This might be better-placed in Tech Matters, but Motorprop has inspired me to ask a question I've often wondered about.
As I understand it, a petrol engine develops a vacuum on the intake manifold when its running. This vacuum can be tapped and used for other purposes; for example, the braking system. (Have I got this correct? Feel free to correct me if not.)
If thats right, why cant diesel engines do the same?
I'm sure this will seem shockingly ignorant to many....
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Because a Diesel engine has no throttle butterfies (honestly) then a vacuum is not created in the inlet manifold in the same way as in a petrol engine. So a vacuum pump is needed to create the vacuum needed for the brake-sevo.
Hope this helps explain?
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Diesels don't usually have a throttle butterfly, so no vacuum to speak of.
EDIT - sorry for the echo, takes me a while to type a sentence.
Edited by Manatee on 09/01/2010 at 09:00
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Yes, thats very clear- thanks to you both!
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In many cases the vacuum is also used to operate the variable-nozzle system on turbochargers, EGR valves and so on.
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Presumably because most diesel engines are turbocharged, so the pressure in the manifold can be well above ambient.
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Diesels don't usually have a throttle butterfly so no vacuum to speak of.
I've not actually looked at them very closely but presumably BMW petrol cars with their clever valvetronic system will need a vacuum pump to run the brake servo too? Presumably there's no negative inlet manifold pressure to feed off?
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i wonder if the new breed of cars with the slowly sinking pedal thats mot compliant have vacuum pumps too
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No i dont think they need it because of valetronic.
Even when the engine switches to running in valvetronic mode (when you first start the engine there's a throttle body does the throttling) and so the throttle body moves (and remains at) the fully open position, you still have a vacuum in the intake.
My understanding is that the vacuum comes from each piston on the intake stroke (piston going down + intake valves open == suction from the intake == negative pressure / vacuum in the intake manifold).
I understand that because of overlap of the cylinders / strokes, there's always one piston sucking in.
I believe theres some kind of really fast (faster frequency than the valves) resonance in the air intake because of the rapid switching between cylinders sucking in (shockwaves of some sort) and somehow these also contribute to make the vacuum even greater. This is how the variable length intake manifolds come into play as well i think.
You can tell from the wooly wording i dont know the full score but would be really happy to hear from someone that can put the above right!
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Valvetronic engines do have a vacuum pump.
en.wikipedia.org/wiki/Valvetronic
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BMW valvetronic engines do indeed have a vacuum pump, mine has been changed!
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