Re: Bomb Gas Pressures

[Eric Maxon <emaxon@megsinet.net>]

Phil and Judy Rose wrote:

> >Date: Tue, 2 Dec 1997 00:01:05 -0800 (PST)
> >From: Orin Eman <orin@WOLFENET.com>
> >Subject: Bomb Gas Pressures
> >
> >OK, so here are the N2 pressures for the bomb, courtesy Mr Bentley:
> >
> >New bomb:  78 - 82 bar (1131-1189 psi).  I'll take that as 80 bar +/-
> 2.
> >
> >Minimum:   30 bar (435 psi)
> >
> >both at 20C/68F.
> >
> >The pressure relief valve in the bomb opens at 150 bar (2175 psi),
> >so the pressure ratio is actually a little less than 2:1 (150:80)
> >for a new bomb, 5:1 for a nearly dead bomb.
> >
> >Orin.
>
> OK. Then the loss of pressure at the defined "dead" point comes to
> about 50
> bar, or more than 1/2 the original amount of nitrogen. If gas loss is
> by
> natural permeation through the membrane and is more-or-less constant
> over
> time, and if the MTBF is assumed to be what, um 5 years, 6 years?
> Let's say
> 6 years. Then the monthly pressure loss works out as about 10 psi. Of
> course the loss of nitrogen isn't going to be linear at all; it will
> depend
> on the gas's partial pressure _differential_ (inside/outside), so that
> it
> should lose nitrogen at a much faster rate in the first few years
> compared
> to later on. Seems that if the manufacturer would ship these buggers
> pressurized on _both_ sides (I assume they're _not_), shelf life
> wouldn't
> be much of an issue. Might not be too practical for ease of
> installation
> though.
>
> Thinking about all this makes me wonder why Audi didn't design the
> hydraulic system to relieve the (hydr) pressure in the bomb when the
> ignition switch is turned off? Is this supposed to be a safety feature
> for
> the truly neurotic? Seems to me that the long periods that the bomb
> spends
> with high hydraulic pressure applied (while car parked and engine off)
> is
> time that it's losing nitrogen at a faster rate than otherwise would
> be. Is
> this, as we pidgin-Deutsch speakers might say:
>
> "Ausdunstung Durch Technik"?
>
> Phil
>
>          **********************************************
>          *  Phil & Judy Rose     E-mail:              *
>          *                       pjrose@servtech.com  *
>          **********************************************

Phil-- pressure in the hydraulic system is maintained by the accumulator
when the engine is off to ensure that you've still got power brakes if
the engine STALLS for any reason.

Have you ever driven a 5000/100/200 with a bad bomb??  I almost lost it
in a customer's 200Q wagon on an off ramp due to the accumulator not
doing it's job.  Trust me, it's not fun losing hydraulic system pressure
while panic braking from 80 mph, which you may have to do some day with
a stalled engine.  (Ever run out of gas on the expressway?  Ever end up
with a dead battery from a bad alternator or broken alternator belt on
the highway?)

>From all of your questions regarding bomb rebuilding, it sounds like
you've got a bad bomb now-- perhaps you haven't really had to stomp on
the brakes yet, but I guarantee you that under heavy braking, you won't
just feel pulsation from the hydraulic pump, you'll LOSE boost.
Suddenly the pedal will get rock hard, and you'll have to stand on the
brakes with all of your weight to avoid mowing down that
deer/child/senior citizen/pole.  Keep in mind that 5/6 "cylinders" in
the pump supply pressurized fluid for the rack, which requires a high
volume of fluid under pressure.  Only one of the pump's cylinders
supplies pressurized fluid for the brake servo, which requires a
comparatively lower volume of fluid.  (Think of the cross sectional area
of the steering rack compared to that of the piston in the brake
servo.)  In addition to dampening pulsations from the pump and
maintaining pressure when the car is off, the accumulator also ensures
that in the rare instance that you need a higher volume of pressurized
hydraulic fluid for really hard braking (the piston in the servo is
extended farther than usual, for more time than usual,) you'll have it.
Give your bomb a big kiss for saving your life.

Eric.