Mahle engineers are regularly called upon for assistance by workshop and engine repairers in diagnosing faults. They encounter small holes that are sometimes found in cylinder liners.
These holes occur only in "wet" sleeves, i.e. sleeves directly flowing through the coolant (type WN) and always in the area of the water jacket, and there, in turn, in the area of the upper and lower dead center of the piston. They are located only on the pressure or back pressure side and increase towards the inside. Are these casting defects? Were there any air bubbles in the cast iron during casting? None of these things. Because MAHLE manufactures cylinder liners by centrifugal casting. The high centrifugal forces during the casting process reliably prevent the formation of bubbles and voids in the molten metal. So these are certainly not factory defects, but a completely different phenomenon - pitting corrosion or cavitation.
The accumulation of numerous small holes in the water jacket area is a clear indication that cavitation was occurring.
At top and bottom dead center, the piston changes the contact surface. In the case of "wet" sleeves, the displacement impulse can cause the surrounding water jacket to vibrate. In the vibration phase, a short-term vacuum is created in the water. This creates vapor bubbles near the outer surface of the cylinder, which implode when the water column returns in the course of vibration. The water is accelerated towards the cylinder liner and hits its surface. Its kinetic energy knocks small particles off the sleeve surface. This can erode the sidewall of the cylinder so much that water gets inside. The fatal outcome: when the cylinder surface is damaged, this creates an additional opportunity for cavitation with subsequent corrosion.
In cross-section, typical cavitation damage is more clearly seen: pits growing inwards
Therefore, MAHLE pays attention to the highest running smoothness when designing the pistons, because the improvement of this smoothness reduces the occurrence of vibrations. To achieve this goal, painstaking research in and out of engines is carried out and analyzed, leading to the appropriate optimization of the piston. It is also important that the cylinder liner is as less susceptible to vibration as possible. This is ensured by its precise fastening at the top of the sleeve flange and at the bottom in the motor body. This guarantees a very precise manufacturing process with tight tolerances.
Engines operating with low coolant temperatures between 50 and 70 °C are particularly susceptible to cavitation. On the other hand, in the range from 90 to 100 °C, overpressure is created in the closed cooling system. The higher this pressure, the less vapor bubbles are formed. Therefore, the cover of the coolant radiator or expansion tank and the bleed screws must be completely tight. If, for example, due to a damaged seal, there is no overpressure in the coolant circuit, the boiling point of the coolant is lowered. In order to quickly reach the required operating temperature and ensure proper temperature control according to the engine load, it is essential to ensure optimal temperature management with accurate thermostats. So when a truck driver removes the thermostat and runs the engine without it, he achieves exactly the opposite effect!
In addition to the primary task of antifreeze, which is to protect the cooling system from freezing, it has the additional task of increasing the boiling point and reducing the risk of vapor bubbles. At the same time, it lubricates the coolant pump seals and prevents corrosion in the engine. Therefore, always add antifreeze to the cooling water – also in summer, in warm countries, and also in the case of generators operating indoors.
MAHLE cylinder liners are manufactured from high-quality materials with tight manufacturing tolerances. In particular, the quality of the structure, external and internal surfaces, and the perfect fit of the O-rings are qualitative features that increase resistance to cavitation.
Good to know: MAHLE designs and manufactures cylinder liners for virtually all engine manufacturers worldwide and develops the alloy composition, molten metal preparation and heat treatment together with its OEM customers, as well as the strict tolerance requirements for machining. Strict quality requirements apply to the production of all MAHLE products, regardless of whether they are destined directly for the belts of engine manufacturers or will be assembled by engine repair shops on the aftermarket.
