Engine Lubrication System: Friction, Oil Circuit, Viscosity and Diagnosis

Introduction

The lubrication system is one of the fundamental systems for engine operation, performance and durability. While the engine runs, many metal parts move against each other under load, at high speed and at high temperature. Without proper lubrication, contact between these surfaces would increase friction, accelerate wear, cause overheating and, in extreme cases, seize the engine.

Engine oil does more than reduce friction. It also helps dissipate heat, cleans the inside of the engine, protects against corrosion, seals, cushions impacts and, in many modern engines, operates hydraulic components such as lifters, tensioners and cam phasers.

That is why correct operation depends on several factors: the right oil, the correct level, efficient circulation, pressure within specification, filtration, temperature, the mechanical condition of the engine and proper maintenance.

1. Friction and the need for lubrication

Friction is the resistance to relative motion between two surfaces in contact. In the engine it occurs between pistons and cylinders, bearings and crankshaft, camshaft, lifters and many other moving assemblies.

Even surfaces that look perfectly smooth have microscopic irregularities (roughness). Contact between these “peaks” is what creates resistance, heat and wear.

Solid friction

Occurs when two solid surfaces come into direct contact. As they move, the roughness peaks collide, creating resistance, heat and wear.

Fluid friction

Occurs when a lubricant film fully or partly separates the surfaces. Friction then happens inside the oil itself, which is much lower than metal-to-metal friction.

The main purpose of lubrication is precisely to let surfaces work separated by an adequate film whenever the design allows.

2. Functions of engine oil

3. Operating principle

In a conventional wet-sump system, oil is stored at the bottom of the engine. When the engine runs, the pump draws oil through the pickup and sends it under pressure to the filter and the oil galleries. After lubricating, the oil drains back to the sump by gravity and the cycle starts again.

Basic path: sump → pickup → pump → relief valve → filter → galleries → components → return to sump.

4. Main components

4.1 Sump and dipstick

The sump (oil pan) is the oil reservoir in wet-sump engines. The dipstick (or the electronic level sensor found on many current cars) is used to check the amount of oil.

Low level can cause air to be drawn into the pickup, pressure loss, higher temperature and lack of lubrication. Level above maximum is also harmful: the crankshaft can whip the oil, creating foam, more crankcase vapors and oil consumption.

4.2 Pickup and suction screen

The pickup is the tube that draws oil from the bottom of the sump. Its inlet has a screen (pre-filter) that protects the pump from large particles. Sludge, excess sealing silicone or pieces of gasket can block this screen: a healthy pump cannot hold pressure if its inlet is restricted.

4.3 Oil pump

The oil pump is a positive-displacement pump: each revolution pushes a fixed volume of oil. In other words, the pump produces flow. Pressure appears because the oil meets resistance in bearing clearances, galleries and the filter. That is why pressure also depends on viscosity, temperature and the condition of the engine clearances.

• External gear: two gears; oil is carried around the outside, between the teeth and the housing.
• Rotor (trochoid or gerotor): an inner rotor with one lobe fewer turns inside an outer rotor, forming chambers that grow and shrink.
• Vane: sliding vanes in an eccentric rotor. Many current engines use variable-displacement vane pumps, which pump only what is needed and reduce fuel consumption.

4.4 Pressure regulator or relief valve

The pump is sized to deliver more oil than the engine needs, especially at high rpm. When pressure reaches the design value, the valve opens and diverts the excess back to the sump or to the pump inlet. A valve stuck open causes low pressure; stuck closed, excessive pressure, which can swell the filter or blow seals.

4.5 Oil filter

The filter traps particles and contaminants carried by the oil. Many filters have two internal valves:

• Bypass (relief) valve: if the element is heavily clogged or the oil is very cold and thick, it lets oil pass unfiltered. Dirty oil is better than no oil at the bearings, but this shows why on-time changes are essential.
• Anti-drainback valve: stops the filter from draining while the engine is off, so pressure builds faster at start-up.

That is why the filter must be the one specified for the engine: the same thread and size do not guarantee valves with the same calibration. Some heavy-duty diesel engines also use centrifugal filters with no filter element: a rotor is spun by jets of oil and centrifugal force deposits the impurities on the wall, forming a cake that is removed during maintenance.

4.6 Galleries, oil jets and oil cooler

Galleries are internal passages that distribute the oil. A blocked gallery causes local oil starvation even with oil in the sump and pressure at the sensor. Oil jets aimed under the pistons help lubricate and cool them. The oil cooler (heat exchanger) limits oil temperature, keeping it away from the range where it degrades quickly. It is common on turbocharged, diesel and high-performance engines.

4.7 Oil pressure sensor or switch

It monitors pressure and controls the warning lamp, the dashboard gauge (common on trucks and machinery) or sends the information to the engine control unit. A simple switch only shows whether there is a minimum pressure: a lamp that is off does not mean the pressure is correct.

