What stops “caries” on cylinder liners?

One thing is clear: cavities on cylinder liners are not a good sign! But how does this damage occur? Where do the small cavities on the outer side of the cylinder liners come from? Were they there from the outset or did they only develop over time? We explain: where these cavities originate from, what encourages their formation, and what needs to be observed in order to possibly avoid this pitting.

Bad vibrations—how vibrations cause cavities

Figure 1: “Pitting” in the area of the top dead center
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First off: the small cavities are not there at the outset. They are caused by the gradual incipient loss of material, or so-called cavitation—which is also simply known as pitting. But how does this happen? The damage solely occurs on “wet” liners that are directly bathed in coolant. And not without reason.

As the piston moves up and down in the cylinder, it changes the contact side at the top and bottom dead center. It is precisely this change that can lead to vibrations, especially in the case of “wet” liners. Because each time the piston changes the contact side, an impulse is generated, which is directly transmitted to the liner and into the cooling water.

Figure 2: Liner with cavitation damage
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During a vibration cycle, a vacuum is temporarily created between the liner and the cooling water. Small vapor bubbles form on the outer side of the cylinder, which implode when the water column swings back. In the process, minute particles are torn from the outer side of the liner. Once the surface has been damaged, the cooling water can penetrate the combustion chamber at this point. If too much cooling water enters the cylinder, a so-called “hydrolock” can occur. In this instance, so much cooling water enters the cylinder, the resulting compression causes the piston and connecting rods to become so severely damaged that the engine is destroyed.

 

Strength lies in calmness

Once you become aware which role vibrations play in cavitation, one thing quickly becomes clear: the quieter the piston runs, the lower the vibration, and the longer the service life. This is precisely one of the reasons why we at MAHLE attach such great importance to absolute running smoothness! To achieve this goal, we have carried out numerous engine tests. The piston is being optimized accordingly. The cylinder liner itself should also operate with minimal vibration. To ensure this, it is secured with maximum precision directly under the liner flange at the top and in the engine block at the bottom. We demonstrate how this works. Fitting recommendations for cylinder liners

Just staying cool doesn’t help either

Figure 3: Cross section depicting the various stages: cavitation in the initial stage (left) and strongly advanced (right). Typical: a cave-like structure, which can extend into the cylinder.
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On the contrary: engines that run with low coolant temperatures ranging between 50°C and 70°C are particularly vulnerable to pitting. The required overpressure in the closed cooling system only starts building up at approx. 80°C—and the higher the pressure, the fewer the vapor bubbles. Therefore, the cap of the radiator or the filler cap of the expansion tank must also be completely sealed when closed. CAUTION: In this regard, antifreeze also fulfills an extremely important task! It raises the boiling point of the cooling water and thus reduces the risk of vapor bubble formation. At the same time, it prevents corrosion in the engine and serves as a lubricant for the water pump, for example. Therefore: never forget to put antifreeze in the cooling water—neither in summer nor in hot countries! What’s more, the leak tightness of the cooling system needs to be checked, because “caries” will develop if there is no pressure.

Premium quality combats “caries” on cylinder liners

MAHLE liners are made from high-quality materials using very narrow manufacturing tolerances. The quality of the microstructure, the outer and inner surfaces, and the perfectly matched seals in particular are our quality characteristics and contribute to the reliable protection against cavitation. In conclusion, it is worth mentioning that cavitation is often confused with casting defects or material inclusions, however, this is not at all possible with our liners! MAHLE uses the centrifugal casting method to produce cylinder liners. During the casting process, the high centrifugal forces completely and reliably prevent air pockets or shrinkholes from forming in the melt.

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