ENGINEMAN 1 & C
(to the atmosphere) during the exhaust stroke; and
some is absorbed by the walls of the exhaust
passages.
Heat losses to the atmosphere through the
exhaust are unavoidable. This is because the
engine cylinder must be cleared of the hot exhaust
gases before the next air intake charge can be
made. The heat lost to the exhaust is determined
by the temperature within the cylinder when ex-
haust begins. The amount of fuel injected and the
weight of air compressed within the cylinder are
the controlling factors. Improper timing of the
exhaust valves, whether too early or too late, will
result in increased heat losses. If too early, the
valve releases the pressure in the cylinder before
all the available work is obtained; if too late, the
necessary amount of air for complete combustion
of the next charge cannot be realized, although
a small amount of additional work may be ob-
tained. Proper timing and seating of the valves
is essential in order to maintain heat loss to the
exhaust at a minimum.
Heat losses due to imperfect or incomplete
combustion have a serious effect on the power
that can be developed in the cylinder. Because of
the short interval of time necessary for the cycle
in modern engines, complete combustion is not
possible; but heat losses can be kept to a minimum
if the engine is kept in proper adjustment. It is
often possible to detect incomplete combustion
by watching for abnormal exhaust temperatures
and changes in the exhaust color, and by being
alert for unusual noises in the engine.
Mechanical Losses
There are several kinds of mechanical losses,
but all are not present in every engine. The
mechanical or friction losses of an engine include
bearing friction; piston and piston ring friction;
pumping losses caused by operation of water
pumps, lubricating pumps, and scavenging air
blowers; power required to operate valves; etc.
Friction losses cannot be eliminated, but they can
be kept to a minimum by maintaining the engine
in its best mechanical condition. Bearings, pistons,
and piston rings should be properly installed and
fitted, shafts must be in alignment, and lubricating
and cooling systems should be at their highest
operating efficiency.
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