What it mean that Lysholm screw-type expander have 94% efficiecy?

[MKimagin]

I kinda puzzle. I read article about Lysholm screw-type expander have 94% efficiency.
What that efficiency represent?
Let say Carnot efficiency will be calculates at 56%, does this mean that the Lysholm expander can deliver up to 94% of the calculated Carnot efficiency? So it mean that such system could deliver up to 50.76% of absolute efficiency?
It look like significantly higher then Otto 30% and Diesel 40%.
So what is the expansion efficiency for real Otto and Diesel.
How about the turbine, how it compare to that?

 

[GregLocock]

My horrible old thermo book seems to say that the efficiency of an expander for a perfect gas is defined as the ratio of the isentropic temperature difference to the actual temperature difference, for a given presure ratio.

I suspect that the 94% efficiency refers to the mechanical efficiency, not the thermodynamic efficiency for a steam engine, which rarely crack 50%.







Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

Oh, so you probably need to knock 6% off the Carnot efficiency.

If you think you are getting a higher Carnot efficiency than a diesel then you've made a mistake - a sensible (turbo) diesel has a higher combustion temperature than any practical steam generator, and an exhaust temperature about the same as any practical condenser.

Boilers are 88% efficient according to my books, that comes straight off the top of the system efficiency.


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[turbomotor]

MK,

Efficiency numbers for compressors can be misleading. Typically, turbo-compressors are compared based on isentropic efficiency. That is, what is the reversible work done on the air in an adiabatic process as compared to the actual work (both reversible and irreversible) done on the gas. The actual work done on the gas in an adiabatic process is the enthalpy rise.

The catch here is the word "adiabatic", which means that no heat goes into or out of the system. In an insulated turbo-compressor, this is not a bad assumption. However, in many positive displalement compressors, the process is anything but adiabatic. An isothermal compression process will require much less work to achieve a given pressure rise than an adiabatic process, all else being equal. Thus when a posiitive displacement compressor efficiency is promoted as an isentropic efficiency, it is meaningless. I once tested a water cooled supercharger than yielded better than 100% efficiency if the efficiency was measured as typical adiabatic reversible efficiency. The resulting data was useful for nothing except perhaps trickery.

The bottom line is, always be skeptical of advertised efficiencies, as the underlying process assumptions determine if the number is actually a useful comparison.

 

[tbuelna]

turbomotor,

Just where did you test that >100% efficiency supercharger?

Sorry to hear about McCready.