Again and again, engine repairers are confronted with cylinder liners that are badly eroded at the surface. The diagnosis: cavitation damage – also called pitting. What causes such damage? And what can your customers do to avoid it?

The cylinder liners have one thing in common: they are all so-called “wet” liners (type WN), which have coolant flowing around them during operation. In this design solution, the generated combustion heat is effectively carried away and dissipated via the heat exchanger.

WHAT DOES CAVITATION DAMAGE LOOK LIKE?
For this type of damage, it can be noticed that the pits are mainly found in the upper and lower dead-centre position of the piston. When these typical pits or erosions are present, we speak of cavitation damage.

An accumulation of small pits in the area of the water jacket – indication of cavitation damage.

WHAT EXACTLY IS CAVITATION? AND HOW IS IT CAUSED AT THE CYLINDER LINER?
Cavitation (lat. cavitare „hollow out “) describes the formation of hollow spaces in (strong flowing) liquids – which mostly dissipate immediately afterwards. This phenomenon is caused by pressure fluctuations that in combustion engines are due to the piston movements. These vibrations are transmitted to the surrounding water jacket, which is then made to vibrate as well. When the cylinder wall moves back during a vibration cycle, a vacuum forms in the coolant and leads to vapour bubbles at that location. When the coolant column vibrates back, these vapour bubbles implode and “blast” individual atoms out of the cylinder liner surface: the result is pitting corrosion.

CAVITATION DAMAGE OR NORMAL CORROSION – HOW TO DIFFERENTIATE
There are two distinctive characteristics of cavitation damage: 1. the pits are only found at the major or minor thrust side of the liner. 2. In contrast to normal corrosion, the pits are getting larger towards the inside. This hollowing out (erosion) has the effect that the wall of the cylinder can be perforated completely – until coolant enters the cylinder. Furthermore, when the surface of the cylinder has initially been damaged due to cavitation, further opportunities for more cavitation damage and also corrosion are opened.

Caries at the cylinder liner: after the liner has been cut open it can clearly be seen that the cavities are becoming larger towards the inside.

PITTING – WHAT ARE THE REASONS?
Insufficient frost protection in the coolant: A common reason for cavitation damage is the composition of the coolant. In many countries of the world, engines are run without antifreeze agent in the cooling water – or with an insufficient proportion.

However, the antifreeze agent does not only protect from frost, but also prevents corrosion in the radiator and engine and lubricates the coolant pump. A suitable antifreeze agent influences the physical and chemical characteristics of the coolant – it lowers the freezing point of the coolant and increases its boiling point. This reduces the tendency to bubble formation and therefore the risk of cavitation damage.

Leakage in the cooling system / insufficient overpressure function: Under normal operating conditions, an overpressure is formed in the cooling system, which reduces the tendency to vapour bubble formation. However, even just a leaking radiator cap prevents overpressure from developing – and can be the reason for cavitation damage at the cylinder liners. Also defective thermostats or viscous couplings of radiator fans can reduce the temperature level of the engine to such an extent that overpressure cannot develop in the cooling system.

Engine operation in the lower temperature range: cavitation damage has especially been observed in engines that operate in the lower temperature range (50 –70 °C). At higher temperatures (90 –100 °C), the increased water pressure prevents vaporous cavitation.

Poor quality products: cylinder liners of inferior quality that cannot be fixed correctly to the cylinder block due to excessive manufacturing tolerances will move in the engine. The increased vibrations result here often in cavitation damage. Also low-grade materials can be the reason for cavitation damage.

ENGINE REPAIR AFTER CAVITATION DAMAGE – TIPS FROM THE PRACTICE
The seat diameter of the lower liner area must not be refaced when the surface is corroded – unless liners with larger fitting diameter are used.

It is crucial that the correct piston mounting clearance is observed – and honing of the liner resulting in increased diameter must be avoided as well as re-fitting of previously used pistons. Instead, either the next oversize should be reached by re-boring (and the relevant oversize piston should be used) or a new assembly should be fitted.

It is essential to use the permanent antifreeze agent with corrosion protection that is recommended by the engine manufacturer – even when engines are only used in warm regions without frost or within buildings (e.g. generator drive). Specifications regarding replacement intervals and alternative additives for certain regions must also be observed.

Also the water quality is important: the use of distilled, strongly alkaline or acidic water is not suitable.

We recommend to check cooling systems, thermostats and radiator fans at regular intervals. The overpressure of the cooling system has to be assured (if necessary replace the radiator cap).

PREVENT CAVITATION DAMAGE – WITH ENGINE COMPONENTS FROM MAHLE
In close cooperation with the engine and automotive industry, MAHLE engineers develop engine components with minimum susceptibility to cavitation damage.

Precondition for long engine life without cavitation damage is a smooth running piston. MAHLE optimizes the piston shape already during the development stage – in many test series at the actual engine. The result is good running smoothness and minimized pulse generation during contact alterations in the cylinder.

MAHLE cylinders ensure optimum functioning, refinement, long service life and reliability of the engine: the most important protection from cylinder liner cavitation is minimising vibration transfer. MAHLE cylinder liners are therefore machined with high accuracy and minimum tolerances – in order to ensure vibration free mounting in the cylinder block and therefore reliable running during the entire lifetime of the engine.