ess lubricating oil in a diesel engine's cylinder is consumed
cleanly to produce power. By contrast, such oil in a gasoline engine's
cylinder is only partly burned. As a result carbon deposits form that
eventually cause malfunctioning of the spark plugs, valves, and
combustion chambers. This advantage accrued to the diesel because it
utilized an excess of air, and in addition its cylinder walls were
hotter. The engine was very clean-running from the standpoint of oil
leakage. This was a safety factor since it eliminated the possibility of
a fire starting on the outside surfaces of the engine, and in addition
it saved the time and money that was normally spent cleaning
engines.[28] Since the diesel utilized its heat of combustion more
efficiently than the gasoline engine, its cooling fin area could be
reduced by 35 percent. This permitted better streamlining. Having less
cooling fin area, it warmed up more rapidly than a gasoline engine.

[Illustration: The PACKARD-DIESEL AIRCRAFT ENGINE

Fire-Safe Fuel

_Furnaces in many a home burn similar oil_

_A lighted match cannot ignite or explode it_

_Saturated cloth can burn only like a wick_

_And the oil itself will quench this fire_

_But only when property atomized the spray may be ignited_

Graphic Proof of fuel safety in the Packard-Diesel Aircraft Engine

Figure 36.--Advertisement emphasizing the advantages of fire-safe fuel.
(Smithsonian photo A48848.)]

Due to the greater simplicity, it was more practical to build a large
diesel than a large gasoline engine. Large airplanes would therefore
need fewer engines if diesel powered. Smaller fuel tanks could be used
because of the greater fuel economy of the diesel, and also because of
the high specific gravity of fuel oil as compared to gasoline.
Furthermore, these smaller tanks could be placed in more convenient
locations. Not having a carburetor the engine could not backfire,
further reducing the fire hazard. The exhaust note was lower because of
the diesel's higher expansion ratio. The absence of an ignition system
permitted the diesel to operate in the heaviest types of precipitation.
Such conditions might cause the ignition system of a gasoline engine to
malfunction. The Packard diesel was flown at times without exhaust
stacks or manifolds; this was practical from a safety standpoint because
of the diesel's lower exhaust temperature due to its higher expansion
ratio. Elimination of these parts reduced th