July 22, 1922] 



NA TURE 



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 some- 

 thing like the pattern on a wall-paper. 



When a spark is passed in a mixture of air and a 

 hydrocarbon 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 di- 

 oxide ; 12 atoms of hydrogen produce 6 molecules of 

 steam. Before ignition there are 41 molecules in- 

 cluding 32 molecules of nitrogen. After the explosion 

 there are 43 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 trans- 

 formed produces 10,000 lb. calories of heat. 



The immediate effect of this production of heat is to 

 increase the velocity of the flying molecules. The 

 actual velocity of the products of combustion in the 

 vessel depends on the mean temperature. Direct 

 measurement of the temperatures of the working 

 charge of a gas engine gives 2570 abs. as a reasonable 

 temperature from which to calculate molecular velo- 

 cities. At this temperature the carbon dioxide mole- 

 cules are moving at 3950 ft. per second, the steam 

 molecules at 6166 ft. per second, and the nitrogen 

 molecules at 4950 ft. per second ; these numbers being 

 the square roots of the mean squares of the actual 

 velocities. 



The next point for consideration is the time taken 

 to effect this change. The time-interval taken by 

 oxygen to combine with carbon and hydrogen lies 

 along a time scale beginning with a detonation and 

 ending with slow burning. In a mixture of air and 

 pentane the oxygen molecules are a long way, on the 

 average, from the carbon and hydrogen of the pentane 

 molecule, and also the freedom of action of the 

 oxygen molecules is clogged by the inert nitrogen 

 present, but the rapidity with which oxygen can 

 combine when the circumstances are favourable is 

 shown by nitro-glycerme. 



Chemists have discovered how to produce this nitro- 

 glycerine molecule so that oxygen lies side by side with 

 the carbon and the hydrogen. Its action is unclogged 

 by any other substance, and the molecular distances 

 have been annihilated, or perhaps it would be better to 

 say that they have become atomic distances. Moreover, 

 the molecule contains almost the exact quantity of carbon 

 and hydrogen required to satisfy the oxygen present. 

 As Lord Moulton once put it, it is a case of the lion and 

 the lamb lying down together. A mechanical shock 

 causes an immediate transformation — the lion devours 

 the lamb ; and the time-interval for the meal is so 

 short that it is not measurable. This is called a 

 detonation. Chemists have by their researches shown 

 how to combine nitro-glycerine with other substances 

 in order to control the rate of combustion. 

 Engineers are also trying to get control of the rate of 

 combustion of some of the mixtures used in the internal 

 combustion engine. Thus the chemist and the engineer 

 are working in different parts of the same wide field 

 of research. 



Experiments initiated by Sir Dugald Clerk are now 



NO. 2751, VOL. I io] 



proceeding at the National Physical Laboratory under 

 the general supervision of the Aeronautical Research 

 Committee for the Air Ministry. Apparatus of the 

 most refined nature has been devised, and the research 

 is being carried out by Mr. Fenning. Various com- 

 bustible mixtures are made up in a bomb. These are 

 exploded and then the time taken for the chemical 

 combination to take place is recorded. Two results may 

 be mentioned : a mixture of one part by volume of 

 hydrogen, 2 J parts by volume of air, was compressed 

 to 64 lbs. per sq. inch and then exploded. Between 

 the passage of the spark and the beginning of the rise 

 of pressure about four-thousandths of a second elapsed. 

 The combination was complete in about the same 

 interval of time. In another experiment the mixture 

 was diluted with one part of hydrogen and 6 parts of 

 air ; this caused delay in the combination, which took 

 six-hundredths of a second to complete. In such 

 diluted mixtures the energy has to be shared by 

 all the molecules which do not take part in the 

 change. 



The engineer is faced with two problems : the 

 problem of a too rapid combustion, becoming a detona- 

 tion, and the problem of a combustion too slow for 

 complete combustion at high speeds. 



In practice the turbulence and eddies caused by the 

 rapid admission of a charge through the narrow 

 annulus of an open admission valve results in quicken- 

 ing the rate of combustion, and it is owing to this 

 cause that the gas engine can run at speeds greater 

 than those corresponding to the measured rate of 

 flame propagation for an efficient mixture. Sir Dugald 

 Clerk found a striking difference in the area of 

 indicator diagrams according to whether the mixture 

 was exploded immediately after the admission valve 

 was closed or whether it was exploded after precautions 

 had been taken to damp out the eddies. 



Among the problems arising from running internal 

 combustion engines at high speeds is that of torsional 

 oscillations, and synchronous oscillations. There is 

 also the balancing problem. The four-cylinder petrol 

 engine is usually constructed so that it is perfectly 

 balanced for primary forces and couples, but ni\es the 

 maximum error for unbalanced secondary forces. At 

 certain speeds a model of this type suspended from 

 springs will oscillate twice as fast as the speed of 

 rotation of the engine, while at the same speed and 

 on the same springs a model, balanced to eliminate 

 the secondary forces, will run steadily at all speeds. 



Other problems have also to be considered. Accurate 

 records of the pressure-volume relation in the internal 

 combustion engine must be obtained, and the diffi- 

 culties are increased owing to the high speed at which 

 the cycles take place. The direct measurement of 

 temperature is also a difficult matter, and there are 

 various fuel problems. 



Sufficient has been said to show that the future of 

 the internal combustion engine is not settled ; it is 

 full of problems requiring continuous and laborious 

 research. The question is what provision has been 

 made for this research. Before the war purely scientific 

 research on the internal combustion engine was focussed 

 largely in the Research Committee of the British 

 Association established at the Dublin meeting in 1908. 

 This Committee was the only one of its kind, and the 



