HUMPHREY INTERNAL-COMBUSTION PUMP 277 



relative results remain approximately the same and for present 

 purposes it is sufficient to take the curves as here given. 



It may be mentioned, however, that with a compression 

 pressure of eleven atmospheres absolute the theoretical thermal 

 efficiency of the Humphrey cycle is 52*5 per cent., whereas that 

 of the Otto cycle is only 40 per cent, when all corrections for 



Compression pressure in lb. per sq. in. above atmosphere. 



20 40 '60 80 100 120 



IS 



V 

 — 30 



ct 

 6 



o 



Formula for Otto-cycle diagram. 



Efficiency = i — — 

 tc 



t = 273 + i7° C. = 290 C. Abs. 

 y = i*4o8. 



Fig. 9. — Theoretical thermal efficiency curves— air standard. 



varying specific heats are allowed for. Such compression 

 pressures have not yet been adopted by Mr. Humphrey but are 

 to be tried in the 1,000 h.p. German pump already referred to. 

 In the meantime, when using only very moderate compression 

 pressures of under 50 lb. per square inch, an actual thermal 

 efficiency of over 23 per cent, has been obtained on a four-cycle 

 Humphrey pump. This corresponds to C95 lb. of anthracite 

 coal per water-h.p.-hour and was obtained on a lift of only 



