crankcase vacuum. why?

[Duff]

i hear people say that you get power gains by having a partial vacuum in your engine.
im trying to understand why.
i can only think of 1 advantage, and that is windage losses being less in lower air pressure. all other thoughts i had resulted in everything cancelling out.
in a vacuum 2 the pistons on the downstroke will have less resistance, but the 2 going up will have more.
there would be slightly more blowby which would be a power loss

an explanation or confirmation would be great

Cheers

[Brian Smith]

the 2 going up will have more.
there would be slightly more blowby which would be a power loss

in a vacuum 2 the pistons on the downstroke will have


And a result would be that less oil will make its way into the combustion chamber. Oil lowers the fuels octane rating. This lets you run slightly higher compression assuming some oil was making its way to the chamber before applying a vacuum.

[Brian Smith]

and oil ring tension can be reduced resulting in less friction.

Applying a vaccum to the crankcase may also help dearrirate (get rid of bubbles in) the oil. This will cause the oil to spend less time being smacked around by the internals. (The froth takes up less space/ sits lower in the pan, away from the crank)

[Wright D]

I have heard that a vacuum in the crank case will reduce blow by. I could be way wrong, some one once told me that piston rings sealed better when there was a vacuum in the crank case. It could be that the gas pressure in the combustion chamber pushes the rings out, sealing them to the side of the cylinders and the piston. If the pressure difference is increased, then the sealing effect might be increased too?

Someone that knows what the hell they are talking about clue us in, cause I'm just guessing.

[VFR750R]

You guys are pretty close. Pan vacuum lowers the pumping losses on the bottom of the piston because the piston moves so fast it creates a head of air on the underside on the way down and sucks a low pressure on the way up. The difference is the power loss. Suzuki GSX-R's try to combat this by cross ventilation between cylinders which allows the air going down to find the piston going up. The lower the density of this air, the less power it takes to move it.

There is also the benefit of keeping the ring sealed better during strokes other then power when the pressure difference across the compression ring is small. This results in better draw on intake and less oil consumption which has the benefits listed above.

I've heard of dozens of cases of 20-30hp on a street engine by hooking up a vacuum pump.

[RawePower]

20-30 hp on a street car seems a little excessive, but I don't have any knowledge to disprove that. A 30 horsepower gain is very common in high power drag racing applications, with 10 to 12 inches of vacuum. With too much vacuum, you can create an oil starvation problem. I believe the advantages of gas ported pistons are also magnified with a vacuum pump.

[VFR750R]

Oops forgot about the max vacuum deal. If you have a wet sump you can run more then about 12in of vac like Ben said otherwise you'll cavitate your pressure pump inlet and....kaboom. In racing applications with a true dry sump there is diminishing returns but generally more is better. I read once in Hot Rod, where they put a Moroso vacuum pump on a 5.0L mustang and got 27hp. Not that it means any engine would get that gain, but it obviously shows that almost any engine would see some measurable gain that can't be overlooked. I've also been told of 20+ on old NASCAR Busch V6 engines by putting on a vac pump.

[Garlic]

I have some real testing experience with this, and from that my conclusions are if you start with engine not designed for a low pressure sump, there are few benefits, especially when power is concerned.

The main benefit is less air resistance for all the moving parts.

The next benefit is better ring seal. But, an engine designed for no negative pressure already has rings designed for that, so ring seal is improved, but was good to start with. Negligible impact on a good engine.

The only real effective benefit for a negative pressure crankcase is utilizing it with low tension rings, that do not adequately seal without a pump. These rings will reduce friction measurably but will only seal with a negative pressure crankcase.

I remember seeing a Hot Rod article with a Big Block engine that made 40 more horsepower with a vacuum pump. What they barely mentioned in the article was that it DID have special rings for negative crankcase pressure. This article has caused lots of confusion the last few years. I haven't seen the 5.0L article.

Other benefits of a vacuum pump is if you use a pressure sensor and log it, it can give you good insight to problems before they crunch your engine.

[LCP_KL]

Hello, i just thinking out loud here and feel free to shoot me down. but what do you think of the below senario?

-build a cylinder around the diffuser of the restrictor, with end caps of cause.
-there will be perforations around the diffuser, so air from this cylinder can pass into the diffuser. but the whole apparatus is sealed from outside air of cause.
-pipe the crankcase ventilation air hose to this cylinder (with air already suitably baffled to remove oil from it).
-the desired effect is that the low pressure of the diffuser will suck air through the perforations, and this air will come from the crankcase. 2 benefits may occur:
1. you delay boundary layer separation in the diffuser, allowing for wider diffuser angles and thus shorter diffuser lengths for packaging.
2. you have a self-sufficient "vacuum pump", increasing in efficiency as rpm increases.

it will probably fail the rules right? but technically, the air that enters the diffuer orginally went through the restrictor right? since crankcase air is effectively piston blow-by (assuming an air tight engine). so if the rules say all air that the engine uses has to go through the restrictor, this is the case... no?

help me find a loop hole in my thinking. judges?



KL
- having PMS (post-Mitchigan Symdroms). sigh... life ain't the same after FSAE...

[VFR750R]

The problem will be the throttle which is before the restrictor. Steady state your idea works although it's doubtful you would make more power doing this as you are using the pistons to pull the pan vac which is the exact reason to pull pan vac. You trade one pumping loss for another, plus to my knowledge EGR hurts power.

Back to the throttle. Since you've linked the crankcase with the intake after the restrictor you just increased your plenum volume by the volume of your crankcase (although with a small orifice connecting them). Transiently, you created a nightmare for yourself. At low throttle settings it will be very hard to control idle, and during prolonged idle the entire crankcase would be sucked down to manifold pressure and when you opened the throttle you would actually be refilling the crankcase back through the holes you are using to pull a vacuum, before the engine got to an rpm where it needed the extra air. Lowering your horsepower more, and creating a lag of sorts.

For all of you thinking about a one way valve or computer controlled valve...remember that you are allowed one manual throttle and an electronic valve between the crankcase and intake might qualify as another throttle.

[Chris Davin]

Sorry to burst your bubble, but rule 3.5.1.5 explicitly prohibits your system:

"Any crankcase or engine lubrication system vent lines routed to the intake system must be connected upstream of the intake system restrictor."

Another point: when you get a chance, take a look at a textbook that deals with compressible flow, and read the section on the behavior of converging-diverging nozzles. Unless you are getting supersonic flow through the diffusor (the design condition for a jet engine or a rocket motor), the static pressure of the stream will be lower in the throat than in the diffusor. The best any Formula SAE-legal engine could hope for would probably be somewhere between the choke point and the supersonic design condition, at very high RPM. In this case, you'll get a pattern of shocks downstream of the throat and the flow will be lossy and unpredictable, so you wouldn't want to be operating your engine in that range anyway - you'd be better off using the throat.

[LCP_KL]

"Another point: when you get a chance, take a look at a textbook that deals with compressible flow, and read the section on the behavior of converging-diverging nozzles. Unless you are getting supersonic flow through the diffusor (the design condition for a jet engine or a rocket motor), the static pressure of the stream will be lower in the throat than in the diffusor. The best any Formula SAE-legal engine could hope for would probably be somewhere between the choke point and the supersonic design condition, at very high RPM. In this case, you'll get a pattern of shocks downstream of the throat and the flow will be lossy and unpredictable, so you wouldn't want to be operating your engine in that range anyway - you'd be better off using the throat."

took that course some time back. correct me if i wrong, cos the only thing i can remember is that supersonic flow (and shock waves) will be present in the diffuser when the pressure in the plenum (in this case) is very low. something about Pb<Pe. yes, this will never happen in FSAE, cos our Pb will never be that low, or our area ratio that high. so i dun think you will get a pattern of shocks downstream of the throat even with this proposed set up. in fact, the addition of bleed air into the diffuser section will make the Pb higher. yes, i agree the pressure in the diffuser is higher than in the throat, and ti increases steadily towards the plenum. but what i was proposing is bleed air just to re-energise the boundary layer and reduce the effect of the adverse pressure gradient. i.e. many small holes. not unlike what you see in external aerodynamics.


to VFR750R: interesting description about the situation when the throttle is sudddenly openened. with air flowing to the crankcase when air is really needed in the cylinders. unless you somehow have a "dumb" valve which has high losses one way and low losses the other... which isn't really a "manual" throttling device...


oh well. this is all in vain, cos the rule prohibit it anyway. guessed i overlooked that rule. sorry. sigh.


but it does bring about an interesting totally fictional idea of locating this "perforated" restrictor totally encased in the actual plenum itself. make the whole thing, say 3 liters. so when the throttle is opened from idle, the "lag" you feel is probably what you get with a 3 liter plenum. of cause, you end up with funny re-circulating flows and vortices in the plenum at high rev and together with energy lost due to air squeezing through the perforations, the air's energy is dissipated as heat. which is what you dun want. seen several set-ups of restrictor-in-plenum designs in FSAEs the past 2 years. wonder if their restrictor are perforated... or did anyone test this.

oh well... just aimless thought experiments from a person with PMS. hahaha.


KL
- having PMS (post-Mitchigan Symdroms). sigh... life ain't the same after FSAE...

[Dan G]

Um, a simple check valve (PCV so that its large enough) would solve the crankcase = more plenum problem. Allow air to only flow out of the crankcase, no more massive plenum.

I don't know if most people are missing this, the biggest reason the turbo shaft seal leaks is because of a pressure differential across the seal. If the bearing housing is significantly higher pressure than the compressor wheel side, you'll get leaks.

With a line connecting the crankcase to the intake AFTER the throttle, you should be able to equalize this pressure anytime the throttle is closed (and the delta P across the restrictor is near zero). Although "negative" compressor surge (high vacuum on the inlet) would still be a problem.

EDIT: I think I got my turbo geometry mixed up. The compressor wheel-side of the shaft seal should see approx. equal pressure as whatever the turbo is sending out. So if there's any boost, thats what it sees, correct? And in that case, boost = no oil leaks since the compressed air would try to be forcing its way into the bearing housing (assuming crankcase pressure ~ ATM).

[Duff]

so general concesus is that, hyperthetically, if we could pull 15in/Hg vacuum in the crankcase, there would be no visible power gains on a dyno?
sounds about right, but i guess its still worth having ago, for thesis content if for no other purpose http://fsae.com/groupee_common/emoticons/icon_wink.gif

[VFR750R]

A true dry sump with 18inches of crankcase vac with lower ring tension could be as much as 4hp in a FSAE car.
The way I figure it, it's not worth doing just a vac pump, but if you're doing a dry sump, it's worth the extra stages to get the vac since you already have done the hard part and spent the time and money getting the pump/tank/pan/ect.