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.

The lower the frictional losses, the more efficient and economical the engine becomes. As most of the engine friction is generated by the piston rings, the MAHLE engineers have taken this as their starting point – and have developed a new coating.

Most of the time, piston rings work in sets of 3. An ingenious interplay assures that the engine oil reaches up to the piston skirt – but does not enter the combustion chamber. In practice, this means that the two upper rings are responsible for the sealing of the combustion chamber between cylinder bore and piston. The lower piston ring serves as oil control ring and keeps the bulk of the engine oil in the interior of the combustion engine. The piston ring team makes therefore an important contribution to reducing fuel consumption and environmental pollution.

An additional task of the piston rings is cooling of the pistons: Due to their contact with the cylinder working surface, the rings transfer the combustion heat from the piston head to the cooled cylinder.

However, good piston rings can do more: They reduce frictional losses in the engine – and increase therefore the engine power, reduce fuel consumption and increase the service life of the engine.

Friction generates costs – minimized friction lowers them

The friction in the internal combustion engine has a significant effect on fuel consumption. MAHLE, one of the worlds largest piston manufacturers and development partner of engine and automobile manufacturers, has been aware of this for quite a long time – and has been working for years on innovative solutions for a comprehensive minimization of frictional losses of piston rings.

With the aid of numerical simulations and engine testing with an integrated approach, MAHLE engineers have been systematically pursuing the development of low friction piston ring sets, especially for modern petrol engines.

About tangential load and surface pressure

The contact force of oil control rings onto the cylinder working surface needs to be relatively high. Only this can prevent that engine oil reaches the piston head and is combusted there. The engineers call this the tangential load.

The tangential load presses the running surface of the piston rings against the cylinder working surface. The force per area results in the specific surface pressure, which is important for good performance. And exactly here lies the difficulty: large tangential loads mean high friction – and this is precisely what is not desired. In order to reduce the tangential load while maintaining the specific surface pressure, the running surface of the piston rings can be reduced – by reducing the ring height. However, this makes them more prone to wear. The solution: a coating that is particularly wear-resistant.

PVD coating: MAHLE at the forefront

Chromium, molybdenum or gas nitriding have been the standard materials and methods for increasing wear resistance of piston rings up to now. The MAHLE engineers have now developed an innovative method for coating piston rings effectively: the PVD method (Physical Vapour Deposition). With this method, derived from motor racing, diamond like carbon coatings are vapour-deposited in a high vacuum. This creates a highly resistant coating with significantly lower frictional losses. However, PVD coatings are not only harder but also more resilient. Piston rings that have been treated this way provide higher wear resistance and improved seizure resistance even under severe operating conditions and reduced oil lubrication. In short: wear resistance and service life of the piston rings are significantly increased.

Moreover, rings with lower height, which are feasible with this coating, have another positive characteristic: they are more flexible. Therefore, the rings align even better to the cylinder surface shape. This in turn makes it feasible to reduce the specific surface pressure – with the possibility of further reductions in tangential load at the same performance.

Piston ring set with PVD-coated compression ring, Napier ring and 3-piece oil control ring.

Frictional loss reduction: lower fuel consumption, lower emissions, better future

The cylinder liner was fitted to a test engine, freely movable in longitudinal direction. With the engine running, force sensors provided information about the frictional forces of the piston rings.

The results show a reduction in frictional losses at the ring package of 20 to 30%, depending on the actual load and engine speed. At low speed and low load – which are the typical conditions in urban driving – the friction reduction was highest.

This friction reduction at the piston rings corresponds to a reduction of total frictional losses in the engine of more than 1% – and therefore also to a reduction in fuel consumption by 1%. An important step in saving resources, complying with strict future exhaust emission standards – and therefore making a contribution to a safe future of the internal combustion engine.

Frictional losses in comparison: The friction force and the Friction Mean Effective Power (FMEP) at a ring set with the dimensions 1.2/1.2/2.0 mm (blue) and one with 1.0/1.0/1.5 mm (red).

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.