. The coated pins showed superior anti-scuffing characteristics over the uncoated piston pins, while preventing building up of transfer layers of the bushing material on itself. It is estimated that the prevention of transfer layer build up is one of the most important factors for the superior anti-scuffing characteristics of the DLC coating for this application.