Renault is introducing a new steel piston on a mass-production 1.5-liter diesel engine for a passenger car, according to a paper presented at the recent SIA Powertrain Rouen 2014 conference. Based on extensive testing, Renault engineers concluded that the new pistons will deliver a fuel savings of some 2%.

Renault’s technical center in Rueil-Malmaison, France led the development of the piston, in cooperation with Renault partner’s and supplier. The new piston features an improved design to minimize weight, low-friction coatings, and a low-tension ring package.

Steel pistons have been used for some time in heavy-duty diesel truck engines due to their ability to withstand high combustion pressure with low heat flux. The main benefit of the steel piston technology is its lower thermal expansion, according to the Renault authors Anthony Hollemaert, Romain Dejean, Jean-Paul Cadalen, and Patrick Gastaldi.

When used in a cast-iron cylinder block, the steel piston enables reduced gap clearance at cold conditions and maintains a positive clearance at hot condition, leading to better NVH behavior during cold start and a better friction benefit under engine running conditions.

The other main benefit of a steel piston compared to an aluminum one is its higher mechanical properties—i.e., yield stress and fatigue. Drawbacks, however, include higher density; lower thermal conductivity; higher chemical reactivity with environment (oxidation); and the presence of scaling temperature limit. Renault dealt with those drawbacks in a number of ways.

• Taking advantage of the mechanical properties, Renault reduced the wall thickness and land height of the new piston by 58% and the compression height by 22% of the piston. This compensates for the higher density, and the resultant assembled mass (piston + pin) is equivalent or lower to that of the aluminum piston.
• Due to a more efficient piston gallery and optimized design, the groove temperatures of the steel piston are equal to or lower than those of the aluminium piston.
• An anti-oxidation coating controls oxidation
• Renault found that the main drawback was the scaling temperature limit. The perlite phase—which endows the steel with its mechanical properties—is destroyed at high temperature. The scaling effect makes the material brittle, which leads to cracking. Renault took special care with piston cooling performance and strategy to meet the maximum piston temperature limit.

To help reduce friction, Renault is using a DLC (Diamond Like Carbon) pin coating; a molybdenum low friction skirt coating; and a low friction ring pack.

Renault tested the steel piston on a single cylinder engine representative of the production 1.5-liter unit. The piston bowl design, including the compression ratio, and the injector remained unchanged between the stock aluminium piston and the prototype steel unit, although the latter used low friction rings.

At 4000 rpm, the air mass flow, the maximum cylinder peak pressure and exhaust temperature were the same while the relative equivalence ratio was varied between 0.6 and 0.7 by changing the injected fuel quantity. For each point, an advantage of 2% in power—or in BSFC—was achieved with the steel piston. A small improvement has also been noticed for smoke emissions but the average level was already very low.

Other results from the testing included:

• A 1.7% improvement in torque and BSFC for the steel piston at 1500 rpm.
• Heat exchange with the walls was globally reduced by 1% with the steel piston, and transferred to the indicated work.
• Results were similar at three operating points, representative of the NEDC cycle. With the same tuning parameters, the BSFC was reduced by 3 to 4% with the steel piston for the same NO_x level.

Steel piston has confirmed its friction reduction advantages versus Aluminium one, but also a combustion slight improvement through an increase in operating temperature. This combustion benefits will be reinforced in future developments using new piston raw material and coatings to allow higher piston Temperature.