Page 50 - E BOOK ENGINE MECHANICAL M2

1. GENERAL

7. DIESEL FUEL
Diesel fuel looks like kerosene, but it is slightly yellowish and is somewhat cloudier and more viscous
than kerosene. The specific gravity of the diesel fuel is 0.82 to 0.87 and its required properties are as
follows.
◇ Good ignitability
◇ Proper viscosity
◇ Least water and impurities
◇ Least sulfur content
◇ Volatility and other properties

(1) Ignitability
Refer to the figure 1-6, it shows dropping diesel fuel and
gasoline fall on a heated iron plate. Gasoline will evapo-
Diesel fuel rate immediately and does not burn, but diesel fuel will
soon produce flame and begins to burn. This is what is
called ignitability and the scale representing ignitability
is igniting point.
The diesel fuel has lower ignition point than the gasoline
as shown in the table below.
Gasoline Diesel fuel
Specific gravity 0.65-0.75 0.82-0.87

Boiling point (℃) 40-200 180-350
Fig. 1-6 Flash point (℃) -40 or higher 50 or higher
Ignition point (℃) under
400-500 300-400
atmospheric pressure
Flammability range
(in volumetric rate %) 1.4-7.6 1.0-6.0

(2) Cetane Number
The highest cetane numbered fuel produces the least diesel knocking. Whether ignition lag is long or
short, it is expressed by cetane number.
The cetane number is determined by the following way. Normal cetane (C16H34) with a short ignition
lag is assigned cetane number 100, whereas alpha methyl naphthalene (C11h10) with a long ignition
lag is assigned cetane number 15. A blend of them is used as a standard fuel, and a test is per-
formed by a CFR (Cooperative Fuel Research) engine. The value of volume (%) of normal cetane in
the blended fuel equivalent in ignitability to the sample is treated as a cetane number.
The diesel fuel with higher cetane number has better ignitability that can suppress the occurrence of
diesel knocking. The cetane number of currently commercially available diesel fuels for diesel engine
is about 55 to 60 in Japan.

<For Your Reference>
Gasoline to be used for gasoline engine has a similar fuel index that is shown by Octane Number,
however the octane number features the resistance degree against the knocking occurrence on gas-
oline engine. When the knocking happens, it seems one of probable causes to use a lower octane
numbered gasoline, because the fuel has higher ignitability.
The use of higher octane numbered gasoline delivers more anti-knocking characteristics on the gaso-
line engine.

Therefore, this section can be summarized as follows:
Diesel fuel: Higher cetane number → better ignitability → Less occurrence of diesel knocking
Gasoline: Higher octane number → lower ignitability → Less occurrence of knocking in gasoline
engine

Pub. No. PTAE1228
1 - 7