Left: Aluminum; Right: Steel.
When trying to "lightweight" something made out of steel, the designer's natural inclination is to turn to aluminum. But the R&D guys over at Mercedes-Benz recently did the opposite of that, and scooped up a Materialica Design and Technology Award for their trouble.
The MDT Awards are part of recently-held trade fair Materialica, which is dedicated to "Materials applications, surface technology and product engineering," and were intended to highlight lightweight design in transportation. To that end Mercedes took an aluminum piston design for a diesel passenger car and replaced it with a redesigned steel one.To the layperson, this doesn't make sense on two levels. First off the cylinder block was left aluminum, which expands and contracts at a different rate than steel, which is why aluminum cylinder blocks are usually paired with aluminum cylinders. Secondly, even your first-year ID or engineering students understand that steel is
heavier denser than aluminum, hardly a good choice of material for a lightweighting competition.
But what Mercedes realized was that by using steel, they could make shorter, more compact pistons (presumably operating within a cylinder of similar displacement), producing the same or better performance while using less overall material and thus, cutting the weight. As for the expansion problem, according to Mercedes, "The lower expansion of steel as operating temperatures rise ensures increasing clearance between piston and aluminium housing. This reduces friction by 40 to 50 percent." The piston rings presumably take up the slack.
The improved pistons are good enough to reduce fuel consumption and CO2 emissions by two to four percent, and "the reduction in fuel consumption is even more pronounced at lower and mid-range engine speeds," writes the company. "These ranges play an important part in everyday driving."
While we assume the automotive future will be electric or at least hybrid, it's interesting to see that innovation in fuel-burning engines is still continuing (perhaps spurred by the non-internal-combustion competition), producing this unexpected steel-for-aluminum swap. With any luck, the materials science will trickle down to other industries.