Monday, January 07, 2013

Oil Confuses Me

I know some things about oil. I know that before start up most of it is in the crankcase, a big metal container-y thing in the middle of my engine. There's a filler neck on the crankcase, capped by the top of the dipstick, and I know which line on the dipstick the oil level should reach. I know how to add more. I'm even certified to do so, because that's considered "elementary work" by Transport Canada. I'm awesome at pouring oil straight from the bottle into the crankcase without spilling any, if the wind isn't blowing. I know that if the wind is blowing, or a helicopter taxies by and I spill oil on the engine it makes a mess, and then the maintenance folk can't tell if the engine is leaking or the pilot is just an idiot. I know how to make a funnel out of an old oil bottle so that I don't make a mess. I haven't yet figured out how to get the oil stain out of my wool sweater from the time I knocked over the funnel, but it just looks like a shadow, anyway. I'm pretty good at estimating how much oil is on the floor, cowling and or belly, if there is a leak. And it doesn't take any special skill beyond resignation to a task to clean oil off the airplane's cowls and belly. When it's time to change it, I can drain the oil from the engine into a bucket without burning myself.

The four functions of oil we learned as student pilots: cleaning, sealing, lubricating and cooling. Thus I know the oil lets the moving parts slide instead of scrape against one another to reduce wear and wasted power; it fills in little gaps around the pistons in the cylinders so the pressure doesn't escape; it exchanges heat between the hot engine and the cold air that blasts in around the oil cooler at 20,000' and 170 kts; and it carries bits of carbon and dirt and whatever with it out of the moving parts and into the oil filter. I can specify the maximum and minimum pressures acceptable oil pressures on the gauge and I even know there's a skinny little line that runs to each cockpit oil pressure gauge along the leading edge of the wings from each engine. It seems too blink-inducingly simple to be the way oil pressure monitoring would work, kind of like using a transparent gas tank for a fuel gauge or a little porthole in the floor to verify gear extension, but I've used such systems, too.

I've always been impressed that the very same oil is also used to control the pitch of my constant speed propellers. A pump steps up the pressure three- to fivefold in order to drive the propeller to fine pitch, and I love those little diagrams of pilot valves opening and closing for on-speed, underspeed and overspeed conditions. (The one on page 72 of this Hartzell manual is, however, astonishingly terrible. You can't even see the way the valve operates. Don't bother looking at it).

So what's my confusion about oil?

It has all these really specific jobs to do and is vitally important to the function of the engine, but after it goes through the pump and filter, it just gets jammed into all the things its supposed to lubricate, and squishes out in a not very orderly fashion. It's supposed to leak in and out of everything, and I'm really not sure how it gets collected back to the sump after its adventures in the engine. Is there a complicated system of check valves and return lines in the oil scavenging system? I know the system used in the Beaver, where the oil just splashes up and drips back down is less advanced than the one in my airplane, but I really don't know how it works. Maybe there are elves. And if so are they the arrogant, bow-wielding kind from The Hobbit or the bumbling ones with oversized hats from Rise of the Guardians. It's probably more complicated than that, because I've watched them cut open the used oil filters and I've never seen any arrows or hats in there.

Also, the FAA mandated this:


I hope there's more than a dashboard placard preventing failure of my crankshafts.


Anonymous said...

Hi Aviatrix,

You'll likely get tons better responses than mine, and I know automotive oiling systems better than those of airplanes, but let me at least give it a try:

Think of the complete oil system as a pressure vessel. The vessel is pressurized once the oil pump successfully pulls oil from the sump (again, not sure how this works in aero engines; in a car, the pickup is sitting near the lowest part of the oil pan) and hydraulic pressure fills all the connected oil passages, lines, the oil filter, etc. with pressurized oil.

Assuming no leaks, this system will stay pressurized as long as the pump is putting pressure into the system. The pump moves oil from the sump, to the filter, through the engine (simplifying this again partly due to ignorance) and back to the sump.

There's typically no need for check valves, etc., as oil will flow the correct way due to the design of the oil system and will flow as soon as pressurized.

Once pressure goes away, gravity and the specific oil route take over, with oil collecting in the lowest spot, gravity-wise.

I hope that helps somewhat. You're waaaay better at making complex subject easily understood than I, but this is my attempt at balancing out all the wonderful knowledge you've transferred my way over the years...


A Squared said...

In a word: Gravity. When the oil is done doing what it does, gravity takes it back to the sump. On Lycoming engines separate oil lines are provided to drain the oil from the cylinder heads, but the oil flows by gravity. On Continentals the camshaft is below the crankshaft and as a result the pushrod tubes are underneath the cylinders. oil drains from the heads thru the pushrod tubes back to the sump.

The Beaver engine does indeed have a pressurized oil system, you won't find any engines more complicated than a lawnmower engine that relies exclusively on splash lubrication.

Aviatrix said...

Sure, sure, there are lines, and gravity is awesome stuff. It's just that when I look at my engine, or a part removed from it, it doesn't seem to be riddled with little return lines. Obviously the oil must be pressured to go up, but there are so many paths it can follow, I don't quite see how it doesn't shortcut back, via gravity, before getting to the more distant moving parts. And there's not just gravity involved, there's airflow, which is why I can get oil all over the top of my cowling when a breather line comes loose. Now if only I could put those elves to work cleaning up the belly where the hydraulic line leaked.

Anonymous said...

Hi, There are passages called Oil Galleries that reticulate the pressurised oil to the places it is needed most, for instance Crank Shaft bearings, and it then leaks out of these and runs back to the sump. As the clearances in a bearing in good condition are quite close, there is enough oil pressure left in the gallery to supply all the bearings etc. along the path. Oil punps have an oil pressure relief valve to ensure that too much pressure is not in the system, which may cause oil seals to blow out for instance.

