Aluminium piston (L), steel piston (R)Germany – Replacing conventional aluminium pistons, the new components will feature in the V6 diesel engine of the E350 BlueTEC passenger car. According to the OEM, this will result in improved engine efficiency, reducing fuel consumption and emissions.

Mercedes-Benz says that, while steel pistons are commonly found in commercial vehicle (CV) engines in combination with heavy cast-iron crankcases, the new pistons can be fitted together with lighter aluminium housings and Nanoslide cylinder bore coating technology which has been developed in-house.
The OEM claims to have tackled the combination of materials with contrasting properties by turning their differences into advantages. Since a steel piston expands with heat to only a quarter of the aluminium equivalent, this enables a tighter fit of the steel piston in the aluminium housing, so that it sits snugly within the cylinder bore. As the temperature rises during engine operation, the housing expands more than the piston, creating greater tolerance of the piston within the cylinder and therefore less friction. This generates a significant improvement in efficiency because the piston/cylinder assembly alone causes 40-50% of the mechanical friction, says Mercedes-Benz.

The OEM admits that manufacturing the new pistons presents a “considerable” challenge to the supplier but says “the effort is worthwhile” because the high-strength steel allows for a more compact design which “more or less compensates for the fact that the material is around three times as dense”. Aluminium pistons in the V6 diesel engine are 71.7mm high, while the steel variety are 58.8mm. Mercedes-Benz says the weight of the unit containing the piston, gudgeon pin and piston rings is “on a par with” the version featuring the aluminium piston.
Meanwhile, the lower thermal conductivity of steel compared with aluminium means that higher temperatures are reached within the combustion chamber. Consequently, the ignition quality increases while the combustion duration decreases, resulting in lower fuel consumption and emissions. The steel pistons feature modified cooling ducts to address the difference in thermal conductivity.