On 09/13/2010 08:47 PM, rampil wrote:
> I am not sure what a cross tube does to "soften" the incoming air pulses.
Ok, I will try to explain some more.
The firing order of the Rotax is 1-4-2-3. The intake order is of course
similar.
Now, let's see what happens.
For ease of explanation, we will start with cylinder 4. The order we
will follow is thus 4-2-3-1.
Intake of #4 opens. Air starts to accelerate through the carb.
At the moment #4 closes, intake #2 opens. This cylinder is on the same
carb as #4. The air is already up to speed through the carb, it only has
to take the next exit.
Intake #2 closes. As on this side of the engine there are no more
intakes, and we now have to wait for the two cylinders on the other side
of the engine to complete their intake cycle, the air comes to a
screaching halt. There is nowhere to go for the air, all intakes on this
side are closed!
At the same time #2 closes, intake #3 opens. This one is on the other
carb. The air has been come to a stop there, because while the engine
was working on #4 and #2, this carb had no throughput at all. So intake
#3 has the task to start accelerating the air. When #3 closes, #1 opens,
same bank, so the air can just go through. And when #1 closes, this bank
is done, air comes to a stop, and we switch over to intake #4. The whole
cycle repeats itself here.
Ok, now with the balance tube. This will fit in between previous paragraphs.
At the moment intake #2 closes, there are no open intakes on that side
anymore. The air however is at speed, and due to inertia the air wants
to keep going. (keep in mind that the engine runs at 5000 rpm, and at
this speed, inertia of the gasses plays a significant role). Luckily
there is a balance tube, and the air still rushing through the carb
(remember, there is a whole column of air up to speed from the airbox
that doesn't know that intake #2 just closed) takes the only exit
remaining: the balance tube.
At the moment intake #2 closed, intake #3 opens. This one is at the
other side. Luckily, there is just a bunch of air arriving through the
balance tube! This air fills up the gap, at the moment the air through
the carb of #3 has to start moving. Of course the flow through the
balance tube doesn't last very long, but at the moment it ceases, the
column of air through its own carb has just accelerated enough to take over.
This is of course a simplified explanation, you'd better think of all
this as pressure waves. To complicate things further, the column of air
rushing in that finds all doors shut, bounces back. This backwards
pressure wave interferes a moment later with the start of a new inlet cycle.
So, this is what a balance tube is for. Maintaning a smooth air stream
is more efficient and is easier on the carbs. Usually this translates in
more power, and a more even distribution of density and mixture.
Unfortunately, the balance tube of the Rotax is very small, and probably
doesn't do much at WOT.
Frans
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