Maximising Crankcase Negative Pressure

[J.A.W.]

Erosion of aluminium alloy engine components exposed to coolant is generally more complex than 'cavitation'.

Coolant pressure was originally raised in aero-engines due to both atmospheric pressure dropping at altitude,
& when use of high temp glycol-based coolant proved problematic.

Emissions controls favour higher coolant temps, but for best power out-put some G.P. mills
operated at low coolant temperatures, ~1/2 the boiling point of water at sea level..

Cavitation damage to austenitic steel propellers of nuclear subs cruising the high pressure ocean depths is
a major problem.

Not many 4.5 litre hemi Daimler mills were built - largely due to the fact that Jaguar bought Daimler from BSA,
& were embarrassed by the 4.5 V8's superiority to their heavy old long-stroke six..

[Ciro Pabón]

It's relatively easy to detect cavitation, because it breaks metal particles out of the system.

I've seen gas turbines that use a "metal detector" in the oil scavenging system. It's as simple as a magnet with a chip. When cavitation occurs, the metal particles build in the magnet and the chip detects them, thus a warning light comes on.

[gruntguru]

No, the accepted notion of windage losses in an engine crankcase or a gearbox housing are the mechanical losses created when a surface moves through a fluid composed of air with a mixture of oil droplets dispersed within it. The transfer of momentum from the moving surface to the mass of air/oil mixture is much greater when there is a larger percentage of oil entrained in the air (ie. when the fluid has greater density). The windage losses in a crankcase filled only with air would be insignificant compared to windage losses in a crankcase filled with a mixture composed of 30% oil and 70% air.

OK so the presence of oil droplets increases the windage losses and may in fact dominate the windage due to air alone. This does not contradict my assertion that the term "windage" refers to aerodynamic losses and is used with respect to machinery where the "windage" is due to dry air alone (e.g. electric motors).

You should rethink the 30/70 air/oil fraction you mention. Even a wet sump system would need ALL the oil in the pan to be mixed into the air to get close to that number, leaving no oil in the pan, pump or oil galleries. 30/70 may be what you see through a scavenge pump but scavenge pickup points are located where highest concentrations of oil are likely to be.

[riff_raff]

Take a look at section 3.4 of this paper to get an appreciation of just how significant the effect even a small increase in the percentage of oil in the crankcase air volume has on windage losses.

http://www.wseas.org/multimedia/journal ... 54-646.pdf

The liquid oil droplets have far different fluid properties than the air. The transfer of energy from the rotating part surfaces to the air or oil fluid they contact is much greater with the liquid oil.

[riff_raff]

It's relatively easy to detect cavitation, because it breaks metal particles out of the system.

I've seen gas turbines that use a "metal detector" in the oil scavenging system. It's as simple as a magnet with a chip. When cavitation occurs, the metal particles build in the magnet and the chip detects them, thus a warning light comes on.

Ciro- Magnetic chip detectors will only detect ferrous debris. They won't detect non-ferrous debris like aluminum, plastic, bronze, austenitic stainless, titanium, magnesium, ceramics, etc.

[J.A.W.]

Take a look at section 3.4 of this paper to get an appreciation of just how significant the effect even a small increase in the percentage of oil in the crankcase air volume has on windage losses.

http://www.wseas.org/multimedia/journal ... 54-646.pdf

The liquid oil droplets have far different fluid properties than the air. The transfer of energy from the rotating part surfaces to the air or oil fluid they contact is much greater with the liquid oil.

& a quote from your cited paper perhaps, r-r..
.. that actually relates to "crankcase air volume" & the O.P. - could even be.. ..more useful ?

[Brian Coat]

To OP:

"* Why would there be a tube running between the oiltank and sump - surely this prevents a low pressure occuring?"

Unless I am mistaking you, there's often NOT such a pipe in dry sump systems.

If you google "dry sump circuit diagram" you will see loads of typical installations and I hope this will make things clearer.

The discussion about why you want negative pressure is interesting. Put simply - any molecules that the crank and rods have to move out of their path will consume valuable energy, so its better to suck them out of the way!

Of course, large drops of dense, viscous oil has more potential for drag than air ...