TM 5-3895-383-24
Most fuels that have a cetane number above 40 will permit
acceptable engine starts in warmer outside temperatures. The
engine will start satisfactorily with this fuel when the engine is
kept warm. The engine can be kept warm by using either a
heated room or a coolant heater.
During average starting conditions, direct injection engines
require a minimum cetane number of 40. A higher cetane
value may be required for operation in high altitudes or for cold
weather operation. The minimum fuel cetane number that is
required for the precombustion engine is 35.
Modifying the Cetane Number
The cetane number of a fuel can be changed if the fuel is
mixed with a fuel that has a different cetane number.
Generally, the cetane number of the mixture will be in direct
relation to the ratio of the fuels that were mixed. Your fuel
supplier can provide the information about the cetane number
of a particular fuel.
Additives can also be used to improve the cetane number of a
fuel. Additives are evaluated through testing in special
engines. However, the fuel characteristics of additives are not
identical to a natural product. While both fuels may be rated as
having the same cetane number, starting may be different.
Cloud Point
It is important to understand that the cloud point of a fuel is
different from the pour point. There is no relationship between
cloud point and the pour point. The cloud point is the
temperature at which some of the heavier paraffin (wax)
components in the fuel become solid particles. This wax is not
a contaminant in the fuel. The wax is an important element of
No. 2 diesel fuel. The wax has a high fuel energy content and
the wax has a very high cetane value. Removal of the heavier
wax lowers the cloud point of the fuel. Removal of the wax
also increases the cost because less fuel can be made from
the same amount of crude oil. Basically, a No. 1 diesel fuel is
formulated by removing the wax from a No. 2 diesel fuel.
The cloud point of the fuel is important because the cloud point
can limit the performance of the fuel filter. The wax can alter
the fuel characteristics in cold weather. Solid wax can fill the
fuel filters. The solidified wax will stop the flow of fuel. Fuel
filers are necessary in order to remove dirt from the fuel. The
filters block foreign material, and the filters protect the parts for
the fuel injection system. Since fuel must flow through the
filters, installing a