The mean effective pressures developed in car engines at the
maximum power output vary from 70 to 110 lbs. per sq. inch,
reckoned on the brake horse power ; this mean effective pressure
is generally denoted by the symbol 77. p, where rj is the mechanical
efficiency, and p the indicated mean effective pressure.
The maximum value for rjp obtained in the Rating of Petrol



206 HIGH SPEED INTERNAL COMBUSTION ENGINES

Engines Committee's tests * was 104 lbs. per sq. .inch at about
1,000 revolutions per minute.
The compression pressures of air-cooled petrol engiaes are lower
than with the water-cooled types, and vary from 70 to 95 lbs. per
sq. inch in modem types, being chiefly Umited by cooling con-
siderations, since high compressions are associated with high
temperatures.
In connexion with two-stroke petrol engines of the three-port
type, the allowable compression pressures with both air and water-
cooled types are generally appreciably lower than with corresponding
sizes of four-cycle engines ; the compressions realized are also
aSeoted by the poorer volumetric efficiencies, which correspond
in effect with throtthng of the charge, poorer exhaust scavenging
and loss of charge, etc.
With the improvement of the two-stroke engine, the compression
pressure has risen, and mean effective pressures varying from 70
to 80 lbs. per sq. inch have been indicated.
Some tests upon exceptionally good two-cycle engines f have
given measured values of rjf varyiag from 80 to 90 lbs. per sq.
inch.
