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I remember taking apart a small lawnmower engine once and saw a little dipper at the bottom of the connecting rod to splash the engine with oil. Lower oil level means less resistance to the dipper, which means better gas mileage.

I am wondering if automotive engine operates partly on splash lubrication, or if all the oil is pumped and filtered.

I assume that the engineers marked the dipstick very conservatively, and that there's no difference in lubrication quality between the low and high mark.

If this is the case, keeping the oil level near the low mark should improve gas mileage without harming the engine.

Thoughts?

Yes, there is oiling from the spashing of the oil in the crank case. The rings are lubed by this, as well as the cam shaft lobes.

Louis B

Depends on the engine... Most autmotive engines have some splash oiling somewhere in the engine. Very few that I know of rely on splash from the sump to lubricate or cool much of anything. Most of the oiling is done by oil passages, and most of the "splash" comes from oil leaking out of the passages onto other stuff.

That sort of splash oiling will continue regardless of the level of oil in the sump, until there isn't oil going through the oil passages. That circumstance is something you want to avoid.

Rings are often lubed by oil that gets thrown off of the crank at the rod journals. Some engines, typically high-performance ones, have dedicated piston squirters in the case that spray the back side of the piston. Some of that oil winds up on the rings. OHC engines will either have oil passages that transport oil up into the cam housing, or will have oil going through the camshafts. That oil--particularly the oil going to the rocker arms--will splash out and get on the valve springs, cooling and lubing them.

There is some "windage" in the crankcase of most motors; oil spray that the crank has to push through when it spins. That's more friction, which means more drag on the engine and higher oil temps. Most engines are designed with ways to reduce that, but it usually cannot be completely eliminated.

-soD

@some_other_dave

I thought dry sump systems were designed to eliminate most of that.

I would be curious to see if a dry sump swap would make any difference as far as fe is concerned.

The question would be to me, is the higher temps on the oil, the posibility of greater wear and tear of parts that are expensive and hard to replace, and the posibility of engine problems worth the small gains in FE. I would not risk it. Replacing an engine ( or any part ) will be more costly than any money saved by it's savings.

A thing to note ! many many many places will fill your oil to what they normally fill the oil. they do take the time to check to see what the proper volume is for your car, they are not all 4 quarts (kicks the pep boys) some of the "techs" are VERY CLUE LESS !

there is another trick, remove the oil pan and replace it with an oil pump , this is a trick they use on many racing cars, or very very low cars that do not have room for the oil pan at the bottom of the car.

AbACUZ, that's the "dry sump" that korenberg mentions in his post just above yours.

korneberg, I am pretty sure that dry-sumping an engine will often result in better FE, all other things being equal. This is an assumption from the fact that dry-sumping a motor typically results in slightly better power output, which says that the power consumed by driving the extra stages of oil pump is more than offset by the power gained by not having so much oil slosh to fight.

Note that the power gain is not universally true, so the FE gain will not be universal either.

Some race cars use vacuum pumps to pull some of the air out of the crankcase, as well. The moving bits then don't have as much air resistance when they're spinning around, and that can result in more power. That may result in better FE as well, but at a potentially significant cost in extra crankcase emissions...

-soD

In most cars, there's an PCV (positive crankcase ventilation) valve that draws the blowby (leakage of compression gas past the piston rings) back into the engine to be burnt. This reduces emissions.

Since the PCV operates on engine vacuum, there there should already be a good amount of vacuum in the crankcase.

Not nearly as much as you can pull with a vacuum pump. Especially if there is much blow-by in an older (or looser-tolerance) engine.

Also, PCV typically only works at a relatively small range of loads. The vacuum pump will operate at all loads and all RPMs.

Again, though--chances are very high that the pump-equipped car will emit much more crankcase pollution than a car with a regular PCV system.

-soD

You don't want the oil to be overfilled or underfilled.

The main purpose of oil is to lubricate the engine but, oil pumps don't quite pick up from the very bottom of the oil resevoir but the oil pump pickup picks up at about 1/2 inch to an inch from the bottom of the pan. The purpose of that is to let the heavy particles, such as microscopic metal particles settle to the bottom.

You want enough oil in the engine to allow it to stay cool, filter and trap metal particles, and lubricate. Too much oil will be added resistance for the engine to cut through .

Oil should be kept pretty near the middle of the hash marks.

Too little, and contaminaints are not filtered AND at lower levels, the engine will blow if you're lucky, and seize if you're unlucky.

Too high and the oil pressure might blow a hose or rod.

The modern wet sump engine is basically a dry sump engine that uses gravity to get the oil back to the oil tank. The crank and rods are at a safe distance above the top of the oil. Lawn mower style splash oiling went out with the Model T.