5. Wet sump and dry sump

6. Lubrication methods

• Pressure-fed: oil is sent through the galleries to main bearings, rod bearings, camshaft and turbo.
• Splash: moving parts throw oil onto cylinders, pins and gears.
• Jets: nozzles aim oil at specific points, such as the underside of the pistons.

These methods are not exclusive: the same engine usually combines all three.

7. Lubrication regimes

Film thickness depends on viscosity, speed and load. This relationship is shown by the Stribeck curve:

7.1 Hydrodynamic lubrication

In a plain bearing, the rotating shaft drags oil into a wedge that forms between the shaft and the bearing shell. This wedge generates enough pressure to lift the shaft and keep it floating on the oil.

7.2 Boundary and mixed lubrication

At start-up, at low rpm or under heavy load, the film becomes thin and roughness peaks touch. This is when most engine wear occurs, and anti-wear additives form a protective layer on the metal. That is why oil pressure must build up quickly after starting.

8. Engine oil: composition

Engine oil is made of base oils (usually 75% to 90%) and an additive package.

• Mineral base oils: refined from crude oil. Paraffinic bases have a higher viscosity index and good oxidation resistance; naphthenic bases flow better in the cold but change more with temperature.
• Synthetic base oils: made by chemical synthesis (such as polyalphaolefins and esters), with more uniform molecules, better thermal stability and better cold flow.
• Semi-synthetics: blends of mineral and synthetic bases.

Even so, choosing an oil just because the label says “synthetic” is not enough. The product must meet the specifications required by the engine manufacturer.

9. Viscosity

Viscosity is a fluid’s resistance to flow. Honey has high viscosity; water, low. In the lab, the most common measurement is kinematic viscosity, in centistokes (cSt or mm²/s), usually at 40 °C and 100 °C.

Temperature changes viscosity a lot: oil gets thick when cold and thin when hot. The engine needs an oil that flows quickly at a cold start and still keeps its film above 100 °C. As a general engineering rule, tight clearances and high speeds call for lower-viscosity oils; larger clearances and heavy loads, higher-viscosity oils. But the engine design decides this, not the workshop’s preference.

10. Viscosity index

The viscosity index (VI) shows how much viscosity changes with temperature. The higher the VI, the more stable the oil.

Multigrade oils use high-VI base oils and VI improver additives (polymers that coil up in the cold and expand in the heat). Do not confuse VI with the SAE grade on the label.

11. SAE classification

SAE (Society of Automotive Engineers) classifies oils by viscosity only.

Common examples: 0W-20, 5W-30, 5W-40, 10W-40, 15W-40 and 20W-50. Monograde oils have a single number (for example, SAE 40). It is essential to understand that SAE does not indicate quality: two 5W-30 oils can have very different performance levels and approvals.

12. API performance classification

The API (American Petroleum Institute) defines performance categories, proven by standardized engine and laboratory tests:

• S series (spark ignition): gasoline, ethanol and CNG engines. It advances alphabetically: older categories (SA to SH) are obsolete; examples of more recent categories are SN and SP.
• C series (compression ignition): diesel engines, such as CI-4 and CK-4.

Each new category was created to keep up with the engines, fuels and emission systems of its time. Even so, do not assume that “the newest letter fits any engine”: always check what the manufacturer requires.

13. Other specifications

Besides API, there are ACEA (European), ILSAC and OEM approvals (each vehicle and engine manufacturer’s own standards). ACEA C-series oils are low in ash, phosphorus and sulfur (low SAPS), because these elements, present in some zinc- and phosphorus-based anti-wear additives, can poison catalytic converters and particulate filters.

Professional oil choice: SAE viscosity + performance level (API/ACEA/ILSAC) + manufacturer approval + operating conditions.

14. Oil additives

The additive package is balanced by the oil manufacturer. Adding products to the oil without a technical recommendation compatible with the engine and the oil is not advisable.

14.1 Properties measured in the lab

• Flash point: the temperature at which oil vapors briefly ignite with a flame. A lower flash point in used oil may indicate fuel dilution.
• Pour point: the lowest temperature at which the oil still flows.
• TBN (total base number): alkaline reserve to neutralize combustion acids. It drops with use.
• Color and smell: do not indicate quality. Dark oil is normal; a fuel smell or milky appearance indicates contamination.

15. Oil pressure

Pressure depends on: engine speed, viscosity, temperature, oil level, condition of the pump and suction screen, pressure regulator, filter, bearing clearances and restrictions in the galleries. That is why low pressure does not automatically mean a faulty pump. An engine with worn bearings, for example, “leaks” oil through the clearances and pressure drops even with a good pump.

15.1 How to measure

1. Before measuring, check oil level and condition.
2. Install a suitable gauge at the point specified by the manufacturer (usually in place of the pressure switch).
3. Warm the engine up to the specified oil temperature.
4. Read the pressure at the specified engine speeds (for example, idle and a higher rpm).
5. Compare with the specification for the engine under test.

