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?
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.
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?
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
What exactly are you trying to establish?
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.
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?
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.
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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