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Marvin Kaye wrote:
Hey guys,
We've been talking about these pop-off valves as safety devices for some time, but the problem always seems to be that the available crop of these things will only work at sea level. This is an obvious shortcoming for airplane applications, as the MAP will always be decreasing with altitude.
I've been kicking around some ideas for awhile and came up with a little drawing of one of these things that, I think, addresses the problem. See the attached picture.
My thinking is that if you create a chamber where you carry around your sea level altitude with you, close it off with a piston, the exposed side of which is open to the atmosphere, then as your altitude increases the piston should move down an amount equal to the change in atmospheric pressure. It then applies the required additional force to our spring and keeps a steady 20psi of force on the valve at the bottom. (This would be the equivalent of 40" hg MAP, but you could make it be whatever you want by changing the spring rate.) As you can see, the exposed sides of both the movable piston and the valve are seeing atmospheric pressure... in this example drawing the piston and valve's areas are both one square inch, just to keep things simple.
Now, for the $64,000 question... what's wrong with this? If it won't work, why not? What do we change to make it work? This can't be that hard. Suggestions????........
<Marv>
It looks to me (with my non-physics-trained eyes) like all this does is shift the popoff pressure up by the pressure in the chamber. Total pressure on the top of the disk would still vary with atmospheric, wouldn't it? What would happen if you closed the 'open to atmosphere' ports & only allowed internal pressure in the chamber to push on the disk?
A more 'universal' problem might be the size of the 'ports'. The turbo is pumping a *lot* of air. As an analogy, consider your air compressor when you use an air nozzle to blow dust off a surface. The pressure in the nozzle drops pretty low, but it takes a relatively long time for the tank pressure to drop noticeably. There has already been a lot of discussion about the need for porting the exhaust side bypass port on the turbine & it's already a lot bigger than the popoff ports being discussed. Will these little ports have any appreciable effect on the total pressure in the intake manifold? Will it happen quickly enough to stop/prevent detonation?
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