32 LIBERATION OF ENERGY 



T — T 



Evidently the fraction ^ is that part of the Q units of heal 



^ 1 



which represents the amount of energy made available for work. 



That is, even under unattainably perfect conditions no more heat 



T — T 



than — ^ of the amount given can be converted into work. 



This equation gives the efficiency of the heat engine. 



The most efficient steam engine yet constructed — a Nordbeg 

 air compressor of 1,000 h.p. — converts 25 per cent, of the heat 

 energy it receives into work. Most steam engines are only 8 to 

 10 per cent, efficient, i.e. only 8 tons out of every 100 tons of fuel 

 burned have their energy converted into work. 



TABLE IV 



Comparative Thermal Efficiencies 



I Compound (non-condensing) - 8-12 per cent. 

 Steam - --; ,, (condensing) - - 10-16 



Parson's turbine _ _ _ 15-18 ,. 



Petrol (motor) - _ _ _ 22-24 



Internal ' ,, (aero) - - - - 26-28 



Combustion -■ Coal gas (stationary) - - 29-31 ,, 



( Diesel - - ' - - - 33-35 



Combined I.e. ) q, n • a-i aa 



J a^ Still engme _ _ _ _ 41-44 



and Steam i "^ 



Animal body _---_-_. 25-34 ,, 



If one were to consider the animal as a heat engine, then it 

 must operate between two temperatures. One of these tempera- 

 tures we know, viz. body temperature, which is 38° C. or 273 + 

 38 = 311° absolute. This is the condenser or " sink " temperature. 

 The other temperature, that of combustion, must be higher. How 

 much higher may be calculated from the equation above. 



Efficiency = ^IziZj _ E, 



or transposing 



T, = T,I{1 - E). 



Suppose we take a low figm-e for animal efficiency, say 20 per 

 cent. Then, substituting, we find that 



T, = ^y_ =:^H= 389° absolute or 116° C. 

 1—0-2 0-8 



