There is a clear trend in the automotive and engine industry: a demand for engines that are more and more powerful. Especially the diesel injection has become the focus of attention of engine designers. An enormous potential has been created in this field in the recent years: thanks to high-pressure injection in combination with the electronic control of the combustion process, the conditions for significantly increased performance of diesel engines have been created.

Of course, it is inevitable that higher specific engine powers also increase the thermal and mechanical loads on pistons. A trend that has to be taken into account especially in the development and design of pistons – as pistons are among the most highly stressed components in the combustion engine. At temperatures of more than 360 °C as they are generated in the piston combustion cavities and ignition pressures of more than 170 bar, pistons made from aluminium alloys reach the limits of their endurance.

New design ideas were therefore needed and the use of new materials was required. The solution from the MAHLE developers: a piston with a crown of highly temperature resistant forged steel and an aluminium skirt – the FERROTHERM^® piston was born.

GOOD GUIDANCE AND HARD WORK
A design that is based on clear task sharing: the aluminium skirt only takes care of guiding the piston in the cylinder bore, while the piston head does the hard work and the sealing. The forged steel of the piston head permits significantly higher operating temperatures and significantly higher surface pressures between boss and piston pin. Also the wear resistance and service life of the ring grooves, which are located in the upper part of the piston benefit from the high strength of the forged steel.

TOP LAND CLEARANCE
An additional, important advantage of steel compared with aluminium-silicon alloys is its significantly lower thermal expansion coefficient, which allows smaller clearance between piston and cylinder at the top land. This again offers great advantages in respect of sealing and reduction of blowby, while the very narrow gap between piston and cylinder supports the piston rings in their work. Small top land clearance has also a positive effect on exhaust emission values – because smaller clearance means less clearance volume and therefore optimal and clean combustion.

THE TRICK WITH THE SHAKER
However, steel has also its disadvantage: its thermal conductivity is worse than that of aluminium. It is therefore necessary to assure a low temperature level by optimising the cooling of the piston. The solution: spring steel plates or a design of the piston skirt that forms a cooling channel through which the engine oil is injected for cooling. Due to the large diameter and the up and down movement of the piston, a so called shaker effect is generated, which cools the piston head sufficiently.

TWO UNEQUAL PARTNERS – ONE SUCCESSFUL COOPERATION
How do you combine two materials as different as steel and aluminium into one functional unit? As can be seen in the picture, the upper part of the piston and the skirt are connected via the pin. The combustion and ignition pressures have their effect on the steel head – and the resulting force is directly transmitted from the head via the piston pin to the connecting rod. A non-ferrous metal bushing is inserted into the pin bore as ideal running partner for the piston pin. In newer designs the pin bore is coated, which improves the interplay between piston and pin even further. A gap is left between the upper steel section and the aluminium skirt. This spatial separation acts as thermal decoupling. The temperatures at the skirt are therefore significantly lower than for a piston made entirely from aluminium. Thanks to the low temperature level at the piston skirt and the decoupling of the power transmission, the skirt design could be optimised in respect of its guiding task. And this can be noticed: in the soft, calm engine running despite harsh diesel combustion – at higher power output and more torque.

THE FERROTHERM^® PISTON: CONVINCING AS ORIGINAL EQUIPMENT …
The FERROTHERM^® piston has been on the market for almost 20 years – but it remains a modern piston design that is used especially in the commercial vehicle sector. Many engines from Volvo, Scania, Detroit Diesel and Caterpillar use FERROTHERM^® pistons from MAHLE as original equipment. Many leading manufacturers still rely on the ingenious principle: currently, engines with two part pistons are being developed for Mercedes- Benz and a large Russian engine manufacturer.

… AND SUCCESSFUL IN THE AFTERMARKET
Also the MAHLE trading programme includes FERROTHERM^® pistons in the current engine component catalogue, recognisable by the 2 raw piston numbers: the steel piston crown has the letter combination “KB”, while the cast aluminium skirt can be recognised by the Letter “L” (see catalogue excerpt).

A look into the MAHLE Original engine component catalogue: the steel head of the two part FERROTHERM^® piston can be recognised by the letter combination “KB”, the aluminium skirt has the designation “L”.

IMPORTANT: THE CORRECT ASSEMBLY
FERROTHERM^® pistons are delivered in dismantled form, with upper part and skirt carefully wrapped and placed side-by-side into the box. When upper part, skirt and connecting rod are assembled, the correct fitting orientation must be observed: the assembly of upper part and skirt can only be done in one direction.

Cutaway section of a FERROTHERM^® piston with piston head of steel.