MAHLE pistons—our competition doesn't want to burn their fingers in Formula 1

Our pistons have to put up with extreme pressures, as their working conditions have become significantly tougher in recent years. For example, thermal loads (with temperatures in the combustion chamber reaching 2,600ºC) and average ignition pressures have increased greatly. The inertial force loads have also increased due to high-speed concepts and a tendency to longer piston strokes. Naturally, the demands on pistons have also risen progressively: they must be lighter, must have lower oil consumption and must be absolutely safe—and this even under extreme conditions such as hot and cold load capacity testing and endurance testing under thermal shock.

And the effects of these demands: the overall height or compression height of the pistons has continuously decreased. New solutions such as the ECOFORM® piston concept developed by MAHLE allow further weight reductions. And with MONOTHERM® pistons, which are single-piece steel pistons that are machined accordingly, it is possible to achieve the same weight as with pistons made from aluminum. Significant improvements have also been made with aluminum pistons such as the MAHLE pistons with cooled ring carriers.

We also greatly benefit in this field from our successes in motorsports—after all, we are the only producer of series products who is gaining experience from Formula 1. As the world's largest manufacturer of pistons, MAHLE is conducting intensive research into the products of tomorrow, and today offers you solutions for almost any engine—fast, reliably and worldwide. With our pistons, we offer the aftermarket the opportunity to benefit from products of the original equipment market and the inherent safety of products that have been tried and tested in practice under the most grueling conditions. We guarantee it.

Two-stroke pistons
These pistons are mainly used in gasoline and diesel engines for passenger cars under heavy load conditions. They have cast-in steel strips but are not slotted. As a result, they form a uniform body with extreme strength.

Cast solid skirt pistons
Piston top, ring belt and skirt form a robust unit. Cast solid skirt pistons have a long service life and can be used in gasoline and diesel engines. Their range of applications extends from model engines to large power units.

Forged solid skirt pistons
This type of piston can mainly be found in high performance series production and racing engines. Due to the manufacturing process, they are stronger and therefore allow reduced wall cross-sections and lower piston weight.

These very quiet running pistons are used primarily in passenger cars. The pistons have cast-in steel strips and are slotted at the transition from ring belt to skirt section.

These pistons are used mainly in gasoline and diesel engines for passenger cars under heavy load conditions. They have cast-in steel strips but are not slotted. As a result, they form a uniform body with extreme strength.

ECOFORMSUP® pistons with pivoting side cores
Weight optimized pistons for passenger car gasoline engines. As a result of a special casting technology these pistons offer low weight and high structural rigidity.

Ring carrier pistons with pin boss bushes
These pistons for diesel engines have a ring carrier made from special cast iron that is connected metallically and rigidly with the piston material in order to make it more wear resistant, in particular in the first groove. With pin boss bushes made from a special material, the load-bearing capacity of the pin boss is increased.

Ring carrier pistons with cooling channel
Ring carrier pistons with cooling channel are used in conditions with particularly high operating temperatures. Because of the high temperatures at the piston top and the ring belt, intensive cooling is provided with oil circulating through the cooling channel.

Ring carrier pistons with cooling channel and crown reinforcement
These pistons are used in diesel engines under heavy load conditions. For additional protection and to avoid cavity edge or crown fissures, these pistons have a special hard anodized layer (HA layer) on the crown.

Pistons with cooled ring carriers
For these pistons, ring carriers and cooling channels are combined into one system in a special production process. This provides the pistons with significantly improved heat removal properties, especially in the first ring groove.

This one-piece forged steel piston has an extremely high structural strength and was developed for modern combustion chambers with pressures of 250 bar and more. The version with connected shaft in combination with a short piston pin makes its weight comparable to that of an aluminium piston.

A steel piston top and an aluminum piston skirt that are flexibly connected via the gudgeon pin. Due to their high strength and lower wear values, these pistons make it possible to comply with low exhaust and emission standards, especially for diesel engines under heavy loads.

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.

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.

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.

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.

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 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.

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”.

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.

The different piston crown markings and what they mean:
Looking at a piston, the markings on the piston crown attract attention. In addition to dimensional and clearance specifications, most pistons show information about their fitting orientation. The pistons are marked with fitting orientations according to specifications from our production customers—the engine manufacturers.

Many customers—this means also many different requests and specifications for piston markings. This multitude of markings may appear to the onlooker somewhat like Egyptian hieroglyphs.

For this reason, we are giving you here an overview of the most important markings and their meaning:

Why is it important to observe the fitting orientation for pistons?
Pistons with asymmetric crown shape or pistons that are designed with different sizes of valve pockets in the piston head can only be fitted to the engine in a particular orientation. This applies also to pistons with combustion cavities that are not in the centre of the piston crown.

However, there are also pistons that appear to be completely symmetrical and have a plain piston crown for instance. Even for these pistons, it may be necessary to observe a particular fitting orientation. The reason for this is the so-called 'pin bore offset'. This means that the piston pin is not exactly in the middle of the piston, but slightly offset to the side. It is not always possible to recognize this with the unaided eye, as the offset may be less than a millimetre, depending on the particular design.

Why do most pistons have a pin bore offset?
Due to the rotation of the crankshaft, the connecting rods are at an angle in most positions. This is pressing the piston towards one side of the cylinder bore during its upward movement. At the upper piston dead centre, the connecting rod stands exactly vertical and then starts to form an angle towards the other side. Also the piston begins now to lean towards the other side of the cylinder bore. In order to soften the impact of the piston against the opposite side and to make it as silent as possible, the pin bore is slightly offset to one side. As a result, the piston tilts slightly around the pin axis during this change of leaning direction. The piston does therefore not hit the cylinder bore with its entire length, but touches the cylinder wall first only with the lower skirt end and settles then into a sliding movement against the other side. This reduces not only engine noise, but also the mechanical stress on the components significantly.

The pin bore is usually offset towards the pressure side of the piston. However, there are exceptions to the rule: some pistons are offset towards the other side, however, the effect is the same.