Date: Wed, 16 May 2012 05:37:49 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Adjustable rate fuel pressure regulator? anybody try one?
In-Reply-To: <4FB34BE5.9040100@turbovans.com>
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At 02:40 AM 5/16/2012, Scott Daniel - Turbovans wrote:
[If the ECU is running closed-loop (listening to the oxygen sensor in
the exhaust) it will dial back the pulse time until the mixture is
correct again. ]
>within a fairly narrow range I think.
No, it's quite a wide range.
>our era vans use narrow range 02 sensors, I believe.
This is true. However it doesn't affect the amount of adjustment the
ECU can apply, only the method by which it does it. And it means
that the ECU can only adjust to one specific mixture, the one at
which the fuel and air charges are balanced so that each one is
completely used up. That mixture is called the stoichiometric
mixture and is about 14.7 parts of air by weight (mass if you're
picky) to one of gasoline. Since that's the mixture that the
catalytic converter needs to work properly, the narrow-band sensor
can succeed. It is in between the best mixture for power (richer)
and the best one for economy (leaner).
The control method is called the "bang-bang" method of servo
control. With that method, the ECU is constantly changing the
mixture. It enriches the mixture until the O2 sensor "bangs" over
into a rich reading, and then leans it out until the sensor "bangs"
back into lean, then starts enriching again, rinse and repeat. The
ECU *always* thinks the mixture is wrong and is trying to correct in
whichever direction will make the sensor flip the other way.
>they run wonderfully without an oxygen sensor anyway, and the 02's
>limited influence is why our vanagons can run so well without a
>working 02 sensor.
Well...the engines run well without the sensor because they're
designed to, just like any engine. It's the catalytic converter that
requires this very precise mixture control, not the engine. I
haven't kept up with recent engine-control techniq and it may well be
that nowadays wide-band o2 sensors are really integrated into the
function of the engine - I don't know. But with engines of our era
the O2 sensor is strictly an afterthought to keep the CAT
happy. Aside from that it's a two-edged sword. It can compensate
for certain engine problems and make other ones worse. It turns an
exhaust leak from a noise problem into a too-rich mixture. It turns
a misfiring cylinder into a too-rich mixture. It turns its own
failure or a ground fault on its lead into a *really* too-rich
<cough, cough> mixture, so rich that the engine can have trouble
getting away from idle.
>'normally' ..on our engines , fuel pressure is held within a fairly
>narrow range...roughly 32 to 38 psi ..something like that..
Actually it's held very closely at a constant pressure. It only
seems to be varying because you're measuring from a different point
than it cares about. Specifically it cares about engine vacuum,
except when you call it that it becomes impossible to talk about
it. So instead we talk about MAP or Manifold Absolute Pressure.
Engine vacuum says how much less pressure there is in the manifold
than in the air outside the car, and it's measured typically in
inches of mercury, because that's how people first measured it.
MAP says how much *more* pressure there is in the manifold than no
pressure at all (i.e. a perfect vacuum), and it's measured in
whatever unit is convenient, which is seldom inches of mercury. So
assuming that we're at sea level and measuring in psi, when the
engine is not running, the MAP is about 14.7 psi. If we hadn't
already specified that we were talking about absolute pressure, we'd
say 14.7 psia, for absolute; which is equal to 0 psig, for gauge. In
other words, psia is referenced to complete vacuum, and psig is
referenced to whatever environment is outside the gauge at the time
of measurement.
Back to injectors and the point of all this. Leave the O2 sensor out
of the system, this engine is running open-loop because it's in an
'84 1.9l Westy in Edzell Scotland in 1991 and they're still using
leaded gas (and it has a cracked 3-4 synchromesh hub but that's
another story). So no CAT and no sensor. The ECU has to do the job
with no hand-holding, which means it has to know how much fuel gets
injected into the cylinders with each injector pulse. The only thing
it can directly control is how long it holds them open, so it's
necessary that they flow at a constant rate no matter what else is
going on with the engine; and the thing that controls flow rate is
the difference in pressure between the two ends of the injector. So
we have to keep that difference constant within fairly small
tolerance. I'm guessing +/- 2% or better.
One end of said injector is in the fuel rail and the other end is in
the manifold, so if we're going to keep the pressure difference
across the injector constant, the fuel pressure is going to have to
follow the manifold pressure very closely; and that's what the fuel
pressure regulator does. It's got a spring-loaded flexible diaphragm
operating a blowoff valve that vents excess pressure (above 2.6 bar?)
into the return line going back to the fuel tank. It's always
venting because the fuel pump shoves a *lot* more fuel through there
than the engine can use. Circulates a full tank about once an hour
IIRC. On one side of the diaphragm is fuel pressure. But on the
other side, the reference side, instead of outside air the diaphragm
is connected into the manifold, and the regulator keeps *that*
difference the same. We've got our little pressure gauge hooked up
to the test tee and it's measuring the difference between the fuel
system and the local atmosphere; so every time something changes in
the engine that gauge jumps around. But if we built an airtight box
around the gauge and connected the box to the manifold, the gauge
should stay rock-steady at 2.6 bar any time the fuel pump is running
- engine on, engine off, full throttle under load, idle, you name
it. It's just inconvenient to do that and expensive to buy a gauge
that's already set up that way, so Bentley gives you a sort of
generally-probably set of ballpark readings. But if you *really*
want to know what the fuel pressure regulator is doing, that's how
you find out.
Yours,
David