- Page 1
- FREE Energy?
- Page 2
- Turbine Theory
- Page 3
- Compressor Side
- Page 4
- InterCooler Wastegate & BOV
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]