Steel pistons for passenger car diesel engines: designed for 95 g CO2/km

KS Kolbenschmidt GmbH was the first to supply steel pistons for the high-volume production of passenger car diesel engines in 2014. They were specifically engineered for a premium German carmaker using an aluminum block with Fe spray coated liner technology. Meanwhile steel pistons have become a major trend for all diesel engine manufacturers. They help to lower fuel consumption and reduce CO2 emissions by three to five percent. This is the case for both high and moderate power densities as well as for rising peak pressures. Kolbenschmidt is developing custom made piston designs to suit all applications and requirements, including a low-cost option for more elementary engine models.

Steel piston with rings and pin

Size comparison: steel vs. aluminum pistons Stahl = Steel Aluminium = Aluminum Kompressionshöhe = Compression height

In order to cope with peak pressures in excess of 180 bar, the company uses its patented single-part steel piston design with connected ring zone. The coolant passage is manufactured with the aid of an unique process and the ring zone is structurally connected to the skirt with a special joining process. This allows a barrier-free coolant passage to be formed which, on the one hand, facilitates effective cooling of the entire ring zone and, on the other, closely follows the contours of the combustion bowl geometry and thus ensures best-possible cooling of the combustion bowl lip, an area that is particularly temperature sensitive.
The high-rigidity ring zone connected in this manner is remarkable for minimum groove deformation at high combustion pressures whilst providing outstanding support to the piston ring function. For engine applications using lower peak pressures, the connection with the ring zone is optional. In this case, the coolant passage is still produced in the same way and delivers the same cooling advantages as on the version with connected ring zone.

The many advantages of steel
Steel is much stronger than aluminum and so the compression height on steel pistons can be reduced by about 30 percent compared with aluminum pistons. The patented Kolbenschmidt design combined with the use of steel means that the minimum compression height is only governed by the geometry of the connecting rod top end, the free passage to the inner form, the bowl geometry, and the necessary piston crown thickness. To the same extent as the compression height can be reduced, the connecting rod can be lengthened as a further step toward curbing fuel consumption. The low compression height is, not least, attributable to an almost 50 percent reduction in top land height compared with aluminum pistons. This, in turn, lowers pollutant and engine-out emissions.

Steel's lower thermal conductivity compared with aluminum, leads to higher surface temperatures at the piston crown on the combustion chamber side and thus boosts the thermodynamic efficiency of the engine. On its series-produced pistons, Kolbenschmidt now uses 42CrMo4 steel. This allows piston crown temperatures to be maximized and the temperature-critical bowl lip to be protected from scaling in the best possible way.

Reduced friction, reduced emissions
The much smaller skirt surfaces on the steel pistons compared with aluminum play a significant role in friction reduction. They also feature the low-friction and wear-resistant Nanofriks coating as standard. The geometry has been optimized for maximum skirt rigidity to allow the piston to be fully guided by the skirt, an additional ring land guidance is avoided. Hence, the ring lands are freely dimensionable and can be perfectly matched for blow-by and oil consumption. This dimensional flexibility merits special mention when installing steel pistons in an aluminum engine block. Here the necessary land clearances can be set to satisfy the particularly critical cold-seizure tests without any negative noise repercussions under hot operating conditions.

Low noise level with aluminum and gray cast iron
Kolbenschmidt's steel pistons for car engines specially support the use with Fe spray-coated aluminum engine blocks. The reduced thermal expansion of the steel pistons compared with the aluminum engine block results in ample fitting clearances under hot operating conditions. Although this does reduce friction, it also calls for precision skirt and skirt connection geometries in order to achieve optimum piston guidance. To this end, extensive noise-reduction measures were carried out operating in mono-aluminum engine blocks, which also have a positive effect on the noise behavior of the gray cast iron blocks. Complete noise level control even with added fitting clearances is one of the challenges confronting Kolbenschmidt engineers in their efforts to exploit further potentials for reducing CO2 emissions.

 

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