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

It has been proposed that downspeeding combined with high boost levels would effectively reduce fuel consumption in heavy-duty diesel engines. Under low-speed and high-boost operating conditions, however, the in-cylinder gas pressure, which acts on the piston crown, is greater than the piston inertia force (such that there is no force reversal), over the entire range of crank angles. Therefore, the piston pin never lifts away from the main loading area (the bottom) of the connecting rod small-end bushing where the contact pressure against the piston pin is highest. In such operating conditions, lubricant starvation is easily induced at the interface between the piston pin and small-end bushing. Through carefully devised engine tests, the authors confirmed that the piston pin scuffing phenomenon arises when the boost pressure exceeds a critical value at which the no-force reversal condition appears. Furthermore, to prevent this piston pin scuffing when there is no force reversal, the authors applied a type of diamond-like carbon (DLC), a hydrogenated tetrahedral amorphous carbon (ta-C:H) coating, to the piston pins and then evaluated its anti-scuffing characteristics in a multi-cylinder engine. The coated piston pins exhibited greatly superior anti-scuffing characteristics relative to the uncoated pins, while preventing the transfer of layers of the bushing material onto the coated piston pins. The authors hypothesize that the prevention of the transfer of layers of bushing material is one of the most important factors leading to the superior anti-scuffing characteristics of the DLC-coated piston pins.