In connexion with aeroplane engines, in which it is essential to
obtain the maximum power output from a given cylinder capacity
and engine weight, the values of t^p for air-cooled engines vary
from 70 to 95 lbs. per sq. inch, being upon the average about 80
lbs. per sq. inch, whilst with the water-cooled types in which higher
compressions are used, the values of ?^p vary from 80 to 110 lbs. per
sq. inch, a good average figure from test results J being 90 lbs. per sq.
inch. It might be here mentioned that most types of aeroplane en-
gines usually run at about half of the maximum speed of car engines,
owing to the fact that aerial propellers work more efficiently at
lower speeds. The speeds of aeronautical engines, in which the
propeller is directly coupled to the crankshaft, vary from 1,100 to
1,400 revolutions per minute ; it has been found possible, however,
in some engines to drive the propeller off the cam-shaft, at half the
engine speed, in which case the engine revolutions per minute vary
from 1,700 to 2,500, and better weight economy results. Engines
of the second type are usually on the Vee cyHnder principle, examples
of which are to be found in the 90 H.P. Renault running at 1,800
revolutions per minute, and the Sunbeam aeroplane engines run-
ning at 2,000 revolutions per minute.
* Proc. Inst, of Aut. Engineers, 1910-11, vol. v.
t " Two-Cycle Engines " (Brewer), Proc. Soc. Engineers, 1912.
j " Aeronautical Engines " (Graham Clark), Proc. Inst. Aut. Engineers,
1912.
PRESSURES AND TEMPERATURES 207

Aeroplane engines are usually of larger horse powers and cylinder
dimensions.
