Steel Pistons for Passenger Car Diesels

Although steel producers seem to be taking it on the chin of late from aluminum, word out of Stuttgart ought to be somewhat encouraging to them. That is, Mercedes has announced that starting next month, steel pistons will be installed in the Mercedes-Benz E 350 BlueTEC engine.

Previously, aluminum pistons were used in the diesel engine.

And now it is going to steel.

The aluminum piston is on the left and the steel on the right.  Turns out for diesels, smaller can be better.

While you might think that steel would be the norm for diesel engines, where there is tremendous pressure involved in the compression ignition, that is the case for commercial vehicles, but not cars, where aluminum pistons have come to the fore.

But Mercedes engineers started examining steel and determined that there is a potential benefit of using steel pistons within aluminum engine blocks, where the bores are coated by NANOSLIDE material, a nano-crystalline iron coating (also developed by Mercedes).

Because the forged steel pistons are higher in strength than a comparable amount of aluminum, they were able to make the pistons smaller: for the V6 in the E 350, the steel piston is 58.6 mm high versus 71.6 mm for an aluminum piston, yet the steel piston lends itself to application even if—or perhaps when—there is an increase of peak pressures inside the engine.

It was necessary to redesign the piston to go from aluminum (left) to steel (right).

There is less thermal expansion of steel compared with aluminum (the steel expands only about a quarter the amount that the aluminum does), so the gap between the cylinder wall and the piston is reduced as far as the first piston ring. A benefit of this is that there is a reduction in pollutants and emissions.

Joachim Schommer, head of basic engine development at Mercedes-Benz sees further application of steel pistons in the diesels that the vehicle manufacturer produces, such as its four-cylinder diesel (used, for example, in the E 250). Schommer said: “We are assuming that pistons made of steel will in future also be widespread use in passenger car diesel engines.”