ENGINE DESIGN

Evening Post, Volume CVI, Issue 56, 15 September 1928, Page 27

ENGINE DESIGN

INCREASING THE OUTPUT AN ENGINEER'S OPINIONS In the opinion of Mr. T. J. Little, one of tho best-knWn of America's motor engineers, it is easier to design an •entirely new engine than to modify an existing model with the aim of getting a greater output, but the latter course commends itself quite frequently, as the expense is considerably less than that of bringing out an entirely now power unit. Mr. Little was addressing a convention of engineers at Quebec. Leaving the bore and stroke unchanged, tho following methods are available for increasing the power developed:—An increase of speed, involving the volumetric efficiency, using a greater compression ratio, improving the thermal efficiency, and trying to get a greater mechanical efficiency. These items are, of course, to a large extent inter-related, as it is impossible to carry out a change of design affecting one of them without to some extent ■influencing the others. The first method, increasing the speed, has been investigated very thoroughly during the past few years, with tho. result that top gear ratios arc now very much lower than was formerly tho case, so that it is quite common for every-day engines to develop their maximum power at 3500 revolutions or more. 'This means that greater attention must bo given to balancing of parts, which- is ono of tho reasons for tho marked increase in the use of sixcylinder engines; Further the valve gear must be made as light as possible and must be provided with powerful sprkigs. Mr. Little forecasts the use of a new design of vibration damper for crankshafts, very much lighter than tho present typo and, of tor stating that the cast-iron piston is .practically obsolete, suggests that beryllium may displace aluminium for the purpose, as it is considerably lighter. As regards the possibility of increasing volumetric efficiency, this is mainly a matter of valve design. The greater the volumetric efficiency the heavier is tho charge of mixture drawn into tho onginc on each suction stroke, so thatan increase of efficiency causes a corresponding increase of power output. As the size of. the ordinary poppet valve is to some extent limited owing to space limitations, designers have been trying the effect of increasing the lift beyond the theoretical limit (one-quarter of the valve diamoter), and have found high lift valves to give a marked increase of. volumetric efficiency at high speeds. HIGH LIFT VALVES. An increase of lift imposes still more difficult conditions upon tho valve gear, and in this connection Mr. Little makes the interesting suggestion that hydraulic means of valve operation may supersede mechanical gears. Tho design, of the. inlet manifold also has a very pronounced influenco upon the volumetric efficiency of a nmlti-cyclindcr engine. As regards the compression ratio, it may bo as well to explain that the higher the pressure to which the mixture is compressed before firing, the greater is the power resulting from the firing stroke, but. the maximum ratio which can be usefully employed is limited by tho occurrence o"f detonation —knocking or pinking. This phenomenon is being suppressed by improved combustion chamber design and tho use of special fuels such a Ethyl and cracked petrol, which show a reduced tendency to detonation. The use of metals of high conductivity for the cylinder head, such as aluminium alloys, is also helpful. Increasing the compression ratio also increases thermal efficiency, which means that more power is obtained from a given quantity of fuel consumed, or conversely, the amount of fuel is decreased for a given rato of powor output. A close control of tho cooling water temperature by a thermostat is advantageous, and it would seem that steam cooling could show improved results as compared with water cooling. Finally, there is mechanical efficiency, which means tho extent to which the engine is usefully employing the power generated on tho working strokes. Oil and air cleaners naturally tend to improve mechanical efficiency by making better lubrication possible, while the powor absorbed in driving auxiliaries would be susceptible to reduction in many instances. Better materials arc also suggested by Mr. Little, such as nitralloy, which has - recently < been developed in Germany... This ,is a steel alloy containing about 2 per cent, of aluminium, which is remarkable for tho fact that it will absorb nitrogen very readily at comparatively low temperature in an atmosphere of ammonia. The impregnated surface can bo skin-hardoned to a point whore it will scratch glass quite readily, and is very much harder than any casehardened surface.