Tribological characteristics and surface interaction between piston ring coatings and a blend of energy-conserving oils and ethanol fuels

Abstract

The friction and wear performance of coatings on nitrided stainless steel (NSS) piston rings and chrome plated stainless steel rings sliding against cast iron cylinder liner segments in fully-formulated engine oils were investigated by using a high frequency reciprocating tribometer. The ring coatings include thermal-sprayed CrN and physical vapor deposited (PVD) diamond-like-carbon (DLC). The tribological characteristics of piston ring coatings were determined by applying several advanced piston ring coatings and energy-conserving lubricants containing friction modifiers. The impacts of E85 ethanol fuel (a fuel blend of 85% ethanol and 15% unleaded gasoline) on scuffing of the chrome-coated piston ring and cast iron cylinder liner segment were studied with different fuels varying in their degree of acidity. The tribological characteristics due to surface interactions between piston ring coatings and energy-conserving engine oils containing molybdenum dithiocarbamate (MoDTC) or an organic friction modifier were compared and analyzed. The test results show that the DLC coating produces the lowest wear on the cylinder liner segment and has a similar ring wear to nitrided and CrN coated piston rings. With MoDTC present in the engine oil, friction and wear are effectively reduced. The acidic fuel promotes the occurrence of scuffing.