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The negative sign results purely from the thermodynamic sign convention of work being done by the system being considered positive, and work done on the system as negative. Thus the power required to drive the compressor is a function of the mass flow rate (m), inlet air temperature, (To,), pressure ratio (Po2l Poj), compressor efficiency (rjis) and specific heat at constant pressure. Equations (2.13) and (2.14) show that low compressor efficiency not only increases the power requirement for a given pressure ratio, but also increases the delivery temperature (T2) and therefore reduces the air density leaving the compressor. It is important to achieve high compressor efficiency for both reasons.

The isentropic efficiency of the turbine may be expressed as the actual power output divided by that obtained from an ideal adiabatic (isentropic) turbine operating with the same inlet pressure and temperature.

actual power

(turbine)

isentropic power

This expression may be developed in a similar manner to the compressor to give fisTT -

Kinetic energy leaving the turbine is wasted through the exhaust pipe, hence the total-to-static efficiency is again most appropriate, although not always quoted by the turbocharger manufacturer. The power output of the turbine is given by:

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