WTF is a catch Tank and do I need one ?

[Bush65]

If you want to do something useful with that bung on the bottom of the catch tank if you have an engine that chucks half of its oil up into the catch tank each event (more for high revving engines on circuits) pipe it back to the sump but it is essential to get the pipe in under the oil level in the sump otherwise the crank pressure will impede the drain back. Under the oil level there is no positive pressure.

The catch tank then still does its job of oil separation but doesn't end up like an oil reservoir

You should rethink what you have stated there about pressure above and below the oil level.

[zoltan]

You should rethink what you have stated there about pressure above and below the oil level.

This is not some theory I have but something I put into practice with catch tanks I have made and fitted successfully to customer cars. If the oil returns above the oil level then crankcase pressure exploits the return line and impedes return. Put the oil in under the oil level and there is no effective pressure impeding return. Sure if you had no other breather on the engine then the oil would run up the return line

We've tried it both ways and under the oil level works, above doesn't.

Or maybe it is magic?

[Lara]

I agree with both of you!

In theory it should,t work like it does,

But in practice the fact that there is only a relatively small pressure in the crank case and not enough to blow the oil up the pipe if fitted below the oil level, this in turn allows the oil to run down the pipe, unaffected by the airflow it would normally encounter if above the oil level.

In practice it does work quite well as Zoltan says.

Lara.

(not 100% sure on that theory but it's the only explanation I can find) and I was on top of the world at the time looking down on my creation!

[Bush65]

This is not some theory I have but something I put into practice with catch tanks I have made and fitted successfully to customer cars. If the oil returns above the oil level then crankcase pressure exploits the return line and impedes return. Put the oil in under the oil level and there is no effective pressure impeding return. Sure if you had no other breather on the engine then the oil would run up the return line

We've tried it both ways and under the oil level works, above doesn't.

Or maybe it is magic?

Pressure in fluids is simple physics - you are trying to convince us the physics that has been taught for hundreds of years is wrong

If you can't understand the physics and how the pressure in the crankcase acts upon the surface of the oil in the sump and adds to the hydrostatic pressure at any depth in the oil, then have a look at how a basic mercury barometer works - how atmospheric pressure acting on the surface of the mercury changes the pressure in the mercury making it rise, thus becoming a measure of the atmospheric pressure.

At any depth below a static fluid level, the pressure is equal to [ (fluid density x depth of fluid above that level x acceleration due to gravity) + pressure acting on the fluid surface ]

[Part1cle]

Perhaps its because oil is being sucked out of the sump, presuming its not dry sumped and has a reservoir in there, and on the examples where it worked that was having the effect of allowing the oil from the catch tank to travel down into the sump?

While there is crankcase pressure when the engine is running the oil is continuously cyclicng around the engine. When the engine stops any oil in the pipe will run down into the sump anyway, perhaps thats what happens irrespective of pressure.

[TheRecklessEngineer]

I think what zoltan is trying to say here, is that if your sump return is above the oil level then any pressurised crankcase gasses will exit up the sump return line and will seriously impede the flow of oil in the other direction.

However, with the return below the oil level, this will not occur.

[zoltan]

Pressure in fluids is simple physics - you are trying to convince us the physics that has been taught for hundreds of years is wrong

If you can't understand the physics and how the pressure in the crankcase acts upon the surface of the oil in the sump and adds to the hydrostatic pressure at any depth in the oil, then have a look at how a basic mercury barometer works - how atmospheric pressure acting on the surface of the mercury changes the pressure in the mercury making it rise, thus becoming a measure of the atmospheric pressure.

At any depth below a static fluid level, the pressure is equal to [ (fluid density x depth of fluid above that level x acceleration due to gravity) + pressure acting on the fluid surface ]

Why the hell are you arguing with me?

I don't have a catch tank problem.

Mine works!

I posted that an effective way to get the oil back in the sump is to put it in below the oil level. No need to enter into a discourse on pressure and physics. This is basic stuff that works. You can complicate the discussion and confuse the hell out of everyone if you wish. (Maybe you ought to check the cheese thread)

Now I don't know if you've ever fitted a catch tank to anything but I have , many times so I have learnt by trial and error what works, what doesn't - simples

We build high revving 4 cylinder 16 valve race engines, small capacity. 1480cc making 195bhp running to 9-9500rpm and they breath - alot. So we have learnt how to prevent the catch tank filling up with oil through trial and error.

Anyway, I don't want to be falling out with anyone so I will go and do something more useful and leave you to it.

Peace

[Lara]

Pressure in fluids is simple physics - you are trying to convince us the physics that has been taught for hundreds of years is wrong

If you can't understand the physics and how the pressure in the crankcase acts upon the surface of the oil in the sump and adds to the hydrostatic pressure at any depth in the oil, then have a look at how a basic mercury barometer works - how atmospheric pressure acting on the surface of the mercury changes the pressure in the mercury making it rise, thus becoming a measure of the atmospheric pressure.

At any depth below a static fluid level, the pressure is equal to [ (fluid density x depth of fluid above that level x acceleration due to gravity) + pressure acting on the fluid surface ]

Hi Bushy,

I don't think that anyone is saying that the physics and theory are wrong.

What is actually happening in the system described is difficult to know for sure but it does work. So somewhere in the line something has changed in order to make it work. That is fact, but "What" is the question!

I think this.

In your barometer example you have a higher pressure acting on the fluid, forcing the fluid up to a lower pressure area. Natural physics. You have a sealed capsule on the top with 1000mb pressure and varying pressure on the base fluid, so creating a moving column.

In the engine, I think that the pressure above the sump oil (vented to the catch tank) and the pressure inside the return pipe dipped into the oil (also vented to the same catch tank) will stay the so close as not to matter. (maybe enough to raise the oil in the tube by an inch) a pressure difference of enough to get gas flow but not a lot else unless we block the other vent to the crank case!

If we leave it like this the oil in the catch tank will drip down this pipe. and the level will equalize with the new level in the pipe.

If we have the drain pipe above the oil level, the pipe will be subject to airflow, (just as the main scavenge pipe) thus carrying with it small droplets of oil that may want to go down and thus hampering the effectiveness of the drain tube.

I have used both systems and they both work, however the system with the tube returning below the oil usually spits less out of the catch tank breather than the other, and keeps the tank empty even at full throttle max revs, where as the other works less and less efficiently the higher the engine breathing! but perfectly at lower revs / loads etc.

However,

With the drain below the oil level,

I have also seen on engines with damaged rings and or holed pistons etc, the oil getting blown up this tube like a fountain!! we do need to treat each case with care and thought! We would not want this on a Diesel

Lara

[Bush65]

I never said anything for or against where the oil drain should return.

Initially I suggested zoltan re-think his claims that the pressure below the oil level was less than in the crankcase above. I never questioned whether installing the catch can as suggested works.

He replied by re-stating the false claims (claims that defy physics) and added the rolleyes emoticon, which suggested I was wrong to question him.

Crankcase pressures are not always high - some engines can have the catch can hosed up so as to create a vacuum in the crankcase under most conditions.

Returning the oil below oil level works, but perfectly good installations can have the oil return into the crankcase above the oil level, but it is best to have a non-return valve in the drain line - the Land Rover 300Tdi is an example of an engine that has the oil drain above oil level.