Why do petrol cars have vacuum pumps?

My Citroen Xsara VTS (cf. Peugeot 306 Gti) has an exaust cam driven
vacuum pump betwixt inlet manifold and servo. I can't see how a
situation could arise where there was insufficient vacuum unless you
drove foot-flat to the floor and controlled your speed purely with the
brake.

From the Pierburg site:

" Especially diesel vehicles, but also more and more vehicles with
petrol engines are being equipped with vacuum pumps from Pierburg, which
supply the necessary vacuum for supporting the brake system at the right
time.

Moreover, vacuum pumps are used in connection with the central locking
system, air conditioning system, automatic transmission, emission
control system and other control components."

Well the latter reasons are certainly red-herrings as my car doesn't
appear to have taps from the vacuum line at all, indeed the thought of
tapping the vacuum lines for the sake of central locking would seem
perverse. Pneumatic door locks have always been Heath-Robinsonesque at
best IMO.

Intrigued,

Thomas Hood

 

Vacuum can be insufficient to meet the needs of the car
under certain circumstances, and the amount of vacuum
capacity available may also not be enough under certain
circumstances. If the car has an aggressive camshaft or
has a very small engine, combined with vacuum driven
accessories that "consume" lots of vacuum then a pump
may be necessary. The engineers make that decision based
upon a comprehensive analysis when designing the car.

JazzMan
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JazzMan wrote in rec.autos.tech

An extreme case was a 63 rambler that I had. It had vacuum operated
windshield wipers. Driving on a flat surface at a steady speed and
they worked fine, going down hill could get them flying. Accelerating
up a hill would almost stop them.
Many cars also use vacuum assisted power brakes, and the concern might
be that the vacum from the engine would not be enough to make that
work.

--
Dick #1349
"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety."
~Benjamin Franklin

Home Page: dickcr.iwarp.com
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You don't tend to accelerate up a hill and use brakes at the same time...
Vacuum is at its greatest on a closed throttle - ideal situation for
brakes.

 

Dave Plowman (News) wrote in rec.autos.tech

You're right. However, the brakes do take quite a bit of vacum,
and usually there is a large canister for the vacum. Could be that
the maker is supplementing that with the vacum pump.


--
Dick #1349
"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety."
~Benjamin Franklin

Home Page: dickcr.iwarp.com
email:

 

I've seen a couple of applications where a vacuum pump is used as part of a
cruise control set up.

John

 

Imagine your surprise as your little lumbering beast is
laboring up the long grade under full throttle, almost
no vacuum available at this point, when some jackass
comes blowing around the corner in your lane. You want
to avoid a head on colision, but when you slam on the
brakes there's no brakes because the engine hasn't
returned to idle yet and hasn't had time to build up
vacuum. Oops.

JazzMan
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But the vacuum servo itself acts as a reservoir. Brakes of this type still
work a couple of times after you stop the engine.
Vacuum wipers are a different matter if you think about it - prolonged use
under some conditions will exhaust any storage facility. Which is why no
decent maker ever used them, and it's why no-one uses them today - despite
vacuum still being used for other things.

Some '50s Vauxhalls had mechanically driven wipers rather than electric.
They just varied their speed with the engine...

 

That's why the servo has a check valve. Full vacuum is generated in the
servo almost instantly when the throttle is shut, and remains there in
the absence of a faulty servo.

The camshaft being too wild is another red-herring I suspect; modern
cars have v.mild camshafts anyway otherwise they wouldn't pass stringent
emissions requirements.

As I said before, my car in particular has no other vacuum driven
accessories. Indeed I used to a have a Carlton GSI 24v which used the
vacuum for both operating an carbon canister and cruise control, which
it did perfectly happily without a pump.

OTTOMH, I can tell you that a 98 BMW 528i, 96 Nisan Almera 1.4, 96 Volvo
T5, all get by without such a pump. It seems strange to add such a
device which introduced another point of failure in the Braking system,
causes a parasitic power loss, and must add significant cost to
manufacture.

Tom

 

- wrote in rec.autos.tech

modern

We are, of course, talking about a Citroen here. .


--
Dick #1349
"They that can give up essential liberty to obtain a little temporary safety
deserve neither liberty nor safety."
~Benjamin Franklin

Home Page: dickcr.iwarp.com
email:

 

That's what the check-valve on the brake booster inlet line is for.

 

Only an optimist would rely on that check valve to save
their life by removing a factory-installed vacuum pump.

I'm not an optimist.

JazzMan
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**********************************************************

 

I've not read that suggested here - have you?

A vacuum pump can fail too.

 

Maybe it is just me, but I remember when power brakes were an option. In my
experience, cars still stop when the brakes have no boost, you just have to
push harder. The only reason I replaced the check valve in my '84 Dodge 600
was that if I got too used to braking with no assist, I would have problems
if I drove a different vehicle.

 

When they were an option, brakes were of such a design that the amount of
boost provided was small - usually drums. Modern ones multiply the effort
by several times, and even the strongest person can't get near the same
pressure.

 

No decent maker? You mean Packard wasn't decent? (scoffs)

of course, they did have an auxiliary vacuum pump on the oil pump
(which caused other problems, but that's irrelevant to this
discussion...) many other mfgrs. used vacuum wipers with an aux.
vacuum pump on the fuel pump as well.

nate

 

I wouldn't know - this is a UK group. But they don't stick in my mind as
being great cars from what little I've heard of them.

And how many do today? If it was so wonderful, they would still be in use.

 

One of my cars weighs 4100 pounds and has MANUAL front disk/rear drum
brakes. I prefer its brake feel to almost any other car I've ever
driven. Manual brake setups (as opposed to power brakes with the boost
disconnected) have a different mechanical ratio between the pedal travel
and the master cylinder piston travel, and a different ratio between the
master cylinder piston area and the total wheel cylinder piston area.

Manual brakes require more pedal MOVEMENT (not necessarily more pressure
at all). The extra movement allows finer driver control of braking
effort, and they respond faster since there are inevitable delays
(milliseconds) in the power booster as valves close and vacuum vents in
order to modulate boost. But you do have to remember to lift your foot
up higher before putting it on the brake pedal!

 

That's about like someone asking sarcastically if I thought Bentley
(back when it was really Bentley) wasn't decent, and me saying "this is
a US group, I wouldn't know."

Sheesh, Packard was one of the top 5 or so of all time. They're the ones
that made many of the manufacturing changes that improved reliability of
the Rolls-Royce Merlin during WWII, after all.

 

But will take for ever to stop from its top speed...