The principal contributor to frictional losses in the engine are the pistons and piston rings. Pistons produce 35% of the energy losses, while piston rings are responsible for a further 26%. To optimise engine performance and achieve government legislated emissions targets, reducing mechanical friction losses are a high priority for automobile manufacturers.
Speaking at F+L Week 2017 on “Engine Tribology Technology Trends and Engine Oil Challenges,” Arup Gangopadhyay, technical leader of Lubrication Science and Mechanical Friction in Powertrain Research and Advanced Engineering at Ford Motor Company, outlined a range of new technologies, materials and coatings, specifically focused on countering piston ring and cylinder bore contact.
Chromium nitride is an extremely hard, inert and thin film coating suitable for high temperature environments and sliding wear applications. PVD chromium nitride use, to lessen frictional loss through piston ring contact, is prevalent in Japan and Europe and increasing in North America.
Gangopadhyay identified nitriding, a process that combines ferrous metal with nitrogen, and Diamond Like Carbon (DLC) coated piston rings as emerging technologies for friction reduction and wear protection, and emphasised a growing trend towards lower piston ring tensions. Although he concedes that the latter creates a “delicate balancing act” when controlling oil consumption.
Plasma Transfer Wire Arc Spray (PTWA), Twin Wire Arc Spraying, and Electric Arc Wire Spraying are three similar technologies gaining prominence for minimising cylinder bore contact. Gangopadhyay highlighted an opportunity to reduce four to six pounds of mass employing these techniques as a cost-effective alternative to speciality cylinder bore liners.
Improvements in form honing and honing finish are also potential contributors in reducing bore distortion. The Ford tribology expert confirmed a move towards mirror finishing the cylinder bore and other parts of the engine.
New wear resistant coatings containing nano particles and polymer coatings have been developed to minimise contact between the skirt and linings. Gangopadhyay also outlined the emerging trends of micro dimpling and texturing, although he says the latter remains at a laboratory stage at this time.