Packard diesel engine compared with
those of a 4-stroke cycle gasoline engine are illustrated in figure 21.

[Illustration: =Brief Analysis of Action in a Four-Cycle Gasoline Engine=

_Mixture of air and gasoline enters cylinder from carburetor._

_Mixture is compressed into smaller volume by piston moving upward._

_An electric spark ignites the compressed mixture causing it to
explode._

_Combustion heat increases the cylinder pressure forcing piston
downward._

_Momentum carries piston upward which pushes burnt gases out through the
exhaust valve._

=Similar Action in the Packard-Diesel Aircraft Engine=

_Atmospheric air only, enters cylinder through single valve._

_Air is so greatly compressed by upward moving piston that it reaches
temperature of 1000 deg. F._

_Just before piston is at dead center fuel oil is sprayed into cylinder
and spontaneously ignited._

_Power of this explosion is passed to crankshaft in conventional
manner._

_Piston forces out burnt gases through same single valve which is cooled
by inrush of new air as cycle repeats._

Figure 21.--Operating cycles. (Smithsonian photo A48846.)]

Although the size, weight, and general arrangement of the Packard diesel
did not differ radically from conventional gasoline engines of a similar
type, there were definite differences caused by the diesel cycle. In the
words of Capt. Woolson:[12]

As this engine operates on an entirely different principle than the
gasoline engines used heretofore in aircraft, it is desirable before
launching into a mechanical description to consider first in a general
way the principles of operation of the Diesel cycle as opposed to the
Otto cycle principle on which nearly all gasoline engines operate.

The real point of departure between the two systems of operation is the
ignition system involved. In the gasoline engine an electric spark is
depended upon to fire a combustible mixture of gasoline vapor and air
which mixture ratio must be maintained within rather narrow limits to be
fired by this method....

In the Diesel engine, air alone is introduced into the cylinders,
instead of a mixture of air and fuel as in the gasoline engine, and this
air is compressed into much smaller space than is possible when using a
mixture of gasoline and air, which would spontaneously and prematurely
detonate if compressed to this degree. The temperature of the air in the
cylinder at the end of the compression stroke of