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Creepybusguy comments on Ranger with 25k miles on it.

Oil is incredibly important to your engine. (And the whole vehicle in other spots but I’m not going to get into it.) It is pumped via specially bored holes and channels through out the whole engine area.
  1. It cleans the gunk and crap away that are by products of combustion and wear. So Carbon bits from combustion and microscopic bits of metal from when your bearing wears and your piston rings slide up and down your cylinder liner. Every oil has soaps and surfactants in it for this purpose. So those guys who filter their oil multiple times are doing themselves a disservice. Even the really big marine diesels that have oil sumps in measured in cubic meters of oil get their oil changed despite extensive filtering. (You should see THAT oil change bill.)
  2. It prevents wear of your bearings and bushings. Your crankshaft has to turn but it turns at a very very high speed. So oil under pressure is forced like this[1]   to the bearings. This pressure actually lifts the crankshaft just a little bit off the bearings and makes a cushion of oil. The bearings I’m talking about look like this[2]   notice the oil groove in it. Well, there’s a hole drilled in your crankshaft (Technically a bunch of them) that also has the oil forced into it via this channel (All while the hydrostatic lift is happening hence the pressure needed) this oil is then forced through to bearings on the piston rod number2021 in this picture[3]   it then travels up to the top of the piston rod lubricates the top end bush. (#16). Again hydrostatic lift like what’s happening on your crankshaft is happening there too! There’s also camshaft bushings, rocker arm bushings that need to be lubricated in this way as well. So what happens with out the lube? Hydrostatic lift doesn’t happen and the metal come into contact and thing go crunch (Which is why it’s good to let your car idle for a minute or so before hitting the gas. It makes sure you have lube to everything) and get hot really quickly.
  3. Which brings me to cooling. People often forget oil cools your engine as well. All that spinning metal makes heat. Even if the metal bits aren’t touching. If you take a stir stick and swirl water round with it it will eventually heat up. Friction happens in liquids too. So the oil takes away the heat from the bearings. It also is forced up through the piston rod and comes out of the #17 position of the previous picture and sprays under the piston. This is from a marine diesel but you get the idea[4]   This removes heat and gives you a better combustion. Too hot and you get weird by-products and you burn out your valves faster. Too low and you get carbon build-up (Why your oil turns black) and power loss and lots of other problems. Oil also cools your turbo bearings. This thing spins at over 20,000rpm (I’m marine diesels and mine weigh a literal ton and spin at that so cars probably go faster) it also takes some time to slow down so let your car idle for about a minute after you stop driving to let it cool off or you’ll cook your bearings there and thrash your fancy turbo.
The oil also sprays on the sides of your cylinder liner to cool it and lubricate it as well. You piston rings are specially designed for oil to go behind them and force them out to seal the piston against the side of the liner.
All this oil then dumps down into the bottom of the crankcase where it’s cooled by either being pumped to a specific cooler or just by the air movement from driving along. I’m not an automotive guy so I’m kind of hazy on that. My oil is pumped through huge plate coolers before being returned to a special tank and then back to the engine.
There’s far, far, far more to oil than that. But it’s the basics. I’ve got a 400 page text book at home that deals with oils and everything about them. So this is the coles notes.
[Here’s a picture of some bearing shells you can see the channels cut in them)(http://www.stationaryengineparts.com/images/P/CS_bearingshells-01.jpg[6]  )

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