"Bernd D. Ratsch" <bernd@dodgetrucks.org> wrote:
> Again, refer to the plastic bottle post. There are things such as positive
> and negative pressure.
I don't know that the bottle analogy holds in this case though
since it concentrates only on the vacuum side of the engine and
ignores the pressure events. That is, we're not trying to suck
through the exhaust, we're trying to blow through it. (Read on below
for my more excruciatingly detailed thoughts on that...) ;-)
> The pistons pulling the air into the cylinders (via the intake) does keep
> vacuum in the intake stream.
Yep - that's the *only* reason there is any vacuum in the engine; the
pistons are pulling against the back of the closed butterflies at idle
with only a tiny IAC passage through which air can pass through; its
basically the same as trying to suck air through a straw - there is a
vacuum created in your mouth. As the throttle blades open, the straw
gets bigger, but there is still some vacuum in your mouth until the
straw gets so big that the restriction becomes less than the power of
your lungs.
> The opening of the exhaust valves releases
> that though the exhaust piping. Plug the pipe and you can't release the
> vacuum.
I'm not sure that I buy that; or, more correctly, I guess my
problem with it is your considering vacuum to be something that can be
"released". I'm more comfortable with the notion that a vacuum is a
*lack* of that something; its a void. You can release pressure, but
vacuum is something that you relase *into*, not something that is
released itself. (Well, I guess except for exposing a weak vacuum to
a strong vacuum, you might think of the strong vacuum being "released"
into the weak vacuum, but I'd prefer to just think about it as a
pressure equalization. Whenever I try to think about vacuum as some
entity moving from place to place, my brain flips out on me.) :-)
Anyway, back to your point. Regardless of how one thinks about
vacuum or pressure, I still don't think your description is correct
because of the combustion event between the opening of the intake and
exhaust valves. Let me try to explain what my thinking is by an
example. Lets consider an engine which has a certain vacuum level in
the intake manifold. The intake valve opens and the piston drops,
which creates a vacuum in the cylinder which is greater than the vacuum
in the intake manifold. This allows air (and fuel) inside the intake
manifold to be sucked into the cylinder (and likewise, air is being
sucked through the throttle body into the intake manifold, and if the
throttle body opening is not sufficiently large, this will cause a
vacuum in the intake manifold). So, at this point, the piston has
moved to the bottom of the stroke, the intake valve is closed, and we
have a vacuum in the cylinder. Now, the piston moves up, and
compression begins. Suddenly, this vacuum that we had has
dissapeared, and we have pressure instead. The spark plug fires, and
now there is a LOT of pressure. This pressure forces the piston back
down, and as it does so, this pressure is reduced. The action of the
piston going down with both exhaust valves closed would normally cause
a vacuum in the cylinder if we were starting out from zero, but we aren't
starting out from zero, we started from pressure, and the combustion
event added a lot more pressure. As the piston gets to the bottom of
its stroke, I must admit that I am not sure if the condition inside
the cylinder is still pressure, neutral, or a vacuum, but I can say
with a good degree of certainty that whatever the pressure, it is
higher than it was at BDC on the intake stroke. The combustion event
"destroyed" some or all of the vacuum. In any case, now the exhaust
valve is open, and the piston rises and pushes the spent mixture out
into the exhaust system. At this point, this mixture is pressurized.
It must be, because if it wasn't at a higher pressure than the exhaust
system, it would stay in the cylinder or the air from the exhaust
system would try to enter the combustion chamber, and neither of those
are going to work to run an engine. :-) So, following this mixture
then, its shoved out into the exhaust system, which if it is not
plugged, allows it to make its way out to the atmosphere. There will
be a slight pressure reading in the exhaust system of perhaps 1-2psi
due simply to the restrictions created by the pipes.
Ok, given the above, now lets throw a clogged cat into the
equation. Suddenly, this low pressure in the exhaust rises because
the pistons are cramming all sorts of air (exhaust gasses) into the
exhaust system, but it just can't flow through the cat fast enough and
it is building up, just like putting a kink in a garden hose. This
means that the pressure just outside the exhaust valve is higher than
before, and less of the mixture inside the cylinder is going to get
pushed out into the exhaust system. This means that compared to
before, as the cylinder is ready to open the intake valve and suck in
a fresh charge, the pressure in the cylinder is slightly higher than
before. The intake valve opens, and the piston drops to create a
vacuum and suck air and gas into the cylinder, but because it is
starting from a slightly higher pressure than before, it cannot create
as large a vacuum as before, and less fuel/air charge is sucked into
the cylinder. This means that some of the air inside the intake
manifold which would normally have been sucked into the cylinder has
stayed inside the intake manifold, and this creates a slightly higher
pressure in the intake manifold, which is where vacuum (or MAP)
readings are taken. This higher pressure in the intake manifold means
that the vacuum reading is lower than before (there is "less vacuum"),
and the MAP reading will be higher (there is "more pressure").
So (finally), *that* is why a plugged cat should result in lower
vacuum readings.
--
-Jon-
.- Jon Steiger -- jon@dakota-truck.net or jon@jonsteiger.com -.
| '96 Kolb Firefly, '96 Suzuki Intruder, Miscellaneous Mopars |
`-------------------------------- http://www.jonsteiger.com --'
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