ON FUEL ECONOMY. *^91 



necessary for really accurate instantaneous observations, and to maintain a 

 true ' istcady state ' in which the temperature of the range itself is kept accurately 

 constant from hour to hour is not possible. Results can only be obtained by 

 prolonging the experiment and obtaining a final result by a process of 

 integration. 



When a charge, liowever small, of fresh solid fuel is added to a glowing 

 fire, the resulting combustion is essentially variable. When tlic rate of stoking 

 is maintained artificially uniform, this variation of the reading takes the form 

 of cyclical variations only approximately of sine character. Another considerable 

 difficulty relates to the intensity of the draught. The same total rate of 

 combustion can be produced in a variety of different ways by varying simul- 

 taneously the thickness of the fire and the intensity of the draught. Each such 

 combustion results in a variation in the observed result. 



The measure of the combustion is effected by analysing the flue gases. It 

 was found impossible, with a range as ordinarily constructed, to obtain any 

 such value of the UO2 content of the flue gases as would correspond to really 

 efficient operations with the ordinary commercial type of range. The general 

 value showed that from eight to ten times the chemically necessary minimum 

 of air was used. A high CO2 value can only be secured by artificially sup]iress- 

 ing the in-leakage of air. The value of the CO2 content in any given appliance 

 was found to be a correct measure of the efficiency. 



With gas there are similar difficulties with regard to the surplus air, but not 

 with regard to constancy of condition. With electricity there are no experi- 

 mental difficulties, provided that the voltage of the supply is maintained 

 constant. 



Experimental Method. 



Oven. — The interior temperature in an oven is a function whose significance 

 is very small, for the following reasons : (1) the reading of the same thermometer 

 m different parts of the oven varies widely (2) several thermometers of different 

 type, all correct, and all, therefore, reading the same, when immersed, for 

 instance, in hot oil, take up widely different readings when placed together in 

 the same oven, essentially because the temperature of any object placed in any 

 conditions is such that the net time rate of gain or loss of heat is zero. 



Heat is lost or gained by any such object by radiation and convection. The 

 relative influence of these two in any given environment depends on tlie size, 

 shape, and radiativity of the surface of the bulb. Apart from any question of 

 dimensions, no measurements of efficiency by means of thermometer readings 

 would, therefore, in any case be possible. There is, in fact, no such function 

 as the ' interior temperature.' and the expression ' mean interior temperature ' 

 is meaningless, except when its physical meaning is arbitrarily laid down. The 

 only function to which this expression can reasonably Be applied is the rate at 

 which heat is transmitted to some extended object of conventional standard 

 size, radiativity of surface and shape, placed in the oven, and of such size as 

 to occupy a large part of the space. The expression ' efficiency ' can only have 

 a definite meaning when the object to which heat is communicated is precisely 

 defined in these respects. 



Exhaustive trials were made with vessels of standard shape filled with water, 

 by observing the rate of rise of temperature, and calculating the transmission 

 therefrom. This general method has the fatal disadvantage that great experi- 

 mental difficulties are introduced, and with a large object in the oven having 

 a rising temperature no sort of ' steady state ' can i)ossibly be maintained. 

 The efficiency also varies considerably according to the temperature of the object 

 to which heat is communicated. 



A more convenient calorimeter is a coil of blackened copper pipe, through 

 which a uniform stream of water is maintained. Special devices were designed 

 and made for maintaining constancy in the water flow. The water may be at 

 any temperature for which the efficiency is required. The flow is regulated so 

 that the rise in temperature between in-flow and out-flow is not great. The 

 coil as a whole can be maintained at a constant mean temperature. This method 

 has the great experimental advantage that one reading of the difference in 

 temperature between in-flow and out-flnw suffices to give the current rate of 



