TRANSACTIONS OF SECTION G. 637 



The following Report and Papers were then read : — 



1. Experimental Investigation of the Thermal Efficiency of a Gas Engine 

 By Professors G. Asakawa and J. E. Petavel.' 



At the Birmingham Meeting of the British Association a preliminary note 

 bearing on the above subject was read. The investigation has been continued 

 during the last two years and has led to the conclusions summarised below. 



The various losses have been separately determined by measurements based 

 not on indicator card readings, but on the rate of change of the kinetic energy 

 of the rotating parts, and thus the errors inherent to the indicator mechanism 

 have been avoided. 



In the following summary the performance of the engine is compared with 

 that of a perfect engine working on the same cycle and with a similar gas 

 mixture. 



Indicated Horse Power. — At full load under the most favourable conditions 

 the indicated horse-power of a gas-engine is 88 per cent, of that of an ideal- 

 engine working with a similar mixture. 



This holds true for all except very weak mixtures, for which the relative 

 efficiency is lower. 



For mixtures containing only a slight excess of air, the above corresponds 

 to an absolute thermal efficiency of 27 per cent, at a compression ratio of 3-75 

 and 33 per cent, at a compression ratio of 5'6 ; for mixtures containing twice the 

 amount of air required for complete combustion the absolute efficiencies are 

 29 per cent, and 36 per cent. 



The indicated efficiency relative to the gas-standard falls from 88 per cent. 

 to 84 per cent, between full and no load. 



Brake Horse Power. — At the full load the brake efficiency relative to the 

 gas-standard varies from 70 per cent, at the compression ratio 3-75, to 67-5 per 

 cent, at the compression ratio 5-6 ; this holds true for all except the weakest 

 mixtures, for which the relative efficiency is lower. The absolute brake 

 efficiency is 21 per cent, at compression ratio 3-75 and 25-5 per cent, at com- 

 pression ratio 5-6 for mixtures containing little excess of air, and 23 per cent, 

 and 27 per cent, respectively for mixtures containing twice the amoimt of air 

 required for complete combustion. The maximum brake efficiency obtained in 

 the present experiments was 27-4 per cent., and occurred at the highest com- 

 pression ratio for a mixture slightly stronger than this. 



At light loads, the brake efficiency relative to the gas-standard decreases 

 more rapidly as the compression ratio rises. For the higher compression ratios 

 the increase of theoretical efficiency is just counterbalanced by the increase in 

 frictional loss, and thus the absolute efficiency remains constant. 



Mechanical Losses. — The mechanical losses increase slightly in absolute 

 amount with the load and with the compression ratio. For the engine imder test 

 (a 25 H.P. National Gas Engine) at normal speed (200 revs, per min.) the 

 mechanical losses amounted to 5-6 H.P. at no load and 6-3 H.P. at full load 

 when the compression ratio is 3-75: and 6'5 H.P. at no load and 7-0 H.P. at 

 full load when the compression ratio is 5- 6. 



The pumping losses are an important part of the total mechanical losses ; at 

 a compression ratio of 4"85 they represented 2-3 H.P. at no load and 2-1 H.P. at 

 full load, or 38 and 31 per cent, of the mechanical losses. 



Thermal Losses. — The loss of power due to thermal losses at full load under 

 most favourable conditions amounts to 12 per cent, of the total available energy, 

 and at no load to 16 per cent. ; of this less than one quarter is due to heat 

 transmission during the expansion. 



2. Rrpnrl on Gaspoiis Explosions. — See Reports, p. 1/)S. 



' See Engineering, September 17, 1915, p. 297. 



