Renault Debuts Steel Piston for 1.5-Liter Diesel

Renault SA is introducing a new steel piston for its 1.5-liter passenger car diesel engine that is quieter and 2% more fuel efficient than a traditional aluminum piston.

Lab tests show the steel design delivered nearly 2% more torque at 1,500 rpm, about 4% lower brake specific fuel consumption and 1% less heat exchange with the cylinder walls.

Renault developers describe the results in a technical paper presented at last month's SIA Powertrain Rouen 2014 conference in Normandy, France.

Steel pistons are common in large truck diesels. Advantages compared with aluminum include the ability to withstand high combustion pressure with low heat flux, plus lower thermal expansion that translates into quieter cold starts and lower friction at normal operating conditions.

But steel pistons also present drawbacks. They are denser and oxidize easier. They also have a scaling temperature limit, above which the piston may crack.

Renault says it overcame the weight challenge by making the steel piston thinner and shorter, resulting in a piston/pin assembly that is equal to or lighter than an aluminum piston. Developers also added an anti-oxidation coating and followed a piston cooling strategy to keep temperatures below the maximum limit.

To reduce operating friction, the Renault system employs a diamond-like carbon pin coating, molybdenum low-friction skirt coating and a low friction ring pack. The piston bowl design and compression ratio are unchanged from the aluminum piston in the previous-generation 1.5-liter diesel.