OT: Diesel Engines (again)

In a modern common-rail injection system, fuel is drawn from the tank via a conventional low pressure pump of the kind not dissimilar to petrol engines, then fed to a high pressure pump which produces absurdly high pressures in the order of 30-40,000 psi which is buffered within the common rail reservoir for onward distribution to the injectors. My question this time is, how is this extreme high pressure generated? What kind of pump can do this and how is it powered?
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On 27/11/2016 16:19, Chris wrote:

Common rail is actually quite an old idea but has only come into vogue recently due in part to emission requirements.
The pump is driven by the engine, the pump is often (always?) a cam driven sprung loaded piston with a non return valve after it, there may be an intermediate pump stage inside the pump to get the pressure up a bit before the serious pushing. The high pump pressures mean that tolerances are very tight and the slightest bit of crap can wreck the pump in seconds.
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On Sun, 27 Nov 2016 16:56:02 +0000, MrCheerful wrote:

OK, so if I have this right.... The whining sound you hear when you turn the ignition on is the electrically powered low-power pump. The high pressure pump becomes active just by engine cranking alone? It's *mechanically-driven* and provides this high pressure regardless of whether any other (electronic) system in the vehicle is functional or not?
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On 27/11/16 17:09, Chris wrote:

Yes....

No so sure that that can't be electric too. Hmm.
A little research suggests that though injector timing is electrically operated, the pumps are normally camshaft operated.

That seems to be the case although without the electronics the injector will never fire to let the fuel into the cylinder.
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On 27/11/2016 17:31, The Natural Philosopher wrote:

as far as I know the hp pump is always engine driven. If there is no lift pump feeding it then it may not produce any pressure at all. It may also have an elctrically operated valve which would release pressure when no electric goes to it (that was how the stop solenoid works on older standard diesel pumps.
However, with no electrics the injectors will not open, so the engine cannot run.
What exactly are you trying to establish?
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On Sun, 27 Nov 2016 17:31:21 +0000, The Natural Philosopher wrote:

I dunno, NP. It's not really my area, but I'm struggling to see how those kind of pressures could be achieved solely from the battery. You have to bear in mind that from a cold start, the battery is already doing a hell of a lot of work in a diesel-engined vehicle. Especially if it really is a cold start in every sense of the word at -15C at 6am. The cold cranking amperage plus whatever the glowplugs consume, plus whatever else is taking current at first switch-on, is going to leave precious little spare capacity for generating 1000 bar (or whatever it is) for the injectors. So for that reason alone I'm guessing (and it is only a guess) that the high pressure pump runs off a purely mechanical mechanism that derives its power directly from the engine's rotation.
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Cursitor Doom wrote:

Which before it starts will be coming from the battery via the starter motor anyway ...
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But you presumably don’t need the highest pressures when the engine is starting, just enough to get the engine started.
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On 27/11/2016 19:52, Rod Speed wrote:

One of the main advantages of common rail for a car engine is that the pressures are consistent across the engine speed range. So it will give the same pressure at cranking speed as at 4,000 rpm
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Yeah, spose so.
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Cursitor Doom wrote :

So far as I know, all HP pumps are mechanically driven. The LP electric pump will build up all the pressure needed, before you turn the key to crank.

Yes, that is correct. The pressure is very high, but the extra mechanical load is not that great, because there is not a great deal of flow.
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The low pressure pump in often/always at the fuel tank under the rear seats. I've always wondered why on my car (a Peugeot 308) the pump *always* sounds for about 30 seconds after I fill up the tank and then start the engine, whereas it often doesn't sound if I start the engine without filing the tank. Could it be that a full tank amplifies the noise of the pump - ie that it *does* always come on as the engine is started, but I only *hear* it when the tank is full?
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On 27/11/2016 20:39, NY wrote:

If the pump is in the tank (likely) then a full tank would damp the sound.
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But he gets the opposite effect, he can hear it when the tank has been filled.
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On 27/11/2016 23:56, Rod Speed wrote:

Pethaps that is to do with location rather than the circumstances.
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Not likely IMO.
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NY explained on 27/11/2016 :

Mine has an in tank pump and a second pump under the bonnet. Neither can be heard unless you listen via the filler/ under the bonnet. They run for 30 seconds then time out if the engine is not cranked before the 30 seconds expires. I would epect a pump to be more audible, with a near empty tank.
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I guess its possible that his is different and the full tank means that the noise the pump makes gets transmitted to the walls of the tank better and so is more noticeable than with a near empty tank given its french and they always do things backwards just because they are frogs.
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Chris wrote on 27/11/2016 :

No - if no pressure is provided by the electric pump, it usually prevents the engine being cranked or stops the engine if it fails whilst being driven.
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On 28/11/16 14:11, Harry Bloomfield wrote:

Or, if you are lucky, and it doesn't fail completely,. allows you to limp home at 30mph..downhill...and almost 10mph...uphill :-)
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