View Full Version : Vac/Bernoulli.........How?

blue up
2nd Nov 2003, 00:29
Strange one, this....but

I've been trying to get to grips with the science of the bernoulli theory to use it in a particular application. I've a vintage car and want to use the exhaust to create a vacuum (ish) to draw fumes out of the crankcase to prevent oil leaks. I've drilled several holes and dropped in several tubes at different angles in an attempt to make the airflow pull air down the pipe.
Can an yone help me decide exactly where the best draw will be (downstream of the thinnest point?) and whether the pipe should point downstream or accross the flow.


Rob (757/767 FO)

[email protected]

2nd Nov 2003, 00:55
are you sure that you can reduce the pressure in the exhaust system to value lower than the crankcase without causing to much back pressure in the system?

2nd Nov 2003, 01:35
Do you have a venturi in your exhaust pipe?

blue up
2nd Nov 2003, 21:08
The system I saw in use pulled 0.5 bar pressure at speed from the crankcase of an otherwise sealed engine block (dipstick replaced by rubber plug etc). The way it works is the same way as the venturi on an old Piper. The fast flow of air through the venturi drew fumes out and thus pulled oil BACK through the tail of the crank. There is a venturi in the exhaust where two pipes come into one at a Y-shaped joint. I can't decide if the little pipe needs to go at a piont of largest cross section or soewhere else. There is an antibackfire valve from a brake servo in the system to stop a tailpipe backfire from igniting the oil vapours in the crankcase.

So.....Do I need the little pipe at the point of largest cross section and does it need to point downstream or at 90 degrees to the flow.

Has anyone got a pictur of a venturi off an old Piper????

3rd Nov 2003, 01:05
The static pressure of the gas leaving the cylinders is higher than atmospheric. This means that your venturi will need to produce quite a drop to get it much below crank case pressure. It can be done but there are easier ways of providing a crank case breather.

Why not connect the tube from your crank case to the inlet manifold like most car manufacturers do? This also has the advantage of burning the crank case fumes before ejecting them into the atmosphere. Crank case fumes often contain petrol vapours, so introducing them directly into the exhaust might produce a form of (noisy but ineffective) afterburning!!!

3rd Nov 2003, 04:25
Across the flow is probably better. Remamber that you are not dealing with a steady state problem and that there is probably reversion at some points in the system.

After an excellent landing you can use the airplane again!

Dick Whittingham
3rd Nov 2003, 06:07
Keith is right, but if you want to go ahead with your exhaust venturi you will find the minimum pressure at the smallest diameter of the pipe, and it doesn't matter much whether you have a flush tapping or a bent pipe pointing downstream. You might hit trouble pointing upstream. Careful with crankcase pressure, too much above or below atmospheric can both cause problems.

Dick W

blue up
4th Nov 2003, 02:52
Thanks guys!!
This engine has always suffered from excess crankcase pressure. It was designed as an APU for a British Army battle tank, where leaks were not a problem. There is not a single non-mechanical seal on the whole unit as it relies on a reverse archimedes screw thread on the crank ends to "wind" the oil back into the block.
Using a fast acting one-way valve from a brake line seems to be enough to prevent any reversion, but I've not had anywhere near as much "vac" as others have had. I've just tried running the end of the stub pipe further downstream into the tail of the "y" piece to the thinnest part and flared the end to further reduce the cross-section of the pipe. Should be able to fire it up again in ther next few days.

Fingers crossed!!