Topics:
- Lubrication system
- Engine oil level
- Functions of lubricating oil
- Properties of lubricating oil
- Additives
- Contamination of the lubrication system (including sludge)
- Consequences of driving too long with contaminated engine oil
- SAE specifications (Viscosity / Viscosity index)
- ACEA specifications
- API specifications (American)
Lubrication system:
The lubrication system ensures that the engine oil is pumped and filtered throughout the entire engine. Because there is so much to explain about this, the topics “oil” and “lubrication system” are separated. On the current page everything about engine oil is explained, and the construction and operation of the lubrication system are explained on the lubrication system page.
Engine oil level:
The importance of the engine oil level is often underestimated. The level must not be too high and not too low.
Many people forget to check their oil level regularly. Especially if the car is not equipped with an electronic oil level sensor, there is a risk that the oil level will drop far below minimum before any action is taken.
The ideal situation is when the oil level is at max (second dipstick from the left in the image).
If the oil level is at minimum (fourth dipstick), half a litre to a full litre often needs to be topped up. First add half a litre and, after checking, top up the oil until it is at max. At an oil level halfway (third dipstick), nothing really needs to be topped up. Of course, it is best to do so anyway.
If the oil level is at minimum, that does not immediately mean that this is bad for the engine. It is in fact the minimum level at which the engine can run. During the design and testing of the engine, the running test engine is held at every possible angle that can also occur in practice. In none of these situations may the oil strainer run dry and the lubrication system must not be put at risk. The minimum oil level is determined using this data.
A minimum oil level does cause the oil in the engine to age faster. The same oil still has to take over all the tasks that would normally be done by more oil. The oil heats up faster and the lubricating properties deteriorate sooner. So it is best to keep the level at, or close to, the maximum marking on the dipstick.
If the oil level is below minimum, or even no longer visible on the dipstick at all, oil must be added immediately to prevent engine damage.
An oil level that is too high is also not good. From about half a litre too much oil (above max.), the crankcase pressure can increase. More oil can then enter the combustion chamber (via the crankcase ventilation into the intake manifold, or past the pistons along the cylinder wall upwards). In the latter case this concerns more than one litre too much oil. With an oil level that is too high, the catalytic converter can therefore be damaged. Due to the increased crankcase pressure, (unburned) oil passes the piston rings, through the combustion chamber into the catalytic converter, where it immediately adheres to the internal structure. This can cause it to fail prematurely. The oil deposits adhere to the internals of the catalytic converter and cannot be reversed.
One or two millimetres above max. is not harmful. If the level is higher than this, some of the oil must be drained or extracted.
Functions of lubricating oil:
Engine components that move past each other must be lubricated. The lubrication system supplies lubricating oil to the parts that move relative to each other. The service life of the engine largely depends on the quality of the oil. That is why it must also be changed periodically. Always replace the oil filter when changing the oil.
Lubricating oil has the following functions:
- Lubrication: The oil must prevent metal-to-metal contact. The oil forms a layer between the components, so friction, and thus wear, remain limited. The image shows a piston in a cylinder. If the piston or the cylinder wall is touched, it seems as if this is completely smooth. If the circled part is greatly magnified under a microscope, it becomes visible that this is not the case. This can be seen after the first red arrow.
- Cooling: Components such as valve guides, pistons, cylinder walls, etc. dissipate part of their heat via the lubricating oil.
- Sealing: The piston in the cylinder is sealed gas-tight by means of the oil. The oil therefore also contributes significantly to the compression.
- Cleaning: Dirt and wear particles that are released are carried by the oil to the filter.
- Noise damping: The oil film between metal parts also partially damps the metal noise.
- Power transmission: The oil must also be able to transmit forces, such as in hydraulic valve lifters. A certain pressure is built up in a lifter, where the oil must not be compressed.
Both the piston material and the cylinder wall contain small irregularities. These irregularities are unavoidable, no matter how well the material has been machined. When these irregularities interlock, which happens if there is no oil film present, friction occurs. The irregularities rub against each other.
By applying an oil film (the right-hand section), the rough parts are pushed apart by the engine oil. The components will not touch each other and the heat that is released is carried by this oil film to the rest of the engine oil. This oil film also provides sealing, so that as few combustion gases as possible can pass from the combustion chamber above the piston into the crankcase.
Properties of lubricating oil:
- Animal oil: This oil is obtained by boiling, extraction or pressing of bones (neatsfoot oil) and animal fat. Animal oil is added to mineral oils and is no longer used as a complete lubricant.