15.2 Oil pressure warning lamp on

With the engine running, a lit lamp must be treated as a potentially serious situation. Professional sequence:

1. Do not keep the engine running unnecessarily.
2. Check oil level and condition and look for leaks.
3. Confirm that the oil and filter are the specified ones.
4. Check the sensor and the electrical circuit.
5. Measure the actual pressure with a gauge and compare it with the specification.
6. Only then investigate internally: pickup screen, pump, valve, galleries and clearances.

Replacing the pump just because the lamp came on is not a diagnosis.

16. Crankcase ventilation: blow-by and PCV

During combustion, a small amount of gas passes the rings and reaches the crankcase: this is blow-by. These gases contain fuel vapor, moisture and combustion products. If trapped, they would raise crankcase pressure and contaminate the oil.

The PCV system separates the oil mist, returns the oil to the crankcase and sends the vapors to the intake, where they are burned. The valve controls the flow according to manifold vacuum and keeps crankcase pressure stable. System faults can cause:

• high internal pressure and leaks at seals and gaskets;
• oil contamination and sludge;
• oil consumption;
• rough idle (unmetered air leak).

Hoses and the separator should be inspected and cleaned periodically.

17. Oil contamination

18. Sludge and deposits

Sludge results from a combination of degraded oil, contaminants, temperature and poor maintenance. It can restrict the pickup screen, galleries, oil drain-backs, hydraulic lifters and variable valve timing mechanisms. In a heavily contaminated engine, careless use of cleaning products can release deposits all at once and cause new blockages: assess the case and, if necessary, mechanically clean the sump and screen.

19. Oil consumption

Possible causes: external leaks, worn rings and cylinders, valve guides or valve stem seals, PCV faults, turbocharger, wrong oil or overfilling. Blue smoke may indicate oil burning, but the source must be identified before replacing components.

20. Turbocharger lubrication

The turbo shaft can exceed 100,000 rpm and works next to very hot exhaust gases. It depends on proper oil supply and drain. Contaminated oil, the wrong specification, restricted lines or lack of pre-lubrication after installation destroy the bearings. When replacing a failed turbo, it is essential to find out why it failed.

21. Oil and filter change

The oil change interval is set by the engine manufacturer, not by the oil manufacturer. It may consider mileage, time, operating hours, operating conditions and the vehicle’s electronic oil-life monitor. There is no universal interval.

21.1 Severe operating conditions

Many manufacturers shorten the interval for frequent short trips, extended idling, heavy traffic, dust, extreme temperatures, towing and intensive commercial use. Many city drivers fall into these conditions. The definition of “severe service” varies between manufacturers.

22. Logical diagnosis of the lubrication system

23. Common mistakes

• choosing oil by brand or SAE grade alone;
• ignoring performance specifications and approvals;
• using thicker oil to hide faults;
• overfilling or running below minimum;
• using a filter that “fits” instead of the specified one;
• arbitrarily extending oil changes;
• adding products without technical need;
• using too much silicone sealant, which can block the pickup screen;
• overtightening the drain plug;
• condemning the pump without measuring pressure;
• ignoring the warning lamp;
• not investigating the cause of contamination or sludge.

24. Greases: the workshop’s semi-solid lubricant

Grease is not used inside the engine, but it is found in wheel hubs, CV joints, tie-rod ends and pivots. It consists of base oil, a thickener (usually a metallic soap, such as lithium or calcium, or a complex thickener) and additives. The thickener works like a “sponge” that holds the oil in place.

Consistency is classified on the NLGI scale (from 000, almost fluid, to 6, very hard). Always use the specified grease: mixing greases with different thickeners can soften them and make them run out.

25. Safety and environment

26. Fundamental rule

There is no universal “best oil”. The correct oil is the one that meets the technical specifications for that application: viscosity, performance, approvals and operating conditions. Likewise, oil pressure should not be judged against a generic number.

Manufacturer’s specification + correct measurement + logical diagnosis = professional service.

27. Quiz — 12 questions

Answer first and then check the commented answer key. These questions are for review and do not replace the Excellence Auto Tech technical assessment.

Conclusion

The lubrication system must be understood as a complete circuit, not just oil, filter and pump. Viscosity, temperature, pressure, clearances, filtration, crankcase ventilation, contamination and additives are all connected.

A qualified technician does not replace parts on assumption: they observe the symptom, check the oil, measure, consult the specifications and determine the cause before repairing.

Technical note: pressures, viscosities, specifications, change intervals and procedures vary by engine. The values mentioned are teaching references. Always consult the manufacturer’s technical documentation. The illustrations are schematic and were created for this lesson.

References: handout Sistema de Lubrificação de Motores; SENAI/Votorantim, Lubrificação; vehicle manufacturers’ repair manuals.