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.
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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. |
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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). |