ON GASEOUS EXPLOSIONS. 197 
and lead to the conclusion that the heat-loss per unit area from a mass 
of glowing gas would go on increasing with the volume of the mass 
until that volume is comparable with the largest sizes of gas-engine 
cylinder now made. David found that the radiation after an explosion 
in a cylindrical vessel 1 foot by 1 foot was nearly twice as great when 
the walls of the vessel were highly polished as when they were black. 
The effect of completely polishing the interior of a vessel is, so far as 
radiation is concerned, much the same as greatly enlarging the volume 
of enclosed gas, so that this experiment gives an idea how far the 
heat-loss from the gas in a cylinder 1 foot in diameter falls short of 
that in a very large cylinder. It is quite clear that in a 12-inch cylinder 
the limit of size beyond which heat-loss per square foot does not 
increase is far from having been reached. 
One practical aspect of this question is the relation between size 
and thermal efficiency. This was fully discussed by Callendar in a 
paper read before the Institution of Automobile Engineers in 1907,° 
who pointed out the probability that some part of the radiation loss was 
proportional to the volume. If heat-loss were simply a question of the 
surface exposed the percentage losses in similar engines should be 
reduced in proportion to the linear dimensions, and there should be a 
corresponding increase in efficiency. But in so far as_heat-flow 
increases with the volume, the efficiency of large and small engines 
will become more nearly the same. Of even greater importance practi- 
cally is the absolute amount of heat-flow per square foot, since it is 
this which determines the internal temperatures and so sets a limit to 
the output of the engine. The results cited show broadly that this 
quantity must be considerably greater in an engine of say 3-feet bore 
than in one whose cylinder diameter is only 1 foot, and that the 
difficulty of designing and working the first is not alone due to the 
greater thickness of metal, but also to the greater heat-flow. 
4. The Effect of Density.—The density of the gas in a gas-engine 
explosion is from four to seven times that of the atmosphere. In the 
Diesel engine it is, of course, very much greater. The effect of this 
factor is greatly to increase the heat-flow as compared with an ordinary 
closed-vessel explosion, where the density is that of the atmosphere 
and the vessel similar in size and shape to the combustion chamber. 
A rough notion of the magnitude of this effect can be obtained by 
comparisons of the jacket-loss in a gas-engine when the total quantity 
of combustible mixture is altered by throttling or otherwise, the com- 
position remaining the same. It has been found that the total heat 
carried away from the jackets increases with the quantity of mixture, 
but not quite in proportion thereto. A similar result is obtained from 
closed-vessel explosions; it is found that the pressure after firing a 
mixture of given composition falls relatively less rapidly when the 
pressure before explosion is higher, but the absolute amount of heat- 
loss in a given time is greater.* The quantitative relation between heat- 
5 Proc. Inst. Aut. Eng., April 1907. 
° The Gas, Oil, and Petrol Engine, by Dugald Clerk, vol. i., chap. vii. (Long- 
mans, 1910.) 
