Question Vaccume

[QQMOON]

Ok Diesel engine why do they run a vaccume pump why does the motor not create vaccume Sorry I dont understand
JR

[Flat Ernie]

Uhhh...they do create vacuum. The pumps are usually because they're running something requiring constant-steady-state vacuum. Most industrials actually run an air pump - not a vacuum pump - for the brakes.

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[willowbilly3]

Engine speed is controlled by the fuel delivery and there is no butterflys or other restrictions in the intake tract so it is hard to pull any vacuum signal strong enough to use for accessories. In the aircraft world it is called manifold pressure and I won't even go there as it gets a little confusing Basically when a piston goes down it creates an atmospheric pressure that is much lower than the basic atmospheric pressure we live in. The vaccum signal is just the resistance to this difference in pressure that the restriction of the intake tract creates. If you just had a piston going up and down in it's bore with no cylinder head there would be very little or no measurable vacuum and this is basically the principle that makes diesels have to need a pump

[Flat Ernie]

willowbilly, I think your explanation is slightly confusing...at least to me.

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Engine speed is controlled by the fuel delivery and there is no butterflys or other restrictions in the intake tract

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True statement. Your throttle typically moves the fuel metering device (barrel, rack, etc) & doesn't control airflow per se.

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so it is hard to pull any vacuum signal strong enough to use for accessories

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I disagree with this statement - hold your hand up to the inlet of any running diesel (Cat, Cummins, Detriot - take yer pick) & come back & tell us there's no vacuum... http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif

What I don't know is what you intended to illustrate with this statement - I think you wandered off into metered vacuum like most of us think in terms of a 'timed vacuum port' for a distributor - like the difference above/below throttle plate. I don't think this is what we're after here though...but it might be!

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Basically when a piston goes down it creates an atmospheric pressure that is much lower than the basic atmospheric pressure we live in

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WARNING - semantics coming at you! When the piston moves, it creates a pressure differential between atmospheric pressure & the pressure observed in the cylinder. This pressure differential is affected by many variables - bore & stroke size (all else being equal, a larger engine will have a greater pressure differential), engine speed, valve timing (even Detroits that only have exhaust valves http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif ), the ability of the inlet tract to adequately fill the volume of the cylinder and equalize the pressure in the time allotted and atmospheric pressure (so Denver & Miami have different effects).

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The vaccum signal is just the resistance to this difference in pressure that the restriction of the intake tract creates.

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I'll go along with that. I don't care for the term 'resistance', but it does describe what's going on. (it's really the ability of the inlet tract to adequately equalize the pressure in the time available, but that's semantics again! http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif )

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If you just had a piston going up and down in it's bore with no cylinder head there would be very little or no measurable vacuum and this is basically the principle that makes diesels have to need a pump

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Here's where you lost me. I agree with your comparison of a piston moving in a bore w/o a head having no 'resistance' to that which generates the vacuum (It's not really a vacuum, but a pressure differential, but that's semantics again), but fail to see your leap to a diesel comparison. That's what confused me & I'm not sure where you're going with it.

My guess on your direction stems from the fact that most industrial diesel engines control engine speed with direct fuel delivery based on fixed relationships (cams, gears, etc) and not metered fuel delivery dependent on a vacuum signal through a primitive carburetor. Consequently, diesel theory focuses on fuel delivery & thinks in terms of pumps, injectors, & fuel timing instead of vacuum signal, venturi size, & runner volume.

BUT that doesn't mean diesels don't generate vacuum.


Tangent time:
Having been a heavy equipment mechanic for several years before going in the Air Force, I can't think of too many applications that had a vacuum pump to begin with. Air can be pressurized quite effecitvely, but a vacuum can be pulled only so far no matter how effective your vacuum pump is. For example, you can push water up quite high with pressure in a tube or pipe. You can't, however, pull water much beyond about 30' vertically with vacuum no matter how good your pump is. Try it sometime.

That's a long way of saying that running a vacuum pump is inefficient. Virtually anything you can do with vacuum, you can do better with pressure - hence why rigs have air brakes & air-powered accessories (and not vacuum).

Going further off on a tangent, I'd take hydro-boost brakes over typical vacuum boosted brakes anyday... http://www.jalopyjournal.com/ubbthreads/images/graemlins/tongue.gif


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[willowbilly3]

I was attemting to explain the reasonning behind diesels needing a vacuum pump. While you did a good job at dismantling some of my semantics maybe you can tell me what the manifold vacuum reading on an idling 6.2 non turbo chevy is. Of course an engine will suck in your hand and that is vacuum and while it was sucking you in there might be a spike on the manifold vacuum guage because you caused a restriction in the airflow which is what it takes to get enough vacuum to run accessories. And which most diesels don't have enough of, hence the need for an auxillary pump.
There are people more adept at explaining the theory, as you are but I thought I could take a stab at it since no one else had.
You could have just posted your version of the facts in a more simple and understandable manner without doing it by putting me down, I was just trying to help.
It doesn't make you look smarter to drag someone else through the mud, even if you are. If you want to be ekevated you don't do it by lowering others.