Engines: Engine Lubrication: Lubrication systems
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Pressure system

 
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Summary
In force-feed lubrication, pressure forces oil around the engine. In a wet-sump system, oil is kept in the sump ready for the next use. In a dry sump system, oil falls to the bottom of the engine and a scavenge pump sends it to an oil tank.
Oil pressure system

Modern vehicle engines use a pressure or force-feed lubrication system where the oil is forced around the engine under pressure. Let’s look at gasoline engines first.

Oil won’t flow up into the engine by itself so a pump collects it through a pickup tube and a strainer, and forces it through an oil filter, then into passageways in the engine block, called galleries.

The galleries allow oil to be fed to the camshaft bearings, the valve mechanism, and the crankshaft main bearings. Holes drilled in the crankshaft webs allow the main bearings to supply oil to the big-end bearings.

After circulating through the engine, the oil falls back to the sump to cool. This is called a wet-sump lubrication system because the oil is kept in the sump ready for the next time it’s used.

Some special engines use a dry sump lubrication system. It uses all of the parts that make up a wet sump system and it lubricates the engine in the same way.

It differs from the wet sump system in the way the oil circulates. In a dry sump system, the oil falls to the bottom of the engine into an oil collection pan. A scavenge pump then pumps it to an oil tank where it is stored until the normal oil pump collects it and pumps it through the filter and engine in the normal way.

Because there is no oil storage sump under the engine, the engine can be mounted much lower than in a wet sump system.

The oil tank can be positioned away from the engine where it can get best cooling. And the amount of oil in the system can be much greater than in the wet sump system.

Diesel engines are lubricated in much the same way as gasoline engines but there are differences.

Diesel engines typically operate at the top end of their power range so their operating temperatures are usually higher than those in similar gasoline engines, so the parts in diesel engines are usually more stressed.

As a result, diesel oils need a different range of properties and are classified differently.

It’s common for some diesel engines to use an oil cooler to cool the oil in the engine. The cooler and oil filter are usually on the same mounting on the cylinder block.