From our insanely detailed guide:

Engine lubrication

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As oil leaves the filter, it makes its way to the following places in the engine:  

Main bearings

The main bearings support the crankshaft and experience the highest load of all the bearings in the engine. Oil travels from the main gallery to the main bearings, where a hole in the engine block, matched with a hole in the bearing shell, allows the oil to be forced between the crankshaft journal and the bearing itself.  

Big-end Bearings

The big-ends of the connecting rods are lubricated with oil that travels up an oilway drilled through the crankshaft. The oil is pushed from the main bearings to the big end bearings. In some engines it will make its way through a passage drilled in the connecting rod to lubricate the cylinder walls.  

Cylinder walls

The cylinder walls require a film of oil for the piston rings to travel smoothly up and down. At the most basic, this film is maintained through oil being splashed onto the walls by the spinning crankshaft. In other systems, oil can be directed onto the cylinder wall by holes in the connecting rods, or by dedicated oil jets, as in our project Miata.  

Pistons

The underside of the pistons can be cooled by a stream of oil. The project Miata has oiljets which direct a constant stream of oil onto the underside of the pistons. These jets have a check valve which only opens once the oil pressure reaches a certain level - protecting the engine components on startup while oil pressure builds up.

Camshaft bearings

Like the crankshaft, the camshaft rotates in bearings which require a constant supply of oil.  

Cam followers

Oil holes in the cam follower bores allow a constant stream of oil to lubricate the clearance between the follower and its bore. That oil drops down under gravity to lubricate the valve spring and stem.

Oil restrictor

Pressure can be maintained in the bottom end of the engine through the use of a restrictor, which is a constricted passageway that reduces the flow of oil between the engine block and cylinder head. In the illustrated engine, the oil restrictor is in the top of the engine block and all the oil for the head flows through this restrictor.

Once ‘consumed’ in those in one of these lubricated parts, the oil makes its way back down to the sump where it sits, cools and is then recirculated. 

Methods of lubrication

There are two mains ways that oil actually reduces the friction between moving parts.  

Boundary lubrication

It’s easy to understand boundary lubrication - if we coat the surfaces of two objects in a film of oil, then they will have reduced friction between them. Inside the engine this type of lubrication is used for all moving parts that aren’t fed with oil under pressure - the cylinder walls and valve stems, for example. 

Hydrodynamic lubrication / force-fed lubrication

For bearing surfaces, which carry heavy loads at high speeds, the oil works in a different way, known as hydrodynamic lubrication. Initially, when the shaft is stationary, it rests on the bottom bearing surface and is separated by a film of leftover oil. As the shaft begins to rotate inside the bearing, it will be lubricated only by boundary lubrication until pressurized oil fills the clearance between the shaft and bearing.

Once up to speed, the shaft will drag around the oil, to create a wedge of lubricant underneath (where pressure is highest) which lifts the shaft and centers it in the bearing. Since the bearing is larger than the shaft, a gap known as the clearance, the type of oil must be matched to the clearance.

Continue reading: Oil pan / Sump