Ring land fractures
DESCRIPTION OF THE DAMAGE
- 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.
DAMAGE ASSESSMENT
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.
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.
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).
POSSIBLE CAUSES FOR THE DAMAGE
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, insufficientpiston 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.
Hydraulic locks:
- 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.
Side info