Mercedes-Benz has taken what, at first, appears to be a step back by replacing the aluminum pistons of its diesel engines with steel versions that slide in cylinder bores coated with the company’s Nanoslide technology. It claims the combination lowers CO2 emissions and improves fuel economy.
According to Mercedes engineers, steel pistons expand only 1/4 as much as their aluminum counterparts. This makes it possible to fit the steel piston more tightly to the bore. This reduced the gap between the cylinder wall and first piston ring lowers the amount of untreated pollutants that can escape into the crankcase. Also, since the bore itself is aluminum, it expands more than the piston as temperatures rise, which reduces friction. With 40%-50% of an engine’s internal friction caused by the piston/bore interface, they saw this as a major opportunity for an improvement in fuel economy.