Aluminium pistons for diesel engines

Injection pressures up to 2,000 bar, mean effective pressures over 20 bar, peak pressures of 170 to 200 bar, specific powers up to 80 kW per liter, and ever lower exhaust gas emissions - these are today's challenges of modern diesel engines for passenger cars. MAHLE is able to meet these requirements for the most diverse engine concepts: whether direct injection or super-charging with charge cooling. MAHLE has developed a series of measures to overcome these loads and high temperatures.



The MAHLE program for diesel engines
MAHLE pistons for passenger car diesel engines are cast from high-strength aluminum alloys with diameters from 65 to 100 mm. Standard features are: ring carriers made from high-strength, austenitic cast iron (Niresist) for increasing the wear resistance of the first ring groove, salt core cooling channels or cooled ring carriers - and a MAHLE GRAFAL® coating for optimizing the skirt performance. For engines with especially high loads, MAHLE also uses bushings in the piston pin bores.

New alloys for the highest demands
Improved material properties are especially required with respect to the high temperature loads in diesel engines - for even greater fatigue resistance over a wide temperature range together with a lower weight. MAHLE utilizes new alloys in response to this. For example, the extremely heat-resistant aluminum alloy "M174+" in piston production for diesel engines. A MAHLE development which fulfills these requirements to the utmost. In addition, MAHLE improved the casting process with its newly developed ADC (Advanced Diesel Casting) method. With ADC, a fine micro structure can be achieved in the high-stress zone of the bowl rim, which improves fatigue resistance and the resistance to temperature fluctuations.

Local reinforcement for maximum strength
To improve the piston properties at critical points beyond the limit of this base material and the casting process, additional parts are cast-in in the piston structure or inserted subsequently.

In addition to the measures described earlier, such as casting-in a ring carrier and inserting bushings, fibers made from aluminum oxides are infiltrated for strengthening the combustion bowl subject to high thermal stresses. This fiber reinforcement enables an increased fatigue resistance, improved rigidity as well as increased thermal shock resistance. MAHLE has manufacturing processes at its disposal that allow a wide application of this technology in future diesel engine concepts.

Cooled ring carriers for critical areas
With its cooled ring carrier, MAHLE has developed a solution for high volume production, which achieves a significant improvement in piston cooling in the critical areas of the bowl rim and first ring groove and which is already used in new engine concepts with high specific powers.

The cooled ring carrier consists of a Niresist ring carrier onto which a thin austenitic steel sheet is welded with inlet and outlet openings. Cast-in in the piston, the combined insert brings the cooling oil even closer to the combustion chamber and the first ring groove.

In contrast to salt core cooling channels, this new MAHLE solution enables a significantly better cooling of the critical areas. Similar to the transition which took place from spray cooled pistons to salt core cooling channels. MAHLE regards the diesel piston with cooled ring carrier as a pioneering innovation in technology. The series production of this piston started in 2001. By 2007, more than ten million pistons with cooled ring carriers had been produced.