- Stress cracks on the edge of the bowl.
- Major crack extends to the piston pin boss.
- Channel burned from the bowl to below the oil control ring by combustion gases streaming through the major crack.
The piston material is heated up significantly in localised areas – at the point where the prechamber jets make contact on prechamber engines (Fig. 3 and Fig. 4), and on the edge of the bowl on direct injection engines (Fig. 1). The material expands more than elsewhere in these areas. As the overheated areas are surrounded by colder materials, the material is plastically deformed here beyond its limit of elasticity. Exactly the opposite happens when it then cools down again: in the areas where before the material was previously buckled and forced away, there is now a shortage of material.
This results in tensile stresses, which ultimately cause stress cracks. If there are superimposed stresses caused by bending of the piston pin in addition to the thermal stresses, then the stress cracks turn into a much larger major crack, which causes complete breakage and failure of the piston.
- Faults in mixture preparation caused by incorrect injection nozzles, malfunctions in the fuel injection pump or damage to the prechamber.
- High temperatures as a result of defects in the cooling system.
- Faults on the engine brake, or excessive use of the engine brake. This results in overheating.
- Insufficient piston cooling on pistons with a oil cooling gallery, caused for example by blocked or bent cooling oil nozzles.
- Temperature fluctuations in engines with frequently changing loads, such as city buses or earthmoving machines.
- Pistons with an incorrect specification, e.g. no oil cooling gallery despite the fact that a piston with oil gallery must be used.
- Pistons made by third-party manufacturers without fibre reinforcement of the edge of the bowl.
- Pistons with an unsuitable bowl shape for the engine (refer to the chapter entitled “Piston head seizure due to the use of incorrect pistons”).
- Ring land fracture on one side of the piston between the first and second compression ring (Fig. 1).
- Fracture, starting at the groove base at the top and running at a diagonal angle into the piston material, emerging at the groove base underneath (Fig. 2).
- Fracture is extended downwards.
- No piston seizure marks or signs of overheating.
Land fractures are not caused by material faults, but by material overload. A distinction can be made between 3 different causes:
1. Knocking combustion:
The octane rating of the fuel was not capable of covering the engine’s needs under all operating and load conditions (refer to the chapter entitled “General information about piston damage due to abnormal combustion in petrol engines”). Ring land fractures caused by knocking combustion usually occur on the pressure side. On a diesel engine, knocking combustion is caused by an ignition delay.
2. Hydraulic locks:
Liquid (water, coolant, oil or fuel) accidentally enters the combustion chamber when the engine is stopped or running. As the liquid is incompressible, the piston and crankshaft drive are subjected to enormous stresses during the compression cycle. This results in ring land fractures, boss fractures or connecting rod/crankshaft damage. Fig. 3 shows the course of a fracture that occurs with knocking combustion and hydraulic locks: the force causing the fracture and acting from above on the ring land causes the fracture surfaces to extend downwards.
3. Installation faults:
If the piston rings are incorrectly compressed, more force is required when installing the piston. Forcibly pressing in or knocking in the piston causes pre-damage to the ring lands in the form of fine hairline cracks. The ring lands fracture in the reverse direction as the pressure comes from below in this case (Fig. 4).
Knocking combustion on petrol engines:
- Fuel without suitable anti-knock properties. The fuel quality must correspond to the compression ratio of the engine, i.e. the octane rating of the fuel must cover the octane requirements of the engine under all operating conditions.
- Petrol contaminated by diesel, which lowers the octane rating of the fuel.
- Excessively high compression ratio caused by excessive machining of the engine block surface and cylinder head mating surface, e.g. for engine reconditioning or tuning purposes.
- Ignition timing too advanced.
- Mixture too lean, resulting in higher combustion temperatures.
- Intake air temperatures too high, caused for example by inadequate ventilation of the engine compartment or incorrect switching of the intake air flap to summer operation (particularly on older carburettor engines).
Knocking combustion on diesel engines:
- Injection nozzles with poor atomisation or leaks.
- Injection pressure of the injection nozzles too low.
- Compression pressure too low due to incorrect cylinder head gaskets, insufficient piston protrusions, leaking valves or damaged/worn pistons.
- Defective cylinder head gaskets.
- Damage to the prechamber.
- Improper or excessive use of starting aids (e.g. starting spray) during cold starts.
- Defective turbocharger.
- Accidental intake of water while driving through water, or as a result of larger quantities of water being splashed up by passing vehicles or vehicles in front.
- While the engine is stopped, cylinder filling up with:
- water, due to leaks in the cylinder head gasket or cracks in components.
- fuel, due to leaking injection nozzles (only applies to petrol engines with a fuel injection system). The residual pressure in the fuel injection system is dissipated through the leaking nozzle into the cylinder.
In both cases, the damage will occur when the engine is started.
- Severe impact marks on the piston head (Fig. 1). Nearly all oil carbon deposits removed.
- Scarring and oil carbon deposits pressed into the piston crown.
- Severe wear on the piston rings, particularly on the oil control ring.
- Imprint of the swirl chamber on the front edge of the piston crown (Fig. 2).
- Imprint of the valve on the right-hand side of the crown.
- First indicators of initial dry running damage due to lack of lubrication on the piston skirt (Fig. 4).
The pistons have struck against the cylinder head/swirl chamber and one of the valves during operation. There have been no fractures yet as a result of these violent impacts. However, the nature of the wear on the piston rings and the piston skirt indicates that one consequence of these impacts has been abnormal combustion due to fuel flooding.
The striking of the piston results in vibrations on the cylinder head. This causes the injection nozzle to vibrate, and it is then unable to hold the pressure when closed and injects fuel into the cylinder in an uncontrolled manner. This causes fuel flooding, which damages the oil film. This damage leads to a higher level of mixed friction and therefore to wear on the piston rings and increased oil consumption. The characteristic damage caused by unburned fuel does not arise until the oil film is impaired by the fuel to such an extent that the piston is running with insufficient lubrication (refer to the chapter entitled “Dry running damage due to lack of lubrication caused by fuel flooding”).
In the initial stages the piston skirt is damaged to a lesser degree, as it is regularly supplied from the crankshaft drive with new oil that is still capable of providing lubrication. Once the abraded particles from the moving area of the pistons mix with the lubricating oil and the lubricating oil loses its load-bearing ability as a result of oil dilution, the wear will spread further.
- Incorrect piston protrusion dimension. The piston protrusion was not checked or corrected during engine reconditioning.
- Connecting rod bush bored eccentrically during replacement.
- Eccentric regrinding of the crankshaft.
- Eccentric reworking of the bearing counter bore (when resinking the crankshaft bearing caps).
- Installation of cylinder head gaskets with insufficient thickness.
- Oil carbon deposits on the piston head and resulting restriction or bridging of the gap dimension.
- Incorrect valve timing caused by incorrect adjustment, chain stretching or a slipped toothed belt.
- Differing connecting rod lengths.
- Excessive reworking of the cylinder head mating surface and the resulting shift in the valve timing. (The distance between the driving pinion/sprocket and the driven pinion/sprocket changes. Depending on the design of the chain or belt adjustment mechanism, it may not be possible to correct this.)
- New valve seat rings have been installed, but care was not taken to ensure that they are correctly positioned. If the valve seat surface is not positioned deeply enough in the cylinder head, the valves will not be recessed correctly in the cylinder head and will protrude too far as a result.
- Over-revving the engine. The valves do not close in time due to the increased inertia forces and strike against the piston.
- Excessive clearances in the connecting rod bearing or a worn-out connecting rod bearing, particularly in conjunction with over-revving when driving downhill.