Auto & Shop InformationLesson 5 of 17
Lubrication Systems
Oil pump, filter, oil grades and what lubrication actually does.
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The cooling system deals with the heat of combustion. The lubrication system deals with the heat and wear of metal moving against metal, and it does more than its name suggests.
The four jobs
Reduces friction. A film of oil separates moving surfaces so they slide on the oil rather than on each other.
Carries heat away. Oil flows past hot parts and carries their heat back to the pan, where it is shed. Bearings and pistons are cooled by oil, not by the coolant.
Cleans. Oil carries combustion byproducts and wear particles to the filter. Detergent additives hold contaminants in suspension, which is why used oil is black - it is black because it is working.
Seals. A film between the piston rings and the cylinder wall helps seal the combustion chamber.
All four are asked, and a question expecting only "reduces friction" is incomplete.
The path
| Component | Job |
|---|---|
| Oil pan or sump | reservoir at the bottom of the engine |
| Pickup screen | coarse strainer at the pump inlet |
| Oil pump | pressurizes and circulates the oil |
| Pressure relief valve | caps the pressure, bypassing excess back to the pan |
| Oil filter | removes particles |
| Main gallery | the main drilled passage that feeds everything |
| Bearings and rockers | where the oil is needed |
| Dipstick | checks the level |
Oil returns to the pan by gravity, which is why the pan is at the bottom and why an engine must be level to read the dipstick accurately.
The pump is usually driven by the crankshaft or camshaft, so oil pressure exists only while the engine is turning. That is why most engine wear happens at startup, before pressure has built and the film has been re-established.
A filter has a bypass valve so that if it clogs, oil still reaches the engine. Dirty oil is better than no oil, which is the reasoning behind that design and is a question.
Oil grades
Viscosity is resistance to flow, and it is what an oil grade describes.
A single-grade oil such as SAE 30 has one viscosity rating.
A multigrade oil such as 10W-30 behaves like a 10-weight oil when cold and a 30-weight when hot.
The W stands for winter, not weight. That is asked directly.
What multigrade solves: oil thins as it warms, as the fluids lesson notes. A plain oil thick enough to protect a hot engine would be too thick to pump at startup on a cold morning. A multigrade flows when cold and still protects when hot, which is why nearly all modern engines use one.
A lower first number means better cold flow. 5W-30 starts more easily in cold weather than 10W-30.
The oil pressure warning light is not the oil level light. It reports that pressure has fallen below a safe threshold, which means oil is not reaching the bearings - and an engine run in that condition can destroy itself in under a minute. The correct response is to stop the engine, not to drive on and check later. Low level is the commonest cause, but the light is reporting pressure.
Symptoms
| Symptom | Likely cause |
|---|---|
| Low oil pressure | low level, worn bearings, failing pump, or oil too thin |
| Blue exhaust smoke | oil burning in the cylinders; worn rings or valve seals |
| Oil level dropping with no leak | being burnt, as above |
| Milky oil on the dipstick | coolant entering the oil, often a head gasket |
| Noisy valve train at startup | oil draining back overnight; slow pressure build |
| Dark oil | normal; detergents are holding contaminants |
Milky oil is the one to recognize. Oil and coolant mixing means a failed seal between the two systems, and the head gasket is the usual suspect.
Blue smoke means oil. Black means a rich fuel mixture; white usually means coolant. Those three colors are a question, and the pairing is worth holding.
Know it by sight
The one picture question in this topic shows the oil pan.
| Item | How to recognize it |
|---|---|
| Oil pan | the shallow metal pan bolted under the engine, with a drain plug |
What you can skip
Across the 59 questions on this topic in our bank:
- Oil service classifications. API ratings and synthetic oils never appear, and SAE is named once.
- Viscosity number systems. Multigrade numbers such as 10W-30 come up three times. Know what the W means and that the second number is the hot viscosity.
Where people lose points
Saying oil only lubricates. Four jobs.
Thinking the W in 10W-30 means weight. Winter.
Thinking dark oil means it must be changed immediately. Dark oil is doing its job.
Saying the oil pressure light means low level. It means low pressure.
Thinking coolant cools the bearings. Oil does.
Assuming a clogged filter stops all flow. A bypass valve prevents that.
Work one in under a minute
An engine's dipstick shows oil that is light brown and milky. What does this indicate, and why is it serious?
Coolant has entered the oil, which is what produces the milky appearance.
The usual cause is a failed head gasket, the seal between the cooling passages and the oil passages.
It is serious because contaminated oil lubricates poorly. Water in the oil disrupts the film, and the bearings are the first thing to suffer.
Note that the color alone gave the diagnosis. Dark oil is normal, milky oil is not, and the difference is the whole of the question.
Where this leads
Viscosity connects to the fluids lesson, and the wear this system prevents is the friction lesson's subject.
Related lessonsReference
- Engine Fundamentals - the parts the oil reaches
- Cooling Systems - the other fluid loop, and where a head gasket fails between them
- Friction - what lubrication is reducing
- Fluids and Buoyancy - viscosity and temperature
Practice this topic
Check that this lesson stuck. Answer questions on lubrication systems only, and see the right answer and why after each one.
Practice Lubrication Systems questions