Crankcase vacuum
- Thread starter TTM
- Start date
George Elliott
New member
No, I don't know.
But a negative pressure in the crankcase is a benefit at higher revs and engines like an RS4 audi V8 have a vac pump I understand in order to ensure there is a constant negative pressure.
Positive pressure obviously leads to oil leakage and air resistance to piston movement. (guess why our dipsticks are designed to be so difficult to remove [8|])
I vaguely recall some effect relating to the piston ring sealing too.
Someone will have a millibar value no doubt.
....You getting the limiter reset to 9000rpm TTM? [
]
George
944t
But a negative pressure in the crankcase is a benefit at higher revs and engines like an RS4 audi V8 have a vac pump I understand in order to ensure there is a constant negative pressure.
Positive pressure obviously leads to oil leakage and air resistance to piston movement. (guess why our dipsticks are designed to be so difficult to remove [8|])
I vaguely recall some effect relating to the piston ring sealing too.
Someone will have a millibar value no doubt.
....You getting the limiter reset to 9000rpm TTM? [
]George
944t
bertelli_1
New member
You will never have a vacuum inside the engine, its inevitable that some of the cylinder pressure will leak past piston rings / valve seals. Its even worse on a turbo car due to the increased cylinder pressures. This is why all modern cars have oil breathers to allow the excess pressure back into the intake.
When I used to work in engine testing we'd connect a gas meter to the oil breather & measure the flow to detect any ring wear. Unfortunately I can't remember the figures.....
When I used to work in engine testing we'd connect a gas meter to the oil breather & measure the flow to detect any ring wear. Unfortunately I can't remember the figures.....
robwright
New member
My diesels at work, Electro-motive 3300HP two stroke v12 units have a yearly engine inspection. As well as ring and liner condition, blow-by test (connecting 60 psi to the cylinder at TDC and listening for leakage), and compression test we also connect a manometer direct to the crankcase when running. We then measure the pressure at idle 200 RPM and full chat 904 RPM. I think the maximum allowed value is -8.5 inH20. If exceed this can indicate worn rings. In the case of my engines the crankcase breathes via an oil seperator and an ejector assembly fed by charge air pressure from the engine side of the aftercoolers into the exhaust stack. So the test can also indicate blocked seperators, ejector or exhaust blockages. You could measure the pressure with a suitable manometer connected to the dipstick tube. They also have a crankcase overpressure detector, which will shut the engine down in the case of extreme crankcase pressure. This usually happens when we blow a cylinder (usually a dropped valve), comprimising the liner and it empties all 1000 litres of it's coolant into the crankcase via the massive hole that used to be the top of the piston []
]George Elliott
New member
Never is a very long time.........[]
]George Elliott
New member
Rob, that negative pressure is induction generated i take it? Mighty engines, very interesting.
I recall being in a WW2 Sherman tank with twin engines, each supercharged 2 stroke. Do you know i cant recall if they were diesel or petrol, It was noisey[]
Any developments TTM?
George
944t
I recall being in a WW2 Sherman tank with twin engines, each supercharged 2 stroke. Do you know i cant recall if they were diesel or petrol, It was noisey[
]Any developments TTM?
George
944t
bmnelsc
New member
Sorry - forgot about the article in Excellence magazine about oil/air separators. According to this article, Porsche provides no specifications for vacuum with properly working oil/air separators but they recommend using BMW test values of approximately 4.5 inches of vacuum at idle. This article appears to be about 911s only but I'm sure the principle is applicable to our 944s.
Remember that suction is applied to the oil/air separators via the intake manifold - we don't have a vacuum pump on the 944s.
Remember that suction is applied to the oil/air separators via the intake manifold - we don't have a vacuum pump on the 944s.
TTM
Well-known member
Bruce, thanks for the info.
The problem here is to link the intake manifold to the AOS on a turbo engine, which makes more vacuum in the intake when the engine is idling than a NA engine.
Georges,
I will update the Engine Build Progress thread in the coming weeks, but you can read the "Smoke at idle" thread on Rennlist to see what I've been working on.
The problem here is to link the intake manifold to the AOS on a turbo engine, which makes more vacuum in the intake when the engine is idling than a NA engine.
Georges,
I will update the Engine Build Progress thread in the coming weeks, but you can read the "Smoke at idle" thread on Rennlist to see what I've been working on.
robwright
New member
ORIGINAL: George Elliott
Rob, that negative pressure is induction generated i take it? Mighty engines, very interesting.
I recall being in a WW2 Sherman tank with twin engines, each supercharged 2 stroke. Do you know i cant recall if they were diesel or petrol, It was noisey[
Any developments TTM?
George
944t
IIRC Sherman's were petrol engined. Knicked from a plane if I am not mistaken.
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