We, recently, came across the video above (yes, we know there is no audio) and wanted to know a little more about it. With no audio, you can see a cutaway of a cylinder with a piston moving up and down. As the piston is moving along the cylinder, the cylinder is flexing. So, what’s the point?
We reached out to our good friend Bill McKnight at MAHLE Aftermarket Inc. to find out what the significance was. McKnight explained the video came from their European offices so the audio was removed. The video is a visual representation to what is happening inside of your engine as you drive. More specifically, it illustrates something called cylinder liner cavitation.
Before we get into McKnight’s explanation, we need to set the stage a little. It can be assumed that all coolant contains tiny air bubbles. It is these air bubbles that can lead to issues.
“Cavitation erosion is the result of a load reversal as the sleeve moves or vibrates as seen in the video. As the sleeve moves minutely outward, the air bubble is compressed as the fluid pressure of the coolant is higher than the air pressure of the bubbles. When the load reverses, the fluid pressure is low as it tries to fill in the void left by the moving sleeve. Low fluid pressure allows the bubbles to expand, the next cycle causes the bubble to collapse or implode. The implosion causes a shock wave to hit the liner with enough force to cause a minute fracture. Repeat this thousands of times and you have cavitation erosion,” explains McKnight.
What does that really mean?
Cylinder liners aren’t paper thin. One or two little tinny erosion spots won’t really affect anything. Over time, on the other hand, one or two erosion spots can turn into hundreds and they begin to penetrate deeper into the liner. Eventually, they can break the surface causing a water in oil problem, pressurized coolant and even catastrophic engine failure.
What can you do to prevent this?
Initially upon assembling the engine, the clearance between the piston and the bore should be checked. The larger the gap, the more impact and deflection of the liner as the piston makes its journey.
Once the engine is assembled, there are other precautions that should be taken. The cooling system should be regularly checked and changed (per the engine mfg’s recommendations). Using a coolant with an anti-corrosion additive will help.
Studies have actually shown Evans Waterless Coolant to outperform any water based coolant in tests conducted by Southwest Research Institute during the ASTM (American Society for Testing and Materials) 250 hour engine dynamometer test. The testing shows 70 percent less pitting with Evans Waterless Coolant.
If you have an engine or situation where cylinder liner cavitation is common, then selecting a liner when rebuilding an engine can be more of a task then just picking up another factory replacement. There are companies like MAHLE Aftermarket that manufacture replacement liners that minimize the liners susceptibility to cavitation erosion.
Not only can the liner itself be designed to minimize the flexing, but the way the liner was manufactured and treated can reduce erosion. Aftermarket companies that recognize issues like this, will or can even opt to use a different type of material for the liner.
Another key component to think about when selecting the parts are the piston’s themselves. According to MAHLE, the shape of the piston can reduce cavitation.
If you would like to find out even more info on cylinder liner cavitation, McKnight passed along a link to a SAE (Society of Automotive Engineers) paper that was written on Cavitation on Wet Cylinder Liners of Heavy Duty Diesel Engines which can be found here.