MUNICH -- Will it be aluminum or steel? That has long been the question in discussing the piston material of the future.
The debate arises from the growing power density of advanced diesel engines, which increases stress on parts.
The trend eventually will force a switch from softer aluminum to steel, which can handle the higher stress. Its mechanical stability is three times greater than that of aluminum, but steel pistons are susceptible to cracking after long use.
Now that the first steel pistons for passenger cars are coming off the production line at German auto suppliers KSPG AG and Mahle GmbH, how quickly can steel pistons totally replace aluminum?
The path to steel
Mahle has proved to be more conservative in its approach to the transition to steel. The company supplies steel pistons for the new Renault diesel engine, which has a power density of about 60 kilowatts per liter of engine displacement.
Stefan Spangenberg, head of engine component development at Mahle, limits use of steel pistons in general to no more than 80 kilowatts per liter.
KSPG is much more optimistic. Since the start of the year, it has converted its entire production at its main factory in Neckarsulm to steel pistons and transferred the manufacture of aluminum pistons to Mexico and other foreign sites.
The company is delivering its first steel pistons to Mercedes-Benz. Mercedes is expected to use them in its new six-cylinder diesel engine that will be introduced this fall.
Both suppliers had a rocky path to developing production-ready steel pistons. Despite steel's greater stability, tests have shown that because of deposits on the cavity rim at high thermal stresses, cracks can form in steel pistons after long-term use.
The damage is caused by steel's lower heat conductivity. Thus, optimized cooling channels have proved extremely important during development. Both suppliers employed the technology to counter scaling and premature oil aging.
Steel is gaining market share for its ability to reduce fuel consumption, rather than for its mechanical stability. The metal reduces friction between the cylinder wall and the piston. However, that advantage is reduced if the cylinder crankcase is made of aluminum and not gray cast iron.
"A CO2 reduction of 3 to 5 percent is realistic from the interplay of rings, bolts, tracks and steel pistons," said Alexander Sagel, who was appointed head of KSPG Hardparts division on July 1.
Mahle's Spangenberg also is betting on the structural advantages of steel pistons. Since they are shorter without sacrificing stability, the entire engine can be smaller, reducing weight and power density.