[Vwdiesel] Mileage test 84 Rabbit. --- ( hagars Pickle Jar day ?

[Val Christian]

> 
> >Like I stated long time ago  ---- flyweights trashing around in oil is a
> power waste.
> >And it is.   Fuel in jar gets very hot.
> 
> 
>   I stand by my theory that almost all of the heat is generated by the high
> pressure diesel fuel leaking (and intentionally spilling past the collar)
> from the high pressure section back to the low pressure in the pump body. It
> takes power to pressurize the diesel fuel, and if it is allowed to vent back
> to a low pressure area without doing any work, that energy is then released
> as heat.

	I would be inclined to argue otherwise.  There is little energy 
	released when a rather incompressible fluid is decompressed.
	This much is calculable.  The number will be small.  And this is
	why hydraulic systems are so neat.

	If a compressed gas were leaking, there there would be more
	energy released, but there would also be a cooling effect
	from the decompression (ref gas laws).


>  I have seen this firsthand with the Rexroth axial piston hydraulic pumps
> that were used to power molding presses where I used to work. The main
> hydraulic reservoir would run between 120 and 130 degrees, but the case
> drain line from the pump body would be uncomfortably hot on a new pump and
> so hot it would burn you immediately on a worn-out pump with lots of leakage
> past the pistons or swashplate. This was further backed up by the amp draw
> on the electric motor - a worn pump would draw more amps than it should and
> would actually overload and trip out the motor if it got bad enough. Why?
> For a given amount of power input (amps) you can have high pressure and low
> flow, or low pressure and high flow, but you cannot get both high pressure
> AND high flow at the same time. With a new pump running at 2000 psi, the
> power required to get full speed and pressure was matched to the electric
> motor driving it. Once the internals got worn and the case drain started to
> pass more and more oil, the motor was now being asked to deliver the same
> 2000 psi AND produce a high volume of oil out the case drain line.
>  One other valuable tip I learned from working with hydraulic systems is
> that if you suspect you have a high pressure internal leak in some component
> in the system but do not know which one is leaking, you can do one of two
> things very easily to pinpoint the problem; if it is quiet enough, you can
> often hear the leak. It will sound like a compressed air leak. The other
> thing you can do is to allow the system to run for a short period of time
> and use your hands or a pyrometer to locate the hottest component, that will
> be where the leak from high pressure to low pressure is.

	I won't begin to dispute your experience with your pumps.
	However, I can tell you that my tractor hydraulic pump, 
	which runs about 3500 psi bypass, doesn't begin to get
	hot.  It's a small pump, and it doesn't seem to get 
	much warmer than the surrounding engine area.  I could 
	measure it sometime, but I doubt if it's 50F hotter than
	the surrounding area, even under heavy load.

	What DOES get hot is the bypass valve area.  That's not
	because of the pressure differential.  It is because of the
	turbulent flow, and the heating secondary to that turbulent 
	flow.

	The VW injection pump has less flow than the tractor hydraulic 
	pump.  And the tractor hydraulic pump creates higher pressure.
	So there's more power put into it.  But the hydraulic pump is
	cooler than the VW injection pump. 

	Why?

	I submit that it is because of turbulent flow.  Yes, some from 
	the release of pump blow-by.  But likely more from the stirring 
	of the fuel with the flyweights.

	Look, I really don't mean to be argumentative.  I'm just sharing
	some of my reasoning, and "common sense" aquired through life.
	Just like your electrically driven pump story.  Perhaps this 
	question would be a good one to have a ME student work out,
	and give us some sample numbers.  Can anyone give a textual
	description of the flywheels, including an estimate of their
	wetted area, and rotational velocities?

	Perhaps engaging someone with a different approach might be 
	enlightening?

	There is a benefit in us knowing what the source of the heating 
	might be.  For example, if, as you suggest, a worn pump requires 
	more energy, and gives off more energy in the form of heat, then
	we would see greater belt torque on a worn injection pump, at 
	operational speeds.  This means that use of a passive IR 
	pyrometer, or other temperature probe, could be employed as
	a first-order check of a pump condition.  We would also be
	able to reduce a possible source of increased belt wear and
	increased failure probability, if true.

	I'm a scientist by profession, and we work lots of things out
	by numbers.  But we also try to shortcut the numbers with 
	intuition and other experience.  I may be real wrong in guessing
	the source of the heat, but my best guess is the turbulent flow,
	vs. the pressure release.  There just isn't much energy to
	derive from simply releasing pressure on a liquid.

	Guessing isn't really necessary, because a student experienced
	with hydraulics, fluid flow, and the like can readily calculate
	the different components involved.  We can then see if his or
	her prediction comes close the the heat gain we see in Hagar's
	test environment.

	Val
	[COM-ASMEL, CFI-AI, N2EPO (work HF at all?)]
> 
>  I don't deny that the flyweights spinning around in the fuel will
> contribute some heat, but I maintain that it contributes only a fraction of
> the total heat produced. Almost all of the heat found in the fuel flowing
> through the return line from the case of the pump comes from the leakage of
> pressurized fuel to a low pressure area.
> 
> 
>   Lee
>   Oo-v-oO
>   PP-ASEL
>   KB1GNI
> 
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