MAHLE cylinder liners—we are strict with our tolerances

The steadily rising demands on internal combustion engines require continuous improvements in the area of cylinder working surfaces. The precise matching of the honed cylinder liners with the pistons and piston rings leads to improved results in the engine.
For cylinder liners made from cast iron, it is therefore necessary to create optimum surfaces that contribute to lower oil consumption and blow-by, produce fewer wear particles and allow shorter running-in times and consequently a longer service life.

MAHLE manufactures cylinder liners for international engine manufactures for series production and the aftermarket—always according to the same strict quality standards. We manufacture our cylinder liners with very strict tolerances. This ensures, among other things, that the cylinder liners can be optimally fitted into the engine block. Additional important quality characteristics of our cylinder liners are the materials, the structure and the surface quality. In close cooperation with engine manufacturers, we determine the composition of the melt, the treatment of the melt and the machining process.

Flake graphite

In addition to aluminum, cast iron alloys are used as materials for cylinder liners.

Flake graphite cast iron is alloyed with phosphorus. Additional alloy materials improve the wear properties and can strengthen the matrix due to bainite and very fine perlite formations.

Good honing has a positive effect on piston ring wear, particle emission, oil consumption and friction.

With further developments and innovations in the honing of cylinder liners, the goal is to keep the running-in phase of the cylinder (engine) as short as possible and achieve improved tribological properties. An important prerequisite for consistent quality honing of the cylinder liners is high material quality. This requires cast material free of voids, with homogeneous structure and uniform hardness, as well as suitable pre-machining of the bore.

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.

The nightmare of every engine repairer: a new cylinder liner is fitted to an engine block. The cylinder head is put in place, the cylinder head bolds are fastened—and suddenly, a faint “ping“ can be heard. The reason for this peculiar cracking sound becomes obvious when the block is dismantled afterwards: the flange of the cylinder liner has snapped off along its entire circumference—and is lying loosely on top. When it is removed, it can be seen that the break happened below the cylinder block face at an angle of about 30°. The fracture surface shows a course break structure. However: material or casting defects cannot be detected even with a magnifying glass or under a microscope. This pattern of damage is typical for a forced fracture.

Cylinder liner flange and flange seat—like lid and pan
The cylinder block has circular, plane recesses, the so-called flange seats. These fix the cylinder liners in axial direction in the cylinder block. The flange of a cylinder liner must fit exactly into such a recess—so that the liner rests on its entire circumference on the flange seat. The cylinder head gasket is then placed on the cylinder block. The sealing of the combustion chamber (for older models a metal border, for newer metal gaskets a profiled surface) has to fit correctly onto the top of the liner flange.

When the bolts are tightened, the cylinder head is pressed firmly against the cylinder block. The cylinder head bolts and the tightening instructions are designed to achieve a firm connection between the cylinder head and the cylinder block—even for ignition pressures of more than 200 bar at times. This means that enormous forces are introduced from the bolts via the gaskets to the liner flange. This makes it even more important that the forces via the head gasket are transmitted vertically to the liner flange. (Sketch 1 shows correct fitting)

Sketch 1: Only correct fitting ensures the right transfer of forces.

Sketch 2: Dirt particles under the liner flange—a cause for dangerous flexing moments.

Danger of damage: Inclined force lines
Cylinder liners are made from grey cast iron. This material has many positive properties—however, flexing is not tolerated by this brittle material. When the force lines are only slightly angled through the cylinder liner flange, flexing results in the upper part of the liner that can lead to fracture of the flange.

Common reasons for bending moments
Dirt particles: to avoid contaminations between the flange and seat in the block (i. e. dirt, chips, gasket residue, sealing material, etc.) cleanliness during fitting is important. Also sealing paste should only be used when it is stipulated by the engine manufacturer—according to the motto „the less the better“.

Our tip: when pre-machined cylinder liners are pressed in, it is a good idea to stop a few millimetres before the block and use compressed air to remove material that may have been scraped off from the gap between flange and flange seat.

A slanted flange seat causes liner flange fracture. (Sketch 3 + 4).

Uneven surfaces and warpage at the flange seat of the cylinder block:
the underlying reasons for this are increasingly lighter engine designs—and the warpage at the component will be worse, the thinner the wall thickness of the block. At the same time, power output, combustion pressures and torques are rising all the time in modern engines. In engines that have been running for 500,000 kilometres (or the equivalent in running time) warpage can be that excessive, that the seat in the block should be reworked. The plane-parallel redressing should be carried out on a boring mill or with a hand operated, mobile flange facing unit. Important: the surface must not be slanted (see sketch 3 + 4)—and after machining, the sharp edge of the seat surface should be chamfered (about 1x45°, see sketch 5). If there is no chamfer, fracture damage can result (sketch 6).

Incidentally: Also the specified liner projection has to be assured before fitting. To adjust the size, a suitable metal ring can be placed under the flange. For many cylinder liners, MAHLE offers also suitable liners with oversized flanges. When the cylinder block is resurfaced, the flange seats have to be redressed by the same amount.

Sketch 5: Chamfer at the sharp-edged flange seat

Sketch 6: Fracture damages due to missing chamfer.

Wrong cylinder head gaskets:
also this can introduce angled force action into the flange (sketch 7), either because the diameter of the combustion chamber border is too small or the gasket has the wrong thickness. Our tip: only use the type of gasket that was originally intended for the engine. Cheap reproduction gaskets can differ in dimensions and material from the original gasket—and this can result in expensive consequential damage.

Sketch 7: Force transfer at an angle due to wrong cylinder head gasket.

Sketch 8: Cylinder head machining: if the grove is not allowed for, the forces are transferred via the fire prevention edge—and the liner fractures.

Incorrect machining:
some cylinder heads, like those for certain Volvo types, have a full perimeter groove into which the fire prevention edge of the cylinder liner projects—and cylinder head and cylinder liner must not come into contact. If the cylinder head is resurfaced due to damage or warpage, the perimeter groove must also be redressed by the same amount. Otherwise, there is a risk that forces are not introduced via the gasket but at an angle via the fire prevention edge of the cylinder liner (sketch 8).

Ignorance becomes expensive
Good hearing is needed when tightening the cylinder head bolts. If the dreaded cracking sound is heard straightaway it is still a blessing in disguise. The cylinder can be dismantled immediately and the cause can be analysed and redressed—and then a new liner can be fitted.

Otherwise, it will get expensive: the broken liner moves gradually in direction crank shaft after the engine is started up again—and as soon as the location of the break is level with the first piston ring in TDC, the piston ring springs open above the break location. At the next downward movement of the piston it pulls the cylinder liner downwards. The rotating crankshaft shatters the liner and piston and connecting rod are also wrecked—and the engine repairer is greeted by a connecting rod that protrudes sideways from the engine block.

A cylinder liner, fractured at the flange due to fitting error.

The coarse break structure—a sure sign of force action.

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.