BTW have you tried eucalyptus oil on your sweater stain. It is excellent for removing oil and tar stains.


Angus said...

Elves, pretty sure of that. According to Wikipedia - The German word for nightmare, Alptraum, means "elf dream". The archaic form Alpdruck means "elf pressure".

Think of the alptraum caused by a lack of alpdruck...

fatfred said...

You have gotten a lot of good partial answers so of course I feel obliged to stick my oar in.
The car system of wet sump lubrication relies on gravity to return the oil to the bottom where it is picked up by the pump, circulated through the full flow filter, and sent to the center of the crankshaft and the oil passages drilled into the block to lube the cam, valve gear and assorted parts. It drips back down in a controlled manner designed to bathe the whole interior of the block and transfer heat to the inside walls of the block and then to the coolant in the water jackets. The return path is almost as important as the supply to ensure even cooling of the moving parts.
In a dry sump system like a race car or an aircraft engine as the engine itself is sometimes moving around so violently that the oil in a wet sump would be tossed away from the pressure pump inlet. The dry sump system uses at least one suction pump about 3 times the capacity of the pressure pump. The suction pump keeps a constant partial vacuum on the crankcase and sends the oil/air mix it picks up to an oil storage, deaereator tank where the foamy oil settles into a liquid.
The pressure pump picks up oil from the bottom of this tank and does the same as the pump in the wet sump system.
All sorts of neat gadgets are used to keep a steady flow of oil to the pressure pump such as baffles in the oil tank and a swivel pick up pipe so you can bank and fly inverted.

Anonymous said...

A well-lubricated post and replies.

A Squared said...

"In a dry sump system like a race car or an aircraft engine....."

The vast majority of horizontally opposed reciprocating aircraft engines have a wet sump system, not a dry sump system.

"The suction pump keeps a constant partial vacuum on the crankcase and sends the oil/air mix it picks up to an oil storage....."

The engines which I am familiar with which have a dry sump system, do not use a partial vacuum for oil return. They are equipped with one or more scavenge pumps which directly pump the return oil back to the oil tank.

A Squared said...

I don't quite see how it doesn't shortcut back, via gravity, before getting to the more distant moving parts

Well, let's take for example the valve train, as that is one of the more complex paths which lubricates the most distant parts. Assume a Lycoming engine with the camshaft above the crankshaft.

There is an oil passage that runs lengthwise along the upper side of the crankcase. This has smaller branch passages that go to each of the bores where the tappets ride. The pressurized oil is directed by the passage into the hydraulic tappet, "inflating" it for proper valve train clearance and flowing around the tappet, lubricating it's movement inside the bore, and lubricating the contact with the cam. Some of the oil is directed by a smaller passage in the tappet, into the pushrod, which is hollow, and has an opening at each end. This oil is pushed through the pushrod to the cylinder head where it emerges in the are of contact between the pushrod end and the rocker, lubricating that point, and also flowing from that point, via gravity down to lubricate the rocker pivoting on it's shaft, and also the end of the rocker where it is actuating the valve. IIRC, there are some engines where the rocker has a passage so that oil flows from the pushrod via the pushrod socket, thru the rocker to emerge at the contact point of the rocker and valve, giving more positive lubrication to that point. In addition to the oil which is directed thru the pushrods, there is also oil that flows around the tappets and then down the pushrod tubes (gravity again) to flow out in a less directed manner over the entire rocker assembly. All of this oil pools at the bottom of the rocker cavity in the cylinder head, where it drains back to the sump (gravity again) thru the oil return lines I mentiond in my earlier post.

Does that shed any light on how the system works?

A Squared said...

In my previous response to fatfred regarding scavenge pumps, that should have read "The aircraft engines with I am familiar with....."

or, correcting for grammar; "The aircraft engines with which with I am familiar...."

Anyway, I wasn't speaking of engines generally, he may very well be correct about oil return on automotive dry sump engines.

Michael5000 said...

You write more entertainingly about ostensibly boring stuff than almost anyone I know.

Anonymous said...

There are no return lines.

All the high-pressure lines end up in bearings, very small holes, etc and then simply "leaks" out to the engine internals and gravity returns the oil back to the reservoir.

Simply the fact that bearings, etc are still leaking a little bit, takes care of the oil circulation.

Of course the leakage locations are carefully designed, to take care as much as possible engine parts are touched by splashing oil and as such lubricated and cooled down.

Oil collects at the lowest location. If this is the "big" reservoir, then it's called a wet sump. If this is only a small reservoir and the oil is low pressure pumped-on right away to another reservoir (for storage and subsequent high pressure distribution), this is called a dry sump. The latter useful when the lower sump does not give a stable oil surface, so for aerobatics.

A Squared said...

Yes, there are return lines on Lycoming engines. Without them the oil in the head would have no path back to the sump. I have personally torn down and reassemblEd these engines, including removing and re intalling the oil return lines. I can assure you with 100 percent confidence that they do exist.

Echojuliet said...

I realize I am a little late to jump in the conversation but... I like the placard. I've spent a fair bit of time the past couple weeks cleaning up paperwork for some of our fleet, including creating a "required placards" document, so we don't have to spend hours digging through all the sources just to sign an item off on the inspection.

majroj said...

This is the first time I have seen the verb "reticulating". Was that "recirculating"? I pictured this basketweave series of channels...

And about the German "alp":üssli_053.jpg/477px-Johann_Heinrich_Füssli_053.jpg

(Apparently "alp" as regards mountains is French/Latin in origin).