In many areas of vehicle construction and components it has become increasingly common for automakers to shift away from steel to embrace aluminum in pursuit of reduced weight. But in one area at least, the reverse is true: Mercedes-Benz is replacing aluminum pistons with a new generation of high-strength steel in some of its car diesel engines.
The material shift will first be found in the V6 diesel powering the E 350 BlueTEC. Mercedes claims some 3% improvement in fuel economy as a result. Power output remains at 190 kW (255 hp) but official combined fuel consumption improves to 5.0 L/100 km.
The newly developed steel pistons are also expected to be used in upcoming four-cylinder Mercedes diesels.
Said Joachim Schommers, Mercedes’ Head of Basic Engine Development: “We assume that in the future steel pistons will be in widespread use in passenger car diesel engines.”
But it’s not quite that simple; considerable amounts of development time and investment have been required. Also, Mercedes has made the change by applying its Nanoslide cylinder bore coating (read more at http://articles.sae.org/10507/). Produced via twin-wire arc spraying to melt iron/carbon wires, the resulting solution is sprayed, using an inert gas flow, onto the inside cylinder walls.
Very fine finishing of the nano-crystalline iron coating creates an “almost mirror-smooth surface with fine pores,” said Schommers. Wear is also reduced. Nanoslide has been in use by Mercedes for eight years, having been first applied to the V8 AMG engine.
Steel pistons are commonly used together with cast-iron crankcases for commercial-vehicle engines application, while lighter aluminum has become the preferred material for cars.
Surprisingly, the pluses and minuses regarding the use of steel or aluminum pistons in cars have now apparently tipped in favor of the former. Schommers agrees that steel expands less than aluminum when heated, does not conduct heat as efficiently, and weighs more: “At first glance this would seem to make the combination of aluminum housing and steel pistons tricky!”
However, R&D work has demonstrated that a steel piston only expands with heat to around a quarter that of a comparable aluminum component. As temperature rises during normal engine operation, the aluminum housing expands more than the steel piston to create greater tolerance within the cylinder and resultant reduced friction—highly significant, as Schommers explains that the piston/cylinder together create up to 50% of the mechanical friction experienced in a piston’s operation.
The downside, though, was that regular steel pistons did not meet Mercedes’ weight and packaging criteria, so a high-strength steel (HSS) solution was developed to offset the denseness of steel against aluminum, which has resulted in piston height being reduced from the 71.6 mm (2.82 in) of the aluminum type used in the V6, to 58.6 mm (2.31 in).
Mercedes states that the weight of the fully dressed HSS piston (piston, piston rings, gudgeon pin) is similar to that of an aluminum component and that it will cope well with future higher in-cylinder pressures as well as providing enhanced efficiency now in terms of increased temperature capability, thanks to the lower thermal connectivity of steel and reduced combustion duration.
And improved resistance to mechanical stresses will aid engine downsizing.
The fuel consumption improvements achieved with the new pistons are most marked in the lower and middle speed ranges.