Whether commercial vehicles or passenger cars – cylinder liners from MAHLE run in many engines Together with the pistons and the cylinder head, the cylinder liners form the combustion chambers of an internal combustion engine. If one considers the high temperatures in the combustion chambers, the high ignition pressures and the up and down moving pistons, it goes without saying that these extreme conditions require high quality cylinder materials. Good sliding properties are provided by grey cast iron and high-grade (but also expensive) alloy materials such as molybdenum, chromium, nickel and manganese increase hardness, wear resistance and corrosion resistance of the cylinder liners – an important prerequisite for lasting and reliable performance.

Fire and flame At the beginning of the manufacturing process are huge melting furnaces in which various steel parts are liquified. Active recycling is put into practice here: for example, pieces of old railroad tracks, shredded steel scrap and recirculated material (chips and remnants from the production process) are melted down.

Alloy materials and special cleaning agents to purify the melt are added. Continuous sampling of material and analysis ensure that the composition conforms exactly to the specifications. The cylinder materials are specific to the engines and are developed in close cooperation with the engine manufacturers. Some engine manufacturers may prefer a higher proportion of a particular alloy material and others specify the degree of hardness that is to be achieved.

In addition, there are many different types of cylinder liners that require special materials. This includes liners that are inserted into the crankcase with seals and have coolant flowing around them. Or there are cylinder liners that are pressed into the engine block bores. Or finned cylinders for air cooled engines. Or cylinder liners that are directly cast-in during the production of aluminium engine blocks. Depending on the application, the optimum alloy material is always used.

As a development partner of the engine and automotive industry, MAHLE supplies cylinder liners for the mass-production of most international engine manufacturers. Also the aftermarket benefits from this. Our partners in trade and workshop can be assured to receive products that have been manufactured in the same plant with exactly the same materials and in the same quality.

The right spin
“This looks like a washing machine“, one might think looking at a centrifugal casting machine. And that is somewhat justified. This is because a drum is rotating in its inside, into which the hot, liquid iron is cast. The metal is then moved outwards due to the centrifugal force. Gas bubbles and slag escape inwards. Due to the rotation, a homogeneous structure is formed. Eight of these “washing machines“ are installed on one carrousel. As soon as one drum is filled with the right amount of cast iron, the carousel turns one step and the next drum is filled. The metal is still glowing light red after a couple of steps, but it has sufficiently cooled for the drum to be stopped and the cylinder blank to be removed. Incidentally: large cylinder liners such as used in engines of giant container ships, must be left for several days to cool down after centrifugal casting.

A good turn
After cooling down, the cylinder blanks are semi-finished all around on lathes. Internal stress in the material is relieved and a homogeneous structure is achieved by annealing them in large electric furnaces at temperatures of around 600 °C for about 20 hours. It can now be guaranteed that no warpage or stress problems occur when the cylinders are machined to their final sizes by precision turning, grinding and honing. The use of furnaces is not always a foregone conclusion. Since the heat treatment requires a lot of electric energy, several suppliers of cheaper products shy away from the high costs involved. The outcome is as one might expect: cheap cylinders often warp inside the engines. This leads to increased oil consumption and dangerous stress cracks.

On the other hand, cylinder liners from MAHLE are distinguished by exact manufacturing processes with adherence to tightest tolerances – thanks to the use of advanced CNC machine tools and continuous quality control.

Honing - for smooth running
For most cylinders, honing is used to achieve a smooth bore surface. Good honing is important for reliable functioning of the internal combustion engine: the bore must have perfect macro geometry – it needs to be round and cylindrical. The surface must be coarse to allow oil to adhere to it and, at the same time, it has to be very fine to allow the pistons and piston rings to glide over it without damage.

These apparently contradictory demands are achieved in several steps of honing. A perfect interplay is required for faultless honing. We ensure that all MAHLE cylinder liners comply with the strict quality standards by using only the best honing machines, high-quality honing stones and carefully selected honing oils.

Customers from the automotive industry and engine repair appreciate the strict overall dimensional accuracy and the tight tolerances in form and position, combined with a first class surface finish – characteristics that are not always obvious to the unaided eye. And our customers know: quality made by MAHLE is a prerequisite for good running quality, low oil consumption and long engine service life.