- Vegetable oil: This oil is made from plants. Vegetable oils are also called fatty oils. They have good lubricating properties, because they adhere well to the material. A disadvantage of vegetable oils is that they age very quickly due to the oxygen in the air. They are therefore not suitable for long-term use. The oxidised oil thickens and contaminates the parts that need to be lubricated.
- Mineral oil: The raw material of mineral oil is crude petroleum, which consists of various compounds of carbon and hydrogen. The advantages of mineral oils are that they are cheaper and that they age less quickly than animal and vegetable oils. The disadvantage is that mineral oils adhere less well to metal.
- Compound oil: This is a composition of mineral oil with 5 to 20% vegetable or animal oil. The addition of the other oils improves adhesion to the metal. Compound oils easily absorb water and age faster than mineral oil.
- Synthetic oil: This oil is produced in the laboratory. This oil is created by combining small molecules into large molecules.
Advantages of synthetic oil:
- Low viscosity (thus less frictional resistance)
- Better resistance to oxidation
- Less sensitive to high temperatures
- Natural cleaning action (detergency)
Additives:
Additives are also added to synthetic lubricating oil, such as:
- Detergent additives act as a kind of dirt solvent; dirt particles loosen from internal engine components and are carried to the oil filter.
- Anti-oxidation additives counteract oxidation. Oxidation means attack by oxygen. When the oil oxidises, the oil ages and becomes thicker, making it more difficult to pump around. Therefore, synthetic oil with these anti-oxidation additives will have a much longer service life than cheap mineral oil.
- Anti-wear additives counteract wear. One additive counteracts wear at high temperatures, the other at low temperatures.
- Viscosity index improver additives ensure that temperature has less influence on the viscosity of the oil. Engine oils with viscosity index improvers are called multigrade oils.
The additives lose their effectiveness as the engine oil ages. If the service intervals are exceeded and the same oil is used for too long between services, the additives will have almost no effect anymore. The engine will become contaminated internally, because the oil no longer loosens the dirt from the components. Changing the oil in time is therefore very important. See the paragraph “Consequences of driving too long with contaminated engine oil”.
Contamination of the lubrication system (including sludge):
After a certain time, engine oil becomes contaminated by, among other things, combustion products, fuel particles and wear particles. Because the oil becomes contaminated, it must be changed after a certain time. If an engine is driven too long with aged oil, or if the wrong oil is in the engine, the lubrication system can become heavily contaminated, and considerably more engine wear will occur. The oil slowly turns into a kind of sludge. This sludge is also called ‘sludge’. There are two types of sludge:
White/grey sludge:
This sludge is a mixture of water vapour and oil. It occurs during a lot of short trips, where the oil does not get properly warm. It deposits on the still cold parts of the engine (usually the valve cover or crankcase ventilation hoses). It is often visible when the oil cap is removed (see image).
If this type of sludge is present in the engine, it is advisable to take a long drive so that the engine gets sufficiently warm for the sludge to disappear automatically. It is best to change the oil as well. There may be a lot of sludge present in the engine, as can be seen in the image below. This is the underside of a removed valve cover of a Mini. This car has been driven over many short distances, so the oil has almost never become properly warm. By flushing the engine with special engine flush (available in many different types) and then changing (flushing) the oil several times, a large part of this sludge can be dissolved. However, it is better to remove the valve cover, as this is usually the coldest part of the engine and will therefore contain most of the sludge. Do not forget to check the crankcase ventilation hoses, as sludge will undoubtedly also be found in them.
When the sludge remains in the engine for a long time, the moisture in this sludge can cause damage to the engine components. Below is a photo of the camshafts from the same Mini from which the valve cover was removed. The coating of the intake camshaft in particular (the upper one in the photo) is badly affected by moisture. The exhaust camshaft looks better, but it also has considerable damage on a number of lobes.
Black sludge:
This sludge is very bad for the engine. It is a black, sticky, hard layer that deposits on internal parts of the engine. Black sludge is visible in the picture below.
The danger is that the oil channels and the oil strainer become clogged, so that the oil can no longer be pumped around properly. This form of sludge is caused by: driving in traffic jams, short-distance driving, an overly rich fuel system setting (carburettor), poor crankcase ventilation, excessively high combustion temperatures in combination with very old engine oil that has not been changed in time.
Black sludge can be removed if the ‘sludge’ is still soft. When a minimal amount of sludge is discovered, the oil must be changed immediately and the filter replaced. If the sludge remains longer in the engine and is completely baked on, it is very difficult to remove (by means of engine cleaning agents and disassembly/assembly of components such as the oil pan, oil strainer, etc.), or in the worst case no longer removable from the oil channels at all.
Consequences of driving too long with contaminated engine oil:
Nowadays, car manufacturers set the oil change interval at a maximum of 30,000 km or 2 years. Sometimes that is still too long and the engine oil has already been contaminated for some time before it is changed. Aged (or poor quality) oil causes black sludge (described above). Moisture will also be absorbed into the engine oil, and it will become acidic due to, among other things, the combustion gases. Oil attracts moisture; so when a car is left standing for a year, it is not the case that the engine oil has not aged during that year. The oil has absorbed moisture and has bonded with oxygen (which makes it thicker). In principle, the oil must be changed, because the lubricating effect can no longer be guaranteed.
When driving many short distances, or when the car is not used frequently, it is sometimes advisable to shorten the service intervals to 15,000 to 20,000 km instead of 30,000 km. Research shows that more than half of cars aged 4 to 6 years are running on bad oil. Even when maintenance is carried out every 30,000 km or 2 years. Driving longer on the oil to save costs is even worse. Oil that has been in use for so long has largely lost its lubricating properties and active additives. The oil then no longer absorbs dirt. The engine will therefore become contaminated internally and wear faster. This leads to considerable repair costs, such as stuck piston rings, worn camshafts, or a defective turbo. A 200,000 km engine that has repeatedly been driven too long with old oil will have the same wear as an engine that has run more than 400,000 km with good oil. It is therefore unwise to economise on fresh engine oil.
When changing the oil, always replace the oil filter as well.
This also applies to the (automatic) transmission oil. This is often indicated as “life time” oil; oil that does not need to be changed. However, this oil also becomes acidic and contaminated. That happens with every oil. Therefore, the transmission oil must also be changed periodically.
SAE specifications (Viscosity / Viscosity index)
The SAE specification indicates the viscosity of the oil (the thickness). Viscosity is the thickness of the oil at a certain temperature. A thick oil has a high viscosity and a thin oil has a low viscosity. Viscosity therefore depends on temperature. Oil is tested at different temperatures, allowing the viscosity index to be determined. The viscosity index code is always shown on the packaging, e.g. 5W30 or 10W40. The number before the W indicates the viscosity at low temperatures and the number after the W indicates the viscosity at high temperatures.
Oil with too low a viscosity can be a cause of low oil pressure. The thin oil is forced too easily through the channels, so there is no proper pressure build-up.
Oil consumption can also increase with too low a viscosity; because the oil can more easily enter the combustion chamber via the piston rings and thin oil can evaporate more easily, after which those vapours enter the combustion chamber via the crankcase ventilation system.
The viscosity index of engine oil cannot be compared with that of transmission oil. For example, the viscosity index of transmission oil is often 75W90. That does not mean that this oil is thicker than 10W40 engine oil. It must be seen as a ratio number. If the transmission oil had the specification 10W40, it could also be mistaken for engine oil by an inattentive person. Because the numbers now differ so much, this is quickly noticeable for an attentive person.
ACEA specifications (European)
ACEA specifications are tailored to European driving and usage conditions. ACEA specifications are the former CCMC specifications (CCMC was changed to ACEA in 1990). There are ACEA specifications for:
- petrol engines
- light-duty diesel engines with a max. speed of more than 3500 rpm, for both direct and indirect injection.
- heavy-duty diesel engines, meaning slow-running, direct-injection diesel engines.
Petrol engines: | Light-duty diesel engines: | Heavy-duty diesel engines: |
ACEA A1-96 | ACEA B1-96 | ACEA E1-96 |
ACEA A2-96 | ACEA B2-96 | ACEA E2-96 |
ACEA A3-96 | ACEA B3-96 | ACEA E3-96 |
|
| TD-4 |
API specifications (American)
The API specifications are tailored to American driving and usage conditions. The API specifications distinguish between oils for petrol and diesel engines.
- Specifications for petrol engines:
SC, SD, SE, SF, SG, SH and SJ - Specifications for diesel engines:
CC, CD, CE, CF-4 and CG-4
The S indicates oil for petrol engines, the letter C oil for diesel engines. The 2nd letter indicates the quality. The higher the letter after the S or the C, the higher the quality requirements.