Steel pistons catch on in Europe

A new generation of steel pistons is gaining favor in diesel passenger vehicles in Europe -- a strategic win in the lightweighting contest among steel, aluminum and other materials.

But it could be a while before the technology reaches U.S. cars, experts say, primarily because diesels have fallen out of favor in the U.S., thanks to the Volkswagen diesel emissions scandal.

Steel pistons are common in medium- and heavy-truck engines in the U.S. and other global markets.

"In Europe it's a big thing. High-speed passenger-car diesels are going in this direction," said Joachim Wagenblast, director of product development for supplier Mahle Engine Components USA Inc. in Farmington Hills, Mich.

He said some automakers have steel pistons "in the development phase" for U.S. diesels, but he declined to identify them.

Mahle has made steel pistons for Renault's 1.5- and 1.6-liter car diesels since 2014.

The pistons offer 3 to 5 percent lower emissions and correspondingly better fuel economy than traditional pistons, according to Mahle and Rheinmetall Automotive AG, another supplier that makes steel pistons through its subsidiary, KS Kolbenschmidt GmbH.

A key factor is size: The steel pistons are shorter.

Because they're stronger than aluminum, steel pistons can work with less length than equivalent aluminum pistons. That potentially helps create a lower-profile engine that can fit under a lower-profile hood, the companies said.

Shorter pistons also mean less surface area to interact with the piston wall, the suppliers said. In addition, steel pistons expand less than aluminum when they're heated. Those two properties spell less friction than aluminum, the companies said.

Part of the new demand comes from tougher European emission regulations.

"Very few companies are using steel pistons so far," but many companies are exploring the concept, said Alexander Sagel, head of the Hardparts Division of Rheinmetall Automotive.

Subsidiary Kolbenschmidt has made steel pistons for Mercedes-Benz since 2014, starting with the V-6 diesel in the Mercedes E350 BlueTec. Mercedes-Benz USA doesn't import that model. In the spring of 2016, Mercedes also added new four-cylinder diesels with steel pistons for Europe, starting with the diesel-powered E class.

Mercedes-Benz USA isn't offering any diesels, said spokesman Rob Moran. He said the U.S. company's priority in the diesel segment is getting U.S. certification for the GLS large crossover. The U.S. gets aluminum pistons, Moran said.

But demand for steel pistons is growing in other markets.

On Nov. 29, Kolbenschmidt said it had landed a six-year contract to supply another premium brand with steel pistons for six-cylinder diesel cars, with production starting in 2019, at a plant in the Czech Republic.

Sagel wouldn't identify the new customer, but said it is a brand that hasn't used steel pistons before.

He said the company has "plenty of requests" from other manufacturers in Europe for information about the technology.