Why do I need a catch tank with a dry sump?

[k80rum]

I'm hoping someone can put me straight as to why I would need a catch tank with my dry sump setup....

 

If I have a D/S tank that in theory has an oil capacity of 7 or 8 litres, why do I need to attach a catch tank to the tank vent, and then have the catch tank vent to the atmosphere? I've seen quite a few dry sump setups using a mocal tank or similar.

 

If I just have a small filter attached to the D/S tank vent, which itself could then vent to the atmosphere, I'd do away with the need for a catch tank (and the expense that brings) wouldn't I

 

Thanks for any light anyone can shed.

 

Darren

[davef]

I think you have to consider what's in the gases being vented from the dry sump tank. This will be a mix of air, blow by gases (partially combusted air fuel mix with water vapour), and oil mist. The ratio of these components varies depending on temp, and whether you've overfilled the dry sump (in which case the proportion of oil can significantly increase)

 

On a normal wet or dry sumped car with plenum induction this mix is/can be passed to the plenum, where it goes through the combustion process.

 

On a dry sumped car, on TBs, the catch tank is there to try catch the liquid parts of this mix (basically the oil).

 

Passing it through just a filter will - esp if you overfill with oil - result in an oily mess - and would not be passed through scrutineering (as would result in oil being deposited on the track).

 

The catch tank can be very cheap - some have used large beer cans for this purpose, and I have the standard Caterham plastic tank, though the Mocal one does look more 'bling'.

 

Finally venting the dry sump tank into the engine bay will result in you breathing some of that mix - not very pleasant (on mine the outlet of the catch tank goes down under the car).

 

In summary I think you should have one.

 

Dave

 

Edited by - davef on 11 Mar 2007 20:24:18

[Roger King]

A well designed d/s system with a healthy engine will not throw any oil into the catch tank. However, if the engine becomes sick and loses ring sealing, it might start to chuck oil out of the main tank. In this case the catch tank not only gives a clue that something may be wrong, but it also provides somewhere for the oil to go. Under the same circumstances if you have only a filter, this will quickly clog and then may start to drip oil, or even worse become blocked allowing the whole system to pressurise with unpredictible results.

 

Many scrutineers will spot the lack of a catch tank and you may find yourself banned from the track.

 

Badly designed dry sump systems, such as may occur when a poorly conceived bellhousing tank is used, often chuck oil out anyway!

[k80rum]

Thanks a million guys, that's helped my understanding. I'm having a D/S tank built up and was going to get a catch tank made up to match it at the same time but managed to confuse myself as to what purpose it was serving.

 

I'll go for the catch tank!

 

All the best,

Darren

[paul jacobs]

I can identify with Rogers line about badly designed DS systems, often using a poorly conceived Belltank system, throwing oil out into the catch tank, on my very expensive Cosworth Duratec. No matter where the oil level is in the DS tank, there will invariably be some oil in the catch tank. It's actually quite annoying having to empty the thing every few hundred miles. Is there anything that can be done on this set up?

 

Paul J.

Loud pipes save lives, but quiet ones save your hearing.

[k80rum]

out of interest, What is it in the design of the tank system (in this example the belltank) that constitutes a bad design? I'd be keen to find out what's likely to cause the problem and how to design it out, as Paul asks.

 

Many thanks,

Darren

[davef]

My system (standard Caterham bellhousing tank) chucks oil if I overfill it - once it finds its level there is little oil deposited in the catch tank. Having said that I'd be interested in knowing the short comings of the system.

 

Dave

 

 

[Englishmaninwales]

Roger and Paul

So what is the downside of the CC belltank system, besides small capacity?

 

Malcolm

[metal mickey]

Retains heat around clutch/transmission.

[Englishmaninwales]

Why is that a problem? Are there any known failures as a direct result? I'm only interested as I've just installed the CC system. There's me thinking how compact and clever it all is ☹️. (I know there are shortcomings with the scavenge pump position).

[Sheds Moderator]

You will as others have said get a bit of oil mist in the breathings, I do. Not much though.

 

As for question 2, are there known failings around the fact that you retain heat around the bellhousing, yes there are but not on the K. *Maybe* this contributes to release bearing (CRB) failure, but who knows as this is a Ford item not designed to go there, don't forget early release forks bent on the K DS (nothing to do with the heat, just underengineered), but the big one is on the VX. The heat cooked out the clutch slave cylinder and made it leak fluid, so no clutch. The K has a cable clutch so no issue.

 

Maybe the heat passes into the gearbox but these get hot enough anyway adn in any case they are VERY heat resistant.

[Roger King]

The tank isn't just a receptacle for the oil. It has the important job of separating the oil from the blowby gases that it will have acquired in the engine. One of the most efficient ways to do this is to feed the oil onto the wall of a circuclar tank and allow it to helter skelter around before dropping onto a baffle plate.

 

Just pumping the oil into an irregular shaped tank will not separate the gases out and may make the situation worse due to severe splashing. This splashing will also tend to bounce oil out of the breather, even if the engine is healthy.

 

You need to remove gas so that you don't end up feeding the bearings with an emulsion rather than liquid oil.

[Sheds Moderator]

Good point Roger, this explains why my DS tank feeds the oil onto a sloping plate. It makes sense too as if you shake a bottle of oil the air bubbles take forever to clear.

[shn7]

Isn't that why CC introduced the conning tower?

 

Steve.

West Sussex AO

Not forgetting Percy the Polar Bear

[k80rum]

Thanks Roger

 

I thought I remembered seeing a cutaway of a dry sump tank some time ago that had a helter skelter but wasn't 100% sure.

 

I'd guess that the apollo has a similar arrangement - I'm half tempted to section mine out of curiosity once the dry sump system's in, but think it'd serve me better if I sold it to recoup my costs.

 

 

Darren E

K80RUM Superlight R #54

 

 

[shn7]

Darren,

 

I've never cut one open but I doubt think the Apollo has an internal helter skelter. I think it relies on the fact that the oil is fed in at a tangent to get the flow to run round and down the sides.

 

Steve.

West Sussex AO

Not forgetting Percy the Polar Bear

[k80rum]

got you Steve...Relying on the centrifugal force generated by the pressure of the scavenge return plus the angle it's entering at, to get a kind of poor-mans helter-skelter effect.

 

That's exactly what the design of my tank (or to be more honest - my own implementation of Dave Jackson's one) was going to rely on, but I think I'll chat to the manufacturer to see if we can get an internal ramp built in as it'd probably be the best approach to adopt.

 

Darren

[Englishmaninwales]

Roger

So does the belltank with a conning tower separate the oil and gases effectively?

Malcolm