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 a Diesel engine
operating with a compression ratio of about 16:1 is approximately 1000
degrees Fahr., which is far above the spontaneous-ignition temperature
of the fuel used. Accordingly, when the fuel is injected in a highly
atomized condition at some time previous to the piston reaching the end
of its stroke, the fuel burns as it comes in contact with the highly
heated air, and the greatly increased pressures resulting from the
tremendous increase in temperature brought about by this combustion,
acting on the pistons, drive the engine, as in the case of the gasoline
engine.
Summing up, the differences between the Diesel and gasoline engines
start with the fact that the gasoline engine requires a complicated
electrical ignition system in order to fire the combustible mixture,
whereas the Diesel engine generates its own heat to start combustion by
means of highly compressed air. This brings about the necessity for
injecting the fuel in a well-atomized condition at the time that
combustion is desired and the quantities of fuel injected at this time
control the amount of heat generated; that is, an infinitesimally small
quantity of fuel will be burned just as efficiently in the Diesel engine
as a full charge of fuel, whereas in the gasoline engine the mixture
ratio must be kept reasonably constant and, if the supply of fuel is to
be cut down for throttling purposes, the supply of air must be
correspondingly reduced. It is this requirement in a gasoline engine
that necessitates an accurate and sensitive fuel-and-air metering device
known as the carburetor.
The fact that the air supply of a Diesel engine is compressed and its
temperature raised to such a high degree permits the use of liquid fuels
with a high ignition temperature. These fuels correspond more nearly to
the crude petroleum oil as it issues from the wells and this fact
accounts for the much lower cost of Diesel fuel as compared to the
highly refined gasoline needed for aircraft engines.