Piston Ring Tribological Challenges on the Next Generation of Flex-fuel Engines 2010-01-1529

With the current use of bio-renewable fuel, the application of Ethanol in Flex-Fuel vehicles presents a very low CO2 emission alternative when the complete cycle, from plantation, fuel production, till vehicle use, is considered. In Brazil more than 80% of the car production is composed of Flex-Fuel vehicles. Due to the lower heating content of the Ethanol, more aggressive combustion calibrations are used to obtain the same engine power than when burning gasoline. Such Ethanol demands, associated with the continuous increase of engine specific power has lead to thermo-mechanical loads which challenges the tribology of piston rings.

The ethanol use brings also some specific tribological differences not very well understood like fuel dilution in the lube oil, especially on cold start, corrosive environment etc. Under specific driving conditions, incipient failures like spalling on nitrided steel top rings have been observed. When running with Ethanol, Flex-Fuel engines present higher PCP and this peak occurs in a crank angle closer to the Top Dead Center. Such condition increases ring wear, scuffing risk and tribological conditions that may lead to cracks on the nitrided case and later spalling.

This paper covers the top ring performance on flex-fuel engines on the tribology perspective. It is discussed how the ethanol use on Flex-fueled engines affects the top ring in terms of wear, scuffing, spalling and friction. For the evaluation of wear and spalling resistance, dyno engine tests are presented. On scuffing, block on ring tests were performed and a rank of ring coatings is presented. Top ring friction behavior and its impact on fuel consumption are also discussed. Test results on engine floating liner are presented. Finally, ring technical solutions to overcome these challenges are discussed. Among them, steel piston ring with an improved nitriding treatment for increased toughness and a PVD coating to improve wear resistance and friction.

It is presented several rig and engine tests to support the discussed technical solutions. Engine tests are based on an abusive procedure to evidence/anticipate the effects on performance caused by the unique characteristics of flex-fueled engines running with Ethanol.