0.004 mm—can make the difference

In modern engines, and for high continuous outputs, our piston rings ensure reliable and precise function of the carriers. As the leading manufacturer of pistons, we constantly work to optimize the performance and running quality of our piston rings. We also set quality standards in this field with state-of-the-art production facilities. With decades of experience in development and production, we can assure optimum compatibility between our pistons and piston rings and can satisfy the highest demands.

Piston rings have three important functions in modern engines:

  • They seal the combustion chamber from the crankcase.
  • They limit and regulate the oil consumption.
  • They dissipate the heat that is taken up by the pistons during combustion to the cooled cylinder bore.

In order to satisfy these demands, the piston rings must fit tightly against the cylinder wall over their entire circumference, even if the cylinder deviates slightly from its ideal form. Due to the high inertial forces and combustion pressures as well as the high wear producing loads, the piston rings have to satisfy high demands in respect to piston ring material (strength/temperature stability) as well as surface finish and shape.

We offer different versions of piston ring sets in original equipment quality as well as for older engines with the aim of reducing their compression loss and normalizing oil consumption. We can supply piston ring sets for almost any gasoline or diesel engine for passenger cars as well as for commercial vehicles, in diameters ranging from 60 to 160.

The MAHLE “N“ piston ring set offers you the piston rings that are also used as original equipment. The MAHLE “V“ piston ring sets have been developed especially for engines that have been running for some time.

It is essential that the TOP markings are observed. Rings with TOP markings have to be fitted with these markings facing upwards in direction piston head.

Only high-grade materials are used for our piston rings. For normal or heavy loads, lamellar graphite cast iron material is used and for the highest demands, nodular graphite cast iron materials or steel material is used.

Special coatings can be applied to the ring faces of piston rings to increase durability and avoid the formation of scuff marks. Chromium coatings or plasma spray coatings of metallic or ceramic materials are used for this purpose.

Piston rings scrape off the engine oil and seal the combustion chamber from the crankcase. This gives them an essential role in the engine compartment.
Piston rings come mostly in complete sets—as a rule in threes. The upper two rings at the piston work purely as compression rings, sealing the combustion chamber between the piston and the cylinder bore. This is a tough task. On the one hand, this is due to the enormous pressures generated in the combustion chamber—especially in diesel engines, where ignition pressures of 200 bar are now almost the norm. On the other hand, the combustion gases are that hot, that even parts of the exhaust system are made to glow. While the piston moves up and down in the cylinder bore, the piston rings glide over the cylinder working surface in order to seal the combustion chamber and to scrape off the oil—and this countless times.

The third piston ring, the so-called oil scraper ring, ensures that the bulk of the engine oil remains in the interior of the engine. All that is needed is a stable oil film between the piston skirt and the cylinder working surface. The small amount of oil that is missed by the oil scraper ring is then taken care of by the piston ring in the middle. And the tiny amount of oil that remains at the cylinder wall above the middle ring is left to the upper ring. This ring has the toughest job of all, as it has to endure the highest pressure peaks and the highest temperatures. A small quantity of the tiny amount of engine oil that lubricates the upper ring evaporates, and is then burned off in the combustion chamber. A little oil consumption is therefore a good sign: it shows that also the upper piston ring is supplied with oil.

Smaller and lighter all the time
Piston rings have to be extremely strong. Only that enables them to generate the necessary forces (tangential and radial forces). Materials of choice are therefore either ductile cast iron or spheroidal graphite cast iron. Also piston rings made from roll-formed steel sections are used. The demanded small size present an additional design and manufacturing challenge: while oil scraper rings had a height of four or five millimetres just some years ago, they are only half this height in today's engines. Only when piston rings can be made extremely small, it is also possible to design small (and therefore light) pistons to match—an essential requirement for engines that are economical and offer high performance at the same time.

Active protection: The coating
The ring faces of the piston rings glide permanently over the cylinder wall. The height of a piston ring is very small compared to the length of the cylinder surface that it glides over. A piston ring has to be protected at its ring face, especially the one at the top (being the ring with the least oil available). This protection can be achieved, for instance, by galvanically applying a layer of chrome, which is a very hard and wear-resistant metal. The thickness of this chrome layer is about 100–200 µm.

Another extremely hard and therefore wear-resistant metal is molybdenum. For instance, the ring faces of compression rings for heavy-load diesel engines are coated with molybdenum. This is achieved by melting molybdenum powder in a plasma jet. The plasma forces the liquid metal droplets onto the ring face. When the metal solidifies, a wear-resistant and hard, yet porous surface is formed. This porous layer picks up engine oil, acting as a kind of spare lubrication: under heavy loads and critical lubrication conditions, the assimilated oil provides lubrication for the ring face. This means reliable performance even under extreme loads. However, also with special chromium coatings (multilayer, chromium ceramics) it is possible to create small breaks and fine channels in the chromium layer to provide room for the engine oil.

Piston, piston rings and cylinder wall—a perfect union
During the design process of the engine, it will be determined what kind of covering is reasonable and necessary for the piston rings. MAHLE is a development partner of the engine supplier industry. MAHLE engineers and specialists develop and test the optimal ring selection for the respective application in cooperation with our worldwide customers in the serial production. Pistons, piston rings and cylinder wall must be matched perfectly. This is the prerequisite for optimal operation of the engine and for long service life. The characteristics of piston rings can also be improved by applying additional coatings over the entire surface of the rings (e.g. phosphate or tin).

The colour of coatings
Occasionally, piston rings may also be modified for existing piston types. If the ring set for an existing piston is altered, MAHLE supplies the up-to-date standard version without delay. For instance, it can happen that a compression ring, which used to be only chromium-plated at the ring face, is now coated with an additional phosphate layer—and the ring that used to look shiny silvery comes now in black. On the other hand, a ring with multilayer coating without phosphate may have replaced a chrome-plated piston ring that was phosphatized—and its colour would therefore have changed from black to shiny silver.

Mahle piston rings—quality in material and workmanship
MAHLE piston rings comply in every respect with the strict original equipment standards—thanks to high-quality materials and precise manufacturing processes. Our references are impressive here: Piston rings made by MAHLE are used as original equipment in vehicles of all leading car manufacturers—from Alfa, Audi and BMW to Mercedes and Volvo. For the aftermarket, MAHLE offers a comprehensive range of piston ring sets in original equipment quality. You can find further information in the current catalogue “Piston ring sets“. This is either available in printed form or on CD-ROM as part of the complete engine components range.