PHYSICS AND THE INTERNAL COMBUSTION ENGINE 27

Percentage of Coeflicient of Expansion
Nickel. ' per degree C.
26 000001312
28 000001131
28-7 000001041
30 -4 0 000004 58
3 I -4 0 00000340
34-6 0-00000137
35-7 0 '00000087
37-3 0-00000356
39-4. 000000537
44 -4. 0 00000856
475 . . 0-00000870
Ordinary Mild Steel 0-00001078
Ordinary Hard Steel 0-00001240

There is little doubt that the progress in the development of the
engine was at times held up almost entirely until a more suitable
material could be found to meet the case.

In the very high speed engines of large output, metals of higher
conductivity were inevitably called for, and those made use of are
special aluminium alloys for pistons, cylinder covers, etc. In the
forged state in particular, such materials give excellent results.

Some progress has taken place also in the use of magnesium
alloys because of their low specific gravity, coupled with very
good heat conductivity. Here again the forged conditions
produce a much more lasting material than with castings.

For crankshafts, connecting rods, etc., special steels were
developed with exceptionally high stress and impact values. An
example is shewn in Fig. 21.

For exhaust valves to resist the great heat, high nickel chrome
alloys have been successfully introduced, a strength of 34-5
tons being retained at 700°C.

The great need for the surface hardening of parts for quick
running engines to resist wear has at last resulted in the invention
of a system whereby, after treating special steel with ammonia gas,
an extraordinary hardness is obtained. Brinell hardness figures
range from 900 to 1,100, equivalent to Shore scleriscope of 102
to I 13 (Fig. 22). This looks to be of real benefit for the wearing