Bruce Giller wrote:
Date: Sun, 29 Jul 2012 11:01:14 -0400
From: Bruce Giller <[email protected]>
Subject: Re: [alfa] over pressurized fuel tank at the track
to paraphrase, "misery, misery, oh misery," on the topic of fuel tank
pressure. I am -not- a fuel system adept, but as I recall the function
of the fuel vapor recovery system, it is to -recycle- the fuel vapors
and burn them. As I recall (to put a huge disclaimer on it), the vapor
canister takes air returning from the engine environment, passes it
through the canister's carbon, which adsorbs the fuel. There is a
finite capacity for fuel, which is far exceeded when there is liquid
fuel in this canister. During running, there is supposed to be an air
flow through the canister to purge it of fuel so that its capacity to
adsorb fuel is restored for the next idle period. It was my
understanding, limited though it be, that a significant overpressure
condition should not happen and that the fuel tank should actually be
under only very slight pressure, or even a slight vacuum. There should
be a power-actuated solenoid valve somewhere (-perhaps- in the vacuum
servo channel) which opens to allow air to pass through this canister en
route to the intake manifold.
If such a path does not exist, then I would understand the thing
malfunctioning. I would not understand where any additional air is
coming from, but I do have experience with gas cans being filled in cool
weather, then bulging nearly to the point of bursting the seams when
heated up. One has the partial pressure of the trapped air plus the
vapor pressure of the fuel summing to normal atmospheric pressure at
cold fill, then the fuel vapor pressure rising significantly as the fuel
heats. If there is a one-way path for bleeding the tank through the
canister to the intake manifold in parallel with a one-way path from the
control valve air intake through the canister, then the canister will be
purged during running conditions, any overpressure in the tank will
slowly be pumped through the engine and burned. There should be a
flow-limiting orifice, which may possibly be just the length of
small-diameter tubing connecting things over many feet of path (the
Bosch Spider spark advance system is flow-limited in this way so that a
leak in the capacitance manometer behind the right rear wall panel does
not overwhelm the air metering).
Some of this may be misplaced due to the limited number of neurons I
have attached to this issue, but a good study on the function and
theoretical flow of this system may tell you in short order what is wrong.
Michael
--
to be removed from alfa, see http://www.digest.net/bin/digest-subs.cgi
or email "unsubscribe alfa" to [email protected]
