Turbo theory question

[Peter & Rebecca Lines]

It was mentioned that expanding gases and not heat is what drives the 
turbo.  This is only partially true.  Obviously, if you blow exhaust gases 
across the turbine, it will spin.  This is the same as a windmill effect. 
 But a properly designed turbine can also extract heat energy directly from 
the exhaust stream.
On my previously mentioned turbodiesel  dyno senior project, we measured 
the temperature and pressure of the exhaust gases before and after the 
turbine (and also at the compressor).  What we found is that the pressure 
drops across the turbine.  That part was expected.  However, there was also 
a large drop in gas temperature across the turbine.  The temperature drop 
was much greater than would be caused by simply allowing the gases to 
expand normally.  This means that not only was the turbine extracting 
kinetic energy, it was also extracting thermal energy.  The total energy 
extracted by the turbine (enthalpy in thermo-geek speak) is transferred by 
the shaft to the impeller and used to compress the intake air.
Another thing we noted is that the amount of heat energy extracted varied 
at different engine speeds.  This explains why the efficiency of the turbo 
usually varies with speed.  Small turbos spoll quickly, but lose efficiency 
at high speed.  Large turbos are the opposite.  It's all a compromise....

Peter Lines ('86 4kq)