Mercedes Switches to Steel Pistons for V-6 Diesel

Daimler AG's Mercedes-Benz unit is replacing the aluminum pistons in the V-6 diesel for its E 350 BlueTEC sedan with more compact steel pistons.

Daimler says the steel assemblies weigh about the same as the aluminum piston sets they replace. Developers capitalized on the differences in properties between the steel pistons and their aluminum engine housings to cut emissions and fuel consumption.

The steel-pistoned engine produces the same power (258 hp) as the earlier V-6 with aluminum cans.

Steel expands only 25% as much as aluminum when heated. It also conducts heat less efficiently. Engineers designed the steel pistons to fit tightly into their cylinder bores, which are coated with Daimler's "nanoslide" material. As the engine heats up, the aluminum block expands more than the pistons do, thus reducing friction.

Daimler notes that an engine's piston/cylinder assembly accounts for 40%-50% of the powerplant's mechanical friction. It says its steel piston/aluminum cylinder design reduces fuel consumption 3%.

Because the steel pistons are less conductive thermally, the revised engine also generates higher combustion temperatures and is more thermally efficient. The result is faster and higher-quality ignition, along with lower emissions and fuel consumption.

Daimler notes that steel also can cope with greater mechanical stress than aluminum can, a feature that may be useful as its diesels are downsized. The company predicts at least some of its future 4-cylinder diesels will switch to steel pistons too.

Daimler's more compact steel piston (right) vs. previous aluminum piston (left).