Oil change

An oil change is the replacement of used lubricating oil with fresh oil in machine transmissions, engines or machines with circulating lubrication.

Change

The time or interval of the oil change and the type of oil to be used are specified by the machine or vehicle manufacturer. This time depends on the ageing of the oil as well as the ambient conditions and the intensity of use of the machine. For vehicles, these intervals are often determined on the basis of mileage or a combination of mileage and time, while for other machines they are determined on the basis of operating hours. Vehicles with Condition-Based Service Display determine the time according to parameters that are recorded and evaluated by the sensors of the vehicle electronics and allow conclusions to be drawn about the actual stress on the oil.

Variable oil change intervals can be checked for maturity using a suitable oil quality probe, which usually also measures the oil level and temperature. In the case of large industrial units, the oil quality is also regularly checked by examining oil samples to determine the optimum time for a partial or complete oil change.

Stress on the lubricating oil in the combustion engine

Tasks of the lubricating oil

In engines with forced circulation lubrication, engine oil is drawn in by an oil pump from a reservoir (oil pan, oil tank) and pumped through an oil filter to the various points in the engine that require lubrication. These are mainly the plain bearings of the crankshaft and camshaft, the connecting rod bearings, the bearings of turbochargers, the valve train and the liners of the engine cylinders. Pressure oil is also used as an operating medium for hydraulic actuators, e.g. for camshaft adjustments. In block engines, it is also used to lubricate the gearbox and clutch. Furthermore, the engine oil - especially in air-cooled engines - serves as a cooling medium that transports heat away from thermally highly stressed regions in the engine, such as piston crowns, cylinder heads or bearing chambers of turbochargers. In order to achieve proper lubrication under all operating conditions, the engine oil must have a certain viscosity - in addition to other defined properties.

Wear and consumption

Engine oil is subject to a constant process of wear and ageing. In addition to absorbing abrasion from the various bearing points, which can only be partially removed by the oil filter, the oil is chemically stressed by blowby gases, which transport unburned fuel or combustion products from the combustion chamber past the piston rings into the crankcase, where the engine oil is located.

It is also necessary for a microscopically thin film of oil to remain on the honed liner wall during each piston stroke. liner wall and is not wiped off in order to lubricate the piston in the cylinder and achieve a fine seal. This oil enters the combustion chamber in this way and is burned there. Other points where oil consumption occurs are the valve stem seal, which act as sliding seals to seal the intake tract at the upper end of the valve stem against the oil chamber in the cylinder head. Oil is also lost through condensation and drip losses, e.g. at shaft seals or housing seals. As a result, the oil level in the engine gradually drops. For this reason, it is advisable to check the oil level regularly. Excessively high consumption can be caused by damage to the cylinder head gasket, valve seals or a piston ring, but rarely also by cracks in the cylinder head or engine block.

Oil dilution

The lubricity of the oil and its chemical composition can also be adversely affected by the introduction of unburned fuel into the engine oil charge. This can be caused by regeneration cycles of diesel particulate filters, among other things, but also during continuous short-distance operation with a cold engine, more condensates of unburned fuel are deposited in the engine oil.

Oil level measurement

The oil dipstick (also known as a dipstick or oil level stick) or a sight glass indicates how much oil is still in the reservoir (e.g. oil pan). The oil level is specified by the manufacturer and should be between a defined minimum and maximum. For measurement, the vehicle should be on a level surface and in a condition specified in the operating instructions - for example, the oil level should often be measured according to the instructions with the engine switched off and at operating temperature. The dipstick should then be pulled out, cleaned and pushed in again completely, as otherwise drops of oil falling from the cylinder head onto the dipstick inside the engine could simulate a higher oil level. The dipstick can now be pulled out again and the oil level read off.

If the oil level is above maximum, the crankshaft constantly hits the oil, causing it to foam up; the oil foam lubricates poorly and can cause damage. In addition, the foam slows down the oil's spray range so that the piston running surface is not sufficiently lubricated. Overfilling has another disadvantage: If the oil level is too high, important ventilation holes in the crankcase can be covered by the oil level, resulting in excess oil entering the intake tract via the crankcase ventilation and being burnt in the engine. This can result in considerably higher combustion temperatures, afterglow on the surface of the cylinder head and, in the case of petrol engines, knocking due to premature ignition in the next cycle and thus damage to many components of the engine, including major engine damage and the catalytic converter.

If the oil level is too low, oil must be topped up to prevent the oil pump oil pump]] drawing in air, which would cause a drop in oil pressure. This is signaled by the oil pressure warning light. If this lights up, the engine is already damaged. For this reason, some vehicles are fitted with oil level sensors that indicate low oil levels even before the oil pressure drops. The oil pressure light is switched from the ignition-switched positive pole of the oil pressure switch to the engine housing as the negative pole. The oil pressure switch is located in the oil duct behind the pump and after the oil filter. The oil pressure opens the contact. Oil pressure switches can be available with different switching pressures with the same dimensions. Multi-pole oil pressure switches have additional switches that are potential-neutral to the housing or sensor contacts for oil pressure and oil temperature measurement or are integrated into a bus system.

Changing the oil in a car

In motor vehicles, an oil change usually means replacing the engine oil. Lubricating oil is also contained in the main gearbox and sometimes in the transfer cases, and in automatic transmissions, power steerings or steering gears, depending on the design, also hydraulic oil; these generally have a much longer service life as they do not affect the combustion process. They do not have to withstand high temperatures and do not have to absorb fuel residues or soot particles.

Even in vehicles that only cover 10,000 kilometers or less per year, the oil should be changed every 12 to 18 months according to manufacturer and workshop specifications, because even with low mileage, ageing leads to a loss of lubricating properties, which is associated with increased wear and the risk of piston seizure. Mineral oils are more subject to these ageing processes than synthetic oils. According to other information, an oil change is only necessary after many years, if at all.

First, the old oil must be drained and collected. The oil drain plug, which is located near the lowest point of the engine, is used for this purpose. It is partially on the side and therefore protected. If this is opened, the oil runs out of the engine and must be collected in a sealed container.

Engine oil must never be allowed to enter the environment (environmental pollution is a criminal offense). [Used oil]] can be disposed of in Germany when purchasing new oil.

In most cases, the oil filter should also be replaced when the oil is changed. This filter catches dirt and metal particles, but can only do this until the capacity of the filter is exhausted. There are filters with the same dimensions and threads, which have different flow resistances.

There are quantity specifications with and without an oil filter for refilling with new oil. The oil level is checked using the engine's own oil dipstick.

Engine-specific oil specifications are always the minimum requirement. This is noted in manuals and operating instructions. If the engine is operated under different circumstances, such as a different climate (which has different viscosity requirements) or with LPG instead of gasoline (gas burns hotter than gasoline), the oil requirements may change. When running on LPG, there is no contamination from fuel components and less soot. When mixing (topping up) oils, the same viscosity should be used. The designation of engine oil types (e.g. 5W-40) should always be interpreted in the same way: the lower the number in front of the letter W, the more fluid the oil is at low temperatures. The second number describes the viscosity at a temperature of 100 °C. In addition to conventional mineral oils, synthetic or low-viscosity oils can also be used. Although all three groups mentioned contain additives, the latter two reduce friction in the engine more intensively, which in turn can have a positive effect on fuel consumption.

For classic cars and small engines such as in lawn mowers that do not have an oil filter, unalloyed engine oils must be used. Unalloyed oils form oil sludge at the bottom of the oil pan, which binds unwanted foreign bodies and engine abrasion to a certain extent. When changing the oil, the oil pan must be dismantled and cleaned of oil sludge.