Mommy, where does engine vacuum come from?

[BLINGMW]

Well I give up. I'm sure I could buy a book or something, but I'm not gonna do that, asking a few others gets me nowhere, and internet searches have failed me. I was even desperate enough to send a request to HowStuffWorks, with no result. Everyone seems to know how to test it, what it does.... but.... Where does engine vacuum come from? and where/why is it used? How (or is) it regulated? OH! And why don't diesels make a vacuum?
From what I've gathered, it comes from the air rushing through the intake. Makes sense that if you hooked a hose into that, the air going by would create a vacuum, and it would increase as load increased, like it does in a car. Case closed? Somehow I don't think so, because why couldn't a diesel do the same thing? And I understand how it's used for driving all kinds of things-- brake booster, timing advance (on old cars like mine), cruise control, all kinds of fun stuff. But why is the top of the head under vacuum? Just to make sure fuel/oil vapors don't build there? Kind of the same reason the crankcase is kept under light vacuum (as regulated by a PCV valve in many cases, which itself is even vacuum driven?) ? Mostly I want to know where it comes from. Hook me up. I hope at least a few others don't know and this isn't just a super dumb question. :lol:

[Mike]

It comes from Oompa Loompa Land... Duh!!!

[Beej]

Generic understanding.

Yeah, the intake generates the vacuum (as the air gets sucked in from the outside through the engine). The breather line from the valve cover is just to vent the vapors, which ingeniously are vented back into the intake after the filter, so hot, greasy air goes into the engine and can build up in the combustion chambers and whatever (see how greasy your TB/intake boot are). It basically does vent the crankcase though. M3's came with a vapor collector though, which would be a cool thing to retrofit.

I can't really see how diesel's wouldn't have a vacuum. Maybe something wierd with the super high compression ratios or something? It still has to suck in air...

It's not used for cruise control, AFAIK.

Just do a 2002 motor conversion. No crappy vacuum advance, just a filter on the valve cover...good times.

[.RJ]

It still has to suck in air...

Air isnt sucked in. It moves due to a pressure differential.

It's not used for cruise control, AFAIK.

On some cars it is. Some cars use vacuum to actuate door locks.... audi still does.

[DierwulfBL]

suc┬Àtion ( P ) Pronunciation Key (skshn)
n.
The act or process of sucking.
A force that causes a fluid, solid, or gas to be drawn into an interior space or to adhere to a surface because of the difference between the external and internal pressures.

the word suck works you nub

[.RJ]

the word suck works you nub

Maybe for your mom.

There's a distinct difference. Channing you took Thermo, you should know this

[CaptainHenreh]

Diesels don't make a vacuum because they are throttled by fuel? I.E. they are at WOT all the time, so there is less of a pressure differential between the "atmosphere" and the internal environment of the engine. Because diesels do create some vacuum, just not enough to do anything interesting with, so you need a vacuum pump.

[.RJ]

Otto Cycle:



Diesel Cycle:

[Feersty]

ISAT 310?

[.RJ]

Unfortunately

[white_2kgt]

Well it looks like you have figured out that the vacuum comes from the pistons moving up and down and the air flowing across the intake manifold (google Manifold Vacuum for more specifics). Now, why that differs from a diesel. In the simple terms (sorry, I'm not an ISAT major) a gas engine is 'air throttled', when you stomp on the 'gas' pedal (incorrectly named) you allow more air into the engine, however a diesel engine is fuel throttled. A turbo-diesel is free to intake as much air as it's intake conduits and turbocharger can supply for the load on the engine. This means that the output of a diesel engine is fuel throttled. So for the most part the diesel engine creates no vacuum while under load, it does produce vacuum at times, but nothing that is really useful like in a gas engine.

-c

[KPWSerpiente]

^^ what he said.

And remember kids, god invented vacuum so your boost gauge would have something to display during low engine load situations.

-T

[.RJ]

I should install a b00st gauge on my R

[Maengelito]

I should install a b00st gauge on my R

next to the vtAk light? you really do have b00st rot...

[BLINGMW]

Thank you for the scary, irrelavant diagrams RJ! :wink:

I think I get what you guys are saying though. The vacuum is created between the pistons and the throttle body, and it's really only there because of the restriction of the throttle body. Diesels don't really have a throttle body that restricts the airflow, so there's no (or little) vacuum. Also makes sense that when the throttle is stabbed, vacuum drops to close to zero, and it's at it's highest at idle when the restriction is highest (I said that backwards before). I like it! Thanks!

Now as far as the difference between suction and a pressure differential, I guess I missed that one. Sounds like semantics to me, but I don't really know if I have to completely understand that. A piston, when moving down, with intake valve open, is creating a low pressure area that demands the "sucking" in of more air. TB restricts that, so the space between the top of the piston and the TB never reaches atmospheric pressure, ie, a vacuum. Good enough. Feel free to explain more RJ if I'm really missing something there.

I would like a vacuum gauge on my E30 come to think of it, sounds like it'd be fun to watch instead of the road... 8)

[.RJ]

I would like a vacuum gauge on my E30 come to think of it, sounds like it'd be fun to watch instead of the road... 8)

Get an Air-Fuel ratio gauge as well and you'll have lots of dancing needles/lights to look at that dont really mean anything

[KPWSerpiente]

Bah. Volt gauges are where it is at. I'm thinking of installing a 5 inch one next tomy 10 inch tach.

-T

[BLINGMW]

awww I miss the volt gauge in my Fiero. Gave me another place to look and see if my lights or turn signals were on. :lol:

[DierwulfBL]

everyone can benefit from an oil pressure gauge

[Kaan]

i dont need no stinking oil... let alone its preasure.