Date: Sun, 17 Nov 2013 13:49:57 -0500
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Arduino and Vanagons
In-Reply-To: <5288631B.2040402@turbovans.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed
At 01:32 AM 11/17/2013, Scott Daniel wrote:
>on a waterboxer..compared to that , it's really crude.
>The ECU know the engine is turning from seeing the igntion pulses from
>the Hall Sender in the distributor.
>It also knows what the rpm is.
>It does not have any idea what cylinder is being fired or where say
>Cylinder 1 is in the 4 Stoke Cycle.
Correct, that is encoded by the distributor position which the ECU is
not aware of. All it knows is that the engine is at the designated
timing point near TDC on some cylinder.
>( I don't know if the ECU can sense 'rate of change of throttle
>position'. I suspect not on Difijet and Digifant.
The ECU *could* calculate rate of change of *airflow,* but I don't
know whether it does.
>and oxygen sensor of course...that comes into play after a short period
>of running. It fine tunes the mixture. They'll run quite well without
>an 02 input, but having one is better of course.
The sensor is needed for correct operation of the catalytic
convertor. It causes the mixture to be set at a point in between the
best-power and best-economy mixtures. If a cylinder is misfiring or
there's an exhaust leak allowing air to be drawn into the exhaust,
the sensor will be fooled and produce bad results.
With no CAT I think it's a mixed blessing, since it can compensate
for some problems but makes other ones worse. None of the European
Digijet Vanagons had sensors, because they were still using leaded
gas over there. My '84 California-spec Westy was delivered in
Germany and ran for the first seven years of its life with no sensor,
until it got shipped over here and VW installed a sensor and CAT in it.
>with that information ....
>the ecu figures out how long to hold the injectors open.
>if the fuel pressure is right ...the right amount of fuel will be injected.
>The ecu has no idea what the fuel pressure is.
>if the fuel pressure is low say ..........for a given amount of injector
>open time...less fuel will be injected.
The ECU "knows" that so many milliseconds of injector time will
result in so many milligrams of fuel injected. That is a constant,
and it's the operation of the fuel pressure regulator that keeps it
constant. Of course as you say, if the regulator is wrong the fuel
delivery will also be wrong.
>The ecu will not even know it, except maybe slightly from the 02 sensor.
The ECU in closed-loop mode will follow the sensor as an adjustment
to what it would have done anyway. If the sensor is out of whack the
adjustment can be extremely large and cause lousy mileage, inability
to idle and such troubles.
>There won't be any codes set or anything like that of course.
Actually the 022F (late 2.1l) ECU has some form of diagnostics
implemented, but the wiring and readout are not in the van.
>OK ..the 'enrichment when opening the throttle' ....the fuel pressure
>regulator has a vaccum line on it, connected to intake manifold vacuum.
>It's built so less intake manifold pressure equals higher fuel
>pressure. When the throttle is opened ..intake manfiold pressure drops
>.....raising the fuel pressure as controled by the fuel pressure regulator.
> if the injectors are held open the same amount of time...more
>pressure means more fuel molecules get injected, thus richening the
>mixture for acceleration. Pretty clever, and fairly crude, but it works
>well.
I'm sorry, the above is not correct. The entire purpose of the fuel
pressure regulator is to maintain a constant pressure across the
injector nozzle under all operating conditions, so that N
milliseconds of injector opening will always deliver X milligrams of
fuel no matter what the engine is doing.
Carburetted engines need an accelerator pump because at the instant
you bang the throttle open manifold pressure rises sharply (or vacuum
drops, same thing) but airflow doesn't rise much because the engine
is not yet turning faster. Under these conditions the carburetor
venturi stops sucking fuel and the engine goes lean and dies. The
accelerator pump forces compensating fuel into the carburetor throat
to keep the engine running until its rpm rise enough to start sucking
fuel again with the wider throttle opening.
But on our fuel injected engines the fuel is pumped in under pressure
all the time and doesn't depend on the carburetor venturi to suck it
in. So when you bang open the throttle manifold pressure still
rises, and the fuel pressure regulator follows that rise so that the
injector pressure remains constant.
The key to understanding the fuel pressure regulator is to recognize
that it's maintaining constant pressure between the inside of the
fuel line and the inside of the intake manifold, or to say it
differently, between the two ends of the injector. When we stick a
gauge on the fuel line we see it wobble all over the place, but
that's only because the manifold pressure is wobbling all over the
place with reference to the outside air. If we enclosed the gauge in
a sealed box and connected the box to the manifold, the gauge would
stay rock-steady within the operating limits of the pressure
regulator, and we would be seeing directly what the regulator is
actually accomplishing.
>Ignitio...base timing is set by the rotational position of the
>distributor in the engine block.
>on 1.9's advance is control by centrifugal weights in the distributor (
>how many people or engine techs even know anymore that felt pad under
>the rotor is for a drop or two of oil to lube the centifugal mechanism
>??? not many I suspect ! )
>Ignition timing is also controlled by the vacuum capsule on the side of
>the distributor.
Yes, 1.9l has centrifugal advance, and both vacuum advance and
retard. The idle stabilizer also advances the timing on a
pulse-by-pulse basis to maintain idle speed if it starts to sag.
>on 2'1's igntion timing is entirely electronically controlled. I'm
>trying to think of how it could be load sensitive ...and can't think of
>how it would know that...seems it's only pegged to rpm. I'll have to
>think about that more.
The ECU is aware of load because of the combination of rpm and air
flow. It adjusts fuel delivery based primarily on those two values,
and it can adjust timing likewise. Whether it actually does I don't
know but I suspect it does.
>Oh ..i get it. By 'discharge' you might mean squirting of fuel by the
>injectors.
>They're all fired like one big one..
>and it's not really timed, even though injection events are triggered by
>ignition pulses.
It's timed in that injection into the manifold happens in a precise
relationship to the Hall pulses from the distributor. However the
timing of it arriving inside the cylinder is of course controlled by
the intake valves and pistons. But the injection is absolutely tied
very directly to the distributor timing pulse.
>if anyone finds this interesting...here is why an Air Mass Meter is a
>huge advance over an Air Flow Meter.
>An AFM has mechanical contacts in it ..those wear. AFM's run 'uneven'
>after a while.
>The flapper door is anther obstruction in the air intake path . A choke
>point.
Another reason is that the mass meter actually measures the mass of
air passing through. With an AFM the ECU has to consult the air
intake temperature and calculate the mass based on temperature.
I'm not absolutely certain of this, but I think the AFM also may not
completely compensate for altitude changes, which the mass meter
does. Does someone actually know the answer to this? In the limit
case of zero pressure the AFM would show zero flow, but is the
relationship linear? I'd sure like to know.
>The way it works is a small current flows through nearly invisible wires
>right in the middle of the air flowing through it.
>The ECU sees changes in resistance in those wires by how much air is
>flowing over the tiny wires.
The one that I read up on was a bit more complicated -- it maintained
the platinum wire at a constant temperature, and measured the current
through the wire required to keep it at that temperature.
The '95 Subaru meter I'm reading about right now for the EJ22 engine
uses a thin-film sensor instead of a platinum wire, but doesn't
otherwise go into details of precisely how the ECU stimulates it.
Yours,
David
