Measurement of Gas-Engine Temperatures. 01 



absorbed in heating it ; for though the motion of the air in 

 the cylinder during this part of the cycle must be very 

 turbulent, the velocities cannot be such as to account for any 

 considerable portion of the energy expended. The temperature 

 of the stream of air just after it has passed the inlet- valve, 

 and has spread out so that its velocity is greatly reduced, will 

 therefore be very nearly equal to the external temperature; 

 and it might be expected at first sight that a thermometer 

 placed anywhere in the cylinder during the suction stroke 

 would show that temperature. As a matter of fact, it wil 

 l)e seen from an inspection of the curve that the gas tempe- 

 rature shown by the thermometer falls in the middle of the 

 stroke to about 10° C, that is some 7° below the temperature 

 of the air in the inlet-pipe. Of this fall there cannot be any 

 donbt. as it was confirmed on a number of different occasions. 

 I think that it is probably due to the fact that the pressure 

 at the middle of the suction stroke is lower than at the 

 beginning, so that the air first drawn in has its temperature 

 lowered by rapid expansion after it has entered the cylinder 

 and has been reduced nearly to rest. Towards the end of the 

 suction stroke the reverse process occurs, since the air which 

 entered at the middle of the stroke has had its pressure 

 increased more or less adiabatically from about 11*3 lbs. to 

 14*7 lbs. per square inch. It will be seen that the temperature 

 shown by the wire at this point is about 31° C. 



The mean suction pressure exerted on the piston during 

 the suction stroke is about 2J lbs. per sq. in., and the work 

 done by the piston is 360 foot-lbs. per cubic foot of stroke 

 volume. The pressure at the end of the suction stroke is very 

 nearly atmospheric, but the air in the cylinder is then a little 

 hotter than it was before it entered the cylinder, because some 

 of the work done by the piston in suction has gone to increase 

 its internal energy. The volume displaced by the piston 

 exceeds the volume of air (reckoned at atmospheric tempe- 

 rature and pressure) which has entered the engine, by the 

 increase of volume due to the rise of temperature, and work 

 is done on the external atmosphere in the course of suction 

 to an amount equal to such increase of volume multiplied by 

 the atmospheric pressure. It is eas}' to see from this that 

 the rise of temperature of the air in the cylinder (assuming 

 that it receives no heat) must be equal to the work done upon 

 it in drawing it in divided by its capacity for heat at constant 

 pressure. Taking the temperature as 17° C, the latter is 

 about 21 ft. -lbs. per cubic foot, and the temperature at the end 

 of the suction stroke should therefore be 14° above the inlet 



