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.

Weight-Saving Features

In order to be successful in aviation use, the modern lightweight diesel
of the time had to have its weight reduced from 25 lb/hp to 2.5 lb/hp.
This required unusual design and construction methods, as follows:

Crankcase: It weighed only 34 lb because of three factors: Magnesium
alloy was used extensively in its construction, thus saving weight as
compared with aluminum alloy, which was the conventional material at
this time. It was a singl