A wet oil sump is within the engine crankcase. However, how is the oil distributed throughout the engine and all the other magical parts? I know for a dry oil sump, it's with a oil pump which holds the oil from a reservoir, but for wet oil sump, how?

  • $\begingroup$ There is still an oil pump if that is what you are asking... $\endgroup$
    – Ron Beyer
    Mar 30, 2017 at 2:20
  • $\begingroup$ There's an oil pump in within the crankcase that pumps the oil around? $\endgroup$
    – nyorkr23
    Mar 30, 2017 at 2:24
  • $\begingroup$ Yes, by definition it is "wet" meaning it is submerged in the oil, but it is still a pump. $\endgroup$
    – Ron Beyer
    Mar 30, 2017 at 2:27
  • $\begingroup$ The pump is submerged in the oil...now that is news to me. Thanks. $\endgroup$
    – nyorkr23
    Mar 30, 2017 at 2:28
  • 1
    $\begingroup$ The crank can fling oil around as well. $\endgroup$
    – acpilot
    Mar 30, 2017 at 2:46

1 Answer 1


The short answer is "The same way as in a dry sump system." – There is an oil pump which sucks oil out of the oil pan and forces it through the engine oil galleries under pressure.

Wet and dry sump oil systems only really differ in where they store "extra" oil.

Wet Sump Systems

In a wet sump system the oil is stored in the oil pan under the engine. The oil pan serves as both the oil tank (holding the supply of lubricating oil) and the oil sump (where oil eventually collects after making its way through the engine). The pressure pump sucks oil directly from the oil pan and circulates it through the engine's oil galleries.
Wet sump oil system

Dry Sump Systems

Dry sump systems store their oil in a separate tank, which feeds the pressure pump. Dry sump engines have a smaller oil pan which only serves as a sump, and a "scavenger pump" that drains the sump back into the main oil tank. They are termed "dry sump" systems because under normal operating conditions very little oil remains in the sump.
Dry sump oil system

Note that there are other engineering differences in dry-sump systems, but they're largely beyond the scope of your question.

Illustrations thieved from https://www.slideshare.net/jesscar/piston-engines-oil.

  • $\begingroup$ One difference in distribution in a wet sump oil system is the inclusion of direct distribution from the sump by the splash method. This is especially true in some older reciprocating designs. See the AMT Powerplant Handbook. $\endgroup$
    – J W
    Mar 30, 2017 at 13:29
  • 1
    $\begingroup$ @JonathanWalters Yes, that isn't well illustrated in the drawings above: In a wet sump design the crankshaft journals and counterweights will splash through the oil in the sump, kicking up oil (and creating a fine oily mist in the crankcase) to lubricate other components like the pistons. In a dry sump system the oil level in the sump may be below the arc of the crankshaft which reduces or eliminates splash lubrication. Dry sump engines often include "piston oilers" & other oil-squirting systems to supplement/replace the lost splash lubrication. $\endgroup$
    – voretaq7
    Mar 30, 2017 at 19:54

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