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Speedy Squirrel Speedy Squirrel is offline
Rocket Scientist
 
Join Date: Jun 2008
Location: Detroit
Posts: 881
Everyone is getting very close to the correct understanding. Let me make a few brief comments to help guide things along:

1.(gskrken) Your thought experiment is in many ways exactly right regarding the effect of addtional header volume. You just have the conditions wrong. When we go from low boost to full boost we do not just have to fill up additional volume, we have to pressurize it. Until we meet the pressure demand of the turbine, the volume continues to need "more filling". Thus, even a relatively small volume can require many liters of exhaust gas compressed into it before the turbine energy requirement is met. When you consider that a .2 second improvement in time to full boost is noticable in the seat of your pants, I think you can see how this can make a difference

2. (lucittm) Your basic grasp of gas dynamics is very good. The individual pipes must be arranged so that, in our case, the pulses are spaced 120 degrees apart. This naturally occurs when each bank is grouped together. An open end reflects back a negative pulse (low pressure). If this negative pulse arrives at the exhaust port just as the exhaust valve is opening, it increases flow out of the cylinder, just as you more or less stated. For all cylinders to be doing the same thing at the same time, the lengths of the tubes have to be the same, as you pointed out.

The condition that we have (and that you recognized) is that we have no open end to reflect a pulse back. It is neither open nor closed. Any pressure pulses are expanded through the turbine and used to compress air. This is the fundamental fact that, unique to turbo engines, we have to recognize. We do not reflect back ANY negative pulses. Without any reflected pulses the length of the tubes becomes non-critical, except from the standpoint of volume that needs to be filled, and surface area for heat loss.

There is only one way to use the pulse energy in a turbo engine, and I mentioned it in my original post. If the individual cylinders of one bank are connected to one side of a divided turbine housing, and the other bank likewise, the pressure of each pulse can be expanded in turn through the turbine, measurably improving its efficiency. This picture is about right (there should be a wastegate on each side, and the divider wall is melted away. It should go all the way to the turbine flange):



One last word, about temperature. Temperature lost is energy lost. How critical it is depends on how much is thrown away.
Old 01-09-2010, 08:00 PM
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