Piston Ring Coating for High-Output GTDI Engines

October 29, 2015

Ryan Gehm

Mahle claims to deliver improved top piston rings using a new thermal spray coating for gasoline turbocharged direct injected engines. Under development since 2011 at Mahle’s thermal spray development labs in Muskegon and St. Johns, MI, the new process initially was designed for high-output GTDI engines currently in production by two U.S. domestic automakers. The new coating also was developed to help meet requirements for new production engines planned for 2018-2020, which call for less cylinder-bore friction and the use of lower-viscosity oils and alternative fuels. According to Mahle, the coating provides the performance of a more costly premium inlaid top ring. Referred to as MSC312, the new coating improves upon the scuff-and-wear capabilities of the supplier’s MSC385 coating because of its chromium nitride (CrN) composition, applied through a high-velocity oxygen fuel (HVOF) method. Tested against the MSC385 (chrome carbide HVOF) currently used in several North American production engines, MSC312 improves wear by up to 25%, the company claims. The thickness of the new coating can be adjusted to meet engine durability requirements. CrN coatings, which are common in the piston-ring industry when applied by physical vapor deposition, provide excellent wear resistance and low friction between piston ring and cylinder wall, and also resist scuffing at the ring-to-wall interface. Mahle claims that its new coating is the first thermally applied piston ring coating to use CrN in its formulation; the addition of molybdenum-chrome provides enhancements to pure CrN. In downsized GTDI engines, conventional top-ring plasma rings are not sufficiently robust to survive high levels of pre-ignition activity associated with low-speed, high-boost conditions, according to Mahle.