The total amount of oil used by an engine is primarily made up of oil consumption (oil burned in the combustion chamber) and oil loss (leaks). The amount of oil that passes the piston rings and cylinder wall into the combustion chamber and is used there is negligible today. As a result of the continuous further development of engine components, material compositions and production processes, the wear on cylinders, pistons and piston rings has been reduced and in turn, oil consumption has also decreased. This is underlined by the high mileages and the reduction of incidents of damage to the crankshaft drive. Although the oil consumption in the combustion chamber cannot be eliminated entirely, it can be minimised: the interacting sliding parts (piston, piston rings and cylinder running surface) require continuous lubrication to ensure smooth operation. During combustion the oil film on the cylinder wall is subjected to the hot combustion. The quantity of engine oil that evaporates or burns here depends on the engine output, engine load, engine oil grade and temperature. In the majority of cases, wear on pistons, piston rings and cylinders and the resulting increased oil consumption is not caused by the components themselves. Instead, wear on these components can nearly always be explained as the result of an external event: abnormal combustion due to incorrect mixture preparation, dirt entering the engine from outside, inadequate engine cooling, lack of oil, use of incorrect oil grades or faults made during installation. The following pages contain detailed descriptions of different types of damage that affect pistons and cylinders.
A separate brochure entitled “Oil consumption and oil loss” has been published on this topic.
- No signs of wear on piston rings or piston (Fig. 1).
- End of the expander spring of the 3-piece oil control ring broken off.
- Scratches in the groove base of the oil control ring.
Due to the overlapping of the expander spring during installation, its circumferential length is shortened. This results in a fracture in the expander spring and/or loss of tension for the 3-piece oil control rings. The blades are no longer pressed tightly against the cylinder wall and no longer scrape off the oil. Oil enters the combustion chamber, where it is burned. This results in excessive oil consumption.
- Incorrect oil control rings.
- Installation faults.
Both coloured parts of the expander spring must be visible after installation of the 3-piece oil control rings. These colour-coded marks should therefore always be checked (even on pre-installed piston rings) before installation of the pistons (Fig. 2a).
- Piston: matt, ground wear pattern on the skirt with fine, small longitudinal scratches on the piston top land and the piston skirt.
- Tool marks created during machining worn away.
- Worn compression ring edges, on the first piston ring in particular as well as ring groove edges (Fig. 2b).
- Axial clearance of the compression rings substantially increased, in particular on the first piston ring.
Abrasive foreign bodies in the oil circuit cause scratches on the piston and piston rings, a matt wear pattern on the piston skirt and roll marks on the ring flanks (Fig. 4 and 5). As the piston rings are worn on the running surfaces and edges, they can no longer seal the cylinder against oil passing into the combustion chamber. At the same time, the pressure in the crankcase increases as a result of combustion gases streaming past the cylinder. This may cause oil to escape at radial oil seals, valve stem seals and other sealing points. Roll marks on the piston rings are caused by dirt particles that become lodged in the ring groove. As the piston ring rotates in the groove it keeps running over the dirt particles, which creates the characteristic roll marks.
- Abrasive dirt particles that enter the engine with the intake air due to inadequate filtration, including:
- Missing, defective, deformed or poorly maintained air filters.
- Leaking points in the intake system, such as distorted flanges, missing gaskets or defective or porous hoses.
- Residual particles of dirt from engine reconditioning. Parts of the engine are often blasted with sand or glass beads during reconditioning work in order to remove persistent deposits or combustion residue from the surfaces. If the blasting material becomes deposited in the material and is not cleaned out properly, it may work its way loose when the engine is running, thus causing abrasive wear. Fig. 6 and 7 show microscopic images of damage caused by dirt, under polarised light. Fragments of the glass blasting material and entire beads of the glass can clearly be seen.
- If the first oil change is performed too late, the abraded particles generated when the engine is run in are spread through the oil circuit to the other interacting sliding parts where they cause more damage. The sharp oil-scraping edges of the piston rings are particularly prone to damage.