590 Professor W. E. Dalby [May 26, 



able achievement for the small internal combustion engine fitted 

 in these vehicles. During 1921 about 800,000 licences were issued 

 to vehicles propelled by internal combustion engines, and the tax on 

 them amounted to about 10 million pounds. 



These brief considerations indicate how profound has been and 

 is the influence of the internal combustion engine in shaping our 

 destinies. It has conquered the air, and given us a prime mover 

 useful in farming and in transport. It is influencing the policy of 

 our railways, and will shortly so transform our outlook and our 

 modes of life that men of to-day will appear to be separated from 

 their boyhood not by a few decades but by a few centuries. 



SOME PKOBLEMS OF THE INTERNAL COMBUSTION ENGINE. 



Considering combustion from the point of view of the Kinetic 

 Theory of gases, but without attempting to explain the nature of the 

 differential attraction between molecules, most of the energy developed 

 in the cylinder of an internal combustion engine arises from the fact 

 that oxygen combines with carbon and hydrogen to develop large 

 quantities of heat. The function of the engine is to convert as much 

 as possible of this heat into mechanical work. 



It can be deduced by the laws of gases that the molecules at 

 22° C. and atmospheric pressure require 729 times the volume they 

 occupied as a liquid. This can be illustrated by " air patterns '* 

 representing the distribution of molecules in the air. Actually the 

 molecules are flying about at a high velocity across the vessel whose 

 sides they are continually bombarding and therefore exerting pressure 

 on them. 



Calculation from the kinetic theory of gases shows that at 22° C. 

 the oxygen molecules in the air are flying at a velocity of about 

 1600 ft. per second, the nitrogen molecules at about 1700 ft. per 

 second. This velocity is not the mean but the square root of the 

 mean square of the actual velocities of the particles. The molecules 

 collide and zig-zag about in the enclosing vessel, so that it is only by 

 imagination that we are able to conceive them as standing still and 

 forming a pattern something like the pattern on a wall-paper. 



When a spark is passed in a mixture of air and a hydro-carbon 

 such as pentane a re-arrangement of the molecules takes place. The 

 5 atoms of carbon in the pentane molecule produce 5 molecules of 

 carbon dioxide ; 12 atoms of hydrogen produce 6 molecules of steam. 

 Before ignition there are 41 molecules, including 32 molecules of 

 nitrogen. After the explosion there are 48 molecules, nitrogen 

 taking no part in the change. Oxygen ceases to exist as a separate 

 entity. The result is that every pound of pentane so transformed 

 produces 10,000 lbs. calories of heat 



The immediate effect of this production of heat is to increase the 



