To meet Euro 6d CO2 emissions targets calling for a fleet average emissions limit of 95g/km, engine OEMs increasingly are calling on suppliers and research specialists for technical help.
Materials development specialist Materion Corp. is doing just that, revealing details of a real-world test program to develop a new metal-silicide-strengthened bronze alloy for enhancing piston compression ring efficiency. The technology, called PerforMet, is aimed particularly at downsized, boosted engines that need to cope with higher pressures and temperatures compared to naturally-aspirated units.
David Krus, Materion’s Director of Business Development, noted that producing equivalent power and torque from smaller engines impacts wear and fatigue resistance characteristics of aluminum alloys used to make pistons.
"This raises reliability issues," he told Automotive Engineering, "and as the piston crown gets hotter, the tendency for premature combustion (knock) increases which affects performance.”
Krus said PerforMet piston rings have been developed having significantly improved thermal conductivity. “They allow unwanted heat to conduct effectively away from the pistons into the engine block and cooling jackets,” he added.
Why wasn’t it tried before? Krus explained that his company's unique alloy cannot be poured directly into near-net ring shapes. Nor is it particularly amenable to the other common process for manufacture of rings—using profiled steel wire. So the idea had to wait for new technology to allow it to be achieved.
Working with Materion was U.K. consultancy GE Precision Engineering and its then Technical Director Scott Bredda, who suggested using that technology for a conductive piston ring.
Explained Krus: “Originally, we had been looking at using the technology for valve seats and guides. Then came the piston ring idea and we said, ‘Great!’. We put it into our thermal model [from the Ultraboost project designed to achieve 5.0-L V8 performance from a boosted 2.0-L I4] and saw temperatures dropping very significantly. So we reckoned we were onto something good and are now developing the wire product."
The improvement is claimed by Materion to be up to four times greater than that of typical iron-based piston rings. This equates to average piston temperatures being reduced by up to 25°C and piston crown temperatures by up to 30ºC, increasing both fatigue resistance of the piston and its combustion efficiency.
The use of PerforMet has been designed to allow engine makers to introduce new solutions, including raising the piston top ring land to a higher location to reduce the cavity between the ring and the crown. Shrinking this crevice volume reduces the amount of fuel that gathers in the space that is not ignited, thus reducing emissions of unburned fuel.
Materion has announced that the PerforMet rings are now being evaluated in real-world ICEs for durability, longevity and wear characteristics. The company announced that two "high volume global car manufacturers" are planning to test PerforMet piston rings in multi-cylinder engines later this year.
Materion expects PerforMet to be used initially in the European market to meet the 2020 legislation. Its North American introduction is likely to be via racing, Krus said.