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Turbo Fundamentals:-2 /4.

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By: Dennis Grant dg50@chrysler.com
June 1997
Subject: Turbo Fundamentals.
PAGE 2/4
 
Turbine Theory
 
Page 1
FREE Energy?
 
Page 2
Turbine Theory
 
Page 3
Compressor Side
 
Page 4
InterCooler Wastegate & BOV

 

  Alright, we determined that a turbo was a device that could be used to get useful work out of otherwise wasted energy, now  we will discuss how that happens in more detail.

It is a common misconception that the exhaust turbine half of a turbo is driven purely by the kinetic energy of the exhaust smacking into it (like holding a kid's tow pinwheel behind your tailpipe) While the kinetic energy of the exhaust flow does contribute to the work performed by the turbo, the vast majority of the energy transferred comes from a different source.

Keep in mind the relationship between heat, volume, and pressure when we talk about gasses. High heat, high pressure, and low volume are all high energy states, low heat, low pressure, and large volumes are low energy states.

So our exhaust pulse exits the cylinder at high temperature and high pressure. It gets merged with other exhaust pulses, and enters the turbine inlet - a very small space. At this point, we have very high pressure and very high heat, so our gas has a very high energy level.

As it passes through the diffuser and into the turbine housing, it moves from a small space into a large one. Accordingly, it expands, cools, slows down, and dumps all that energy - into the turbine that we've so cleverly positioned in tho housing so that as the gas expands, it pushes against the turbine blades, causing it to rotate. Presto! We've just recovered some energy from the heat of the exhaust, that otherwise would have been lost.

This is a measurable effect: Stick an EGT upstream and downstream of the turbo, and you see a tremendous difference in temperature.

So, in real world terms, what does this tell us?

All else being equal, _The amount of work that can be done across an exhaust turbine is determined by the pressure differential at the inlet and outlet_ (in English, raise the turbo inlet pressure, lower the outlet pressure, or both, and you make more power) Pressure is heat, heat is pressure.

Raising the inlet pressure is possible, but tough. Lowering the outlet pressure is easy - just bolt on a bigger, free flowing exhaust. I've seen a couple of posts from people who added aftermarket exhausts, who report "my turbo spools up faster now" Well, that's because by lowering the outlet pressure, you increased the pressure differential, and now the exhaust gas can expand more, and do more work. That increased work pushes harder on your turbo, and it spools up faster. You should also see less boost drop at redline, because if an exhaust system is flow-limited, once you pass the flow limit of the system, any additional gasses you try and force through it only raise the outlet pressure. Higher outlet pressure, lower pressure differential, less work, less boost.

[Note that the compressor side comes into play here too - that's another story]

That covers Turbine Theory. Next - the Compressor Side.


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