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National Advisory Committee for Aeronautics, Technical Notes - Factors Affecting Heat Transfer in the Internal Combustion Engine

There are several methodslfor evaluating heat losses
in the internal~combustion engine. These methods mav be
oroadly divided into two categories: one based upon theI
thermodynamic analysis of indicator diagrams (references
1 and 2), and the other based upon the measurement of the
total amount of heat absorbed by water Jackets (references
3, 4. and 5) or cooling air (ref_erence 6). The changes
that actually occur in- the engine cylinder are so compli~
cated in nature that no method of theoretical analysis or
experimental investigation may claim'to have yielded en—
tirely satisfactory-results.

Investigators of the first category often contend
that the direct measurement of heat absorbed by jacket
water or cooling air has- no actual bearing on the thermo—
dynamics of the engine, as an unknown proportion of ex—
haust heat absorbed by the exhaust port and_its surround—
ings, as well as the uncertain amount of piston—friction
heat, inevitably creep into the measurement. This arqu~
ment is quite correct. However, the analysis based upon
the indicator diagram does not appear to give results
which finally Justify- the amount of labor called for, and
it does not give a good picture of the mechanism of heat
transfer and factors controlling same, so that the prac—
tical application.of such analysis is rather limited. It
would be of great thermodynamic interest if sufficiently
reliable indicator diagrams were obtainable at high speeds.
so that instantaneous gas temperatures could be computed
from these diagrams without serious error.

The present investigation is an attempt to evaluate
the effects of'air consumption, mean piston speed, mix-
ture ratio, and compression ratio on the mean heat trans—
fer from the hot gases'to a heat collector screwed into
the combustion chamber of theispark-ignition engine. Heat
exchange between the cylinder walls and the heat collector
was reduced to the passible minimum by the provision of a
dead~air space around the heat collector, and by equaliz-
in€;the temperatures of the two bodies.