Why do petrol cars have vacuum pumps?

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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
 
- said:
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.
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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
 
JazzMan wrote in rec.autos.tech
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.
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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.
 
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.
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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 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.
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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.
 
I've seen a couple of applications where a vacuum pump is used as part of a cruise control set up.

John
 
Dave said:
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.
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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
 
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.
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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...
 
JazzMan said:
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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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
cars have v.mild camshafts anyway otherwise they wouldn't pass stringent emissions requirements.
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We are, of course, talking about a Citroen here. :-).
 
JazzMan said:
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.
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That's what the check-valve on the brake booster inlet line is for.
 
Steve said:
That's what the check-valve on the brake booster inlet line is for.
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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
 
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.
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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.
 
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.
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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.
 
Dave Plowman (News) said:
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...
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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
 
No decent maker? You mean Packard wasn't decent? (scoffs)
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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.
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.
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And how many do today? If it was so wonderful, they would still be in use.
 
Dave said:
When they were an option, brakes were of such a design that the amount of boost provided was small - usually drums.
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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!
 
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.
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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.
 
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.
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But will take for ever to stop from its top speed...
 
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