Aluminium piston (left) the new steel piston (right). Click to enlarge.

The new pistons contribute to the maintenance of the same engine output (190 kW/ 258 hp), but with fuel consumption of around 5.0 liters of diesel fuel per 100 kilometers (47 mpg US)—the saving achieved through the use of the steel piston is around 3%.

Steel pistons are already commonly found in commercial vehicle engines, where they are combined with heavy cast-iron crankcases, while aluminium pistons have over the years gradually become the norm for passenger car diesel engines. The steel pistons that Mercedes-Benz has now developed from scratch harmonize with the much lighter aluminium engine housings and the NANOSLIDE cylinder bore coating technology also developed by Mercedes‑Benz.

Exploiting material advantages. Steel and aluminium have significantly different characteristics: steel expands less than aluminium when hot, conducts heat less well and is heavier. Mercedes-Benz engineers discovered opportunities by using the apparent differences between the properties of the respective materials to their best advantages.

A steel piston expands with heat only to about a quarter of the extent of its aluminium equivalent, allowing the engineers to fit the steel piston more tightly within the aluminium housing, with the effect that it sits very snugly in the cylinder bore. However, as the temperature rises during operation of the engine, the aluminium housing expands more than the steel piston—and the result is greater tolerance of the piston within the cylinder and thus less friction. As the piston/cylinder assembly alone causes between 40 and 50% of the mechanical friction, the potential for efficiency here is significant.

Steel pistons used up until this point were, however, little suited for the combination with engine housings made of aluminium, Mercedes-Benz said. So, Mercedes‑Benz redesigned the piston. The modern versions of the steel pistons that will now be fitted for the first time as standard in the V6 diesel on board the E 350 BlueTEC, within an aluminium crankcase with NANOSLIDE cylinder bore coating technology, are forged out of high-quality, high-strength steel. The challenge for the piston supplier is considerable, since the manufacturing process for the new, high-strength steels is similarly complex.

The higher strength of the modern steel allows for a more compact piston design, which more or less compensates for the fact that the material is around three times as dense. The advanced steel pistons that will in future be used in the V6 diesel engine are as much as 13 millimeters lower than the aluminium equivalents used until now (aluminium pistons in the V6 diesel approx. 71.6 mm high, steel pistons only 58.6 millimeters).

Due to this changed geometry and design, the weight of the unit comprising piston, gudgeon pin and piston rings is on a par with that of the version with aluminium piston. Mercedes-Benz has thus been able to compensate almost completely for the weight disadvantage of steel and even to ensure reserves of strength for peak pressures that may become even higher in future.

Higher efficiency, lower consumption. At the same time the use of steel pistons has enabled the engineers to improve the level of efficiency, since the lower thermal conductivity of steel compared with aluminium means that higher temperatures are reached within the combustion chamber. The ignition quality thus increases, while the combustion duration is reduced. The result is lower fuel consumption and pollutant emissions.

Mercedes-Benz took account of the lower thermal conductivity of steel with design modifications such as modified cooling ducts in the pistons.

Experience has shown that the innovative steel pistons optimize thermodynamic performance and, at the same time, reduce friction significantly. Furthermore, measurements showed that significant consumption benefits can be achieved in the lower and middle speed ranges important in everyday driving.

The engine designers at Mercedes-Benz see further possible advantages in the use of the high-tech steel pistons:

• Steel not only allows the piston to be made smaller, but also offers greater reserves for coping with mechanical stresses. This is particularly advantageous for further downsizing concepts.

• Since steel pistons are stronger than aluminium ones, a diesel engine fitted with them can operate at higher temperatures and thus achieve a higher level of thermodynamic efficiency.

• Since the lower thermal expansion of steel pistons compared with aluminium also means that the engine designers are able to reduce the gap between the cylinder wall and the piston as far as the first piston ring, it has been possible to reduce both pollutants and untreated emissions.

Mercedes-Benz is envisioning further application of the new pistons in its four-cylinder diesel engines.

We are assuming that pistons made of steel will in future also be in widespread use in passenger car diesel engines.

—Joachim Schommers, head of basic engine development at Mercedes-Benz