Steel pistons for passenger car diesels

  • 06-Oct-2009 03:08 EDT
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A new monobloc steel piston for passenger car diesel engines with high output rates and high peak pressures is ready for series application in the near future.

Engine component supplier Kolbenschmidt Pierburg is paving the ground for what could be a paradigm change in European diesel engine technology. Together with European vehicle and engine manufacturers, the supplier has developed a new generation of monobloc steel pistons for passenger car diesel engines. The supplier first exhibited this new technology at the Frankfurt Motor Show.

In Europe, passenger car diesel engines are equipped with aluminium pistons. “However, initial testing has shown that direct-injection diesel engines with 80-kW/L output or more and ignition pressures beyond 200 bar benefit from the higher performance of steel,” said Rainer Fluhr, head of the management board of KS Kolbenschmidt.

In commercial engines, steel pistons have long proven their performance.

It would be a change for a whole industry: Cast aluminum pistons are the established standard and the technology of choice for diesel engine makers in Europe. Of course, the fast manufacturing processes, design freedom, limited need for machining, and lightweight properties of aluminum alloys still apply.

“Yet, for applications with severe conditions, we are now ready to advance steel piston technology to passenger car diesel engine series production,” said Fluhr.

So far, aluminum has helped to meet ever more stringent emissions legislation by limiting the mass of the reciprocating piston. Optimized geometries with undercut areas have helped to minimize the inertia that the piston undergoes during each stroke. Steel pistons seemed to be too heavy to compete in this area. With the constant need to limit a new passenger car generation’s weight gain, adding weight to a component that directly influences the engine’s efficiency seemed taboo.

Now, peak temperatures in the diesel combustion chamber gradually rising to around 400°C (752°F) and peak pressures beyond 200 bar (2900 psi) could well take aluminum to the limits of durability. The current efforts to make aluminum pistons durable under such adverse conditions are intensifying. Cooled steel ring carriers, coolant passages, laser remelting of the piston bowl, coating processes, and oil spray cooling in combination with optimized inner piston geometry to extend the oil retention time (shaker pistons) help to extend the aluminum piston’s service life in highly efficient modern diesel engines, but even then, aluminum will only go so far.

Also, one of the most critical areas of the aluminum piston is the pin bore. Due to the high effective forces, this area has to be massive enough to survive the load from the expanding gases. If one looks at the whole picture and compares the piston and pin assembly, an all-steel assembly can be more slender in this critical area due to the higher material strength and better material match with the steel pin. Consequently, the pin could be shorter; the weight disadvantage of a steel piston may therefore be smaller than expected.

Another plus: the identical thermal expansion of a steel engine block and a steel piston facilitate smaller tolerances and lower pretensioning between piston rings and cylinder wall. Less blow-by and lower inner friction are among the potential benefits.

According to Fluhr, the new monobloc steel pistons are ready for series application in the near future.

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