MAHLE cylinder liners: precisely round
MAHLE develops and tests its cylinder liners in close cooperation with all international engine manufacturers. Every customer has a unique set of expectations regarding the type or hardness of the alloy. But all of them expect that MAHLE will provide precise manufacturing while complying with minimal tolerances—and their expectations are met, thanks to well-trained employees, CNC machines, and constant in-line quality inspection. And the same goes for the repair shops in the aftermarket.
The demands on our cylinder liners are pretty tough
Cylinder liners are exposed to extreme temperatures, severe changes in pressure, and continuous friction. The stresses grow with each new engine generation. Nevertheless, the cylinder liners hold up under these conditions because MAHLE produces them from carefully cast and cooled iron or aluminium alloys. To make sure they are perfectly round, they are then honed with extreme precision. This makes the surface so fine that the piston and piston rings can slide easily over it. It is also rough enough to take up engine oil and form a perfect lubricating film.
Materials: for the safety of people, engines, and the environment—only the best
MAHLE uses alloyed cast iron and aluminium as the base materials for cylinder liners. The cast iron is alloyed with phosphorous and refined with lamellar graphite. Additional alloying elements may include chromium, nickel, molybdenum, or manganese. They strengthen the surface and thus reduce wear, for example by forming bainite and ultrafine pearlite.
Different types of cylinder liners require very special materials. For example some liners are cast directly into the aluminium engine block during the manufacturing process, while some are pressed into the engine block bores. Others are inserted into the crankcase with gaskets so that they can be bathed in coolant. Finally MAHLE also manufactures finned cylinders for air-cooled engines.
Depending on the application, the optimal alloy is used. They are melted in large centrifugal casting vessels and then cooled in a precisely defined process. The raw castings then head towards the first lathe before they are annealed for many hours at 600°C again. This relieves stresses in the material and ensures that the microstructure is homogenous. Cheap suppliers skip the annealing step, but MAHLE will not take the risk of distortion. By the way: large cylinder liners, such as those for the gigantic engines in container ships, need to cool for several days after centrifugal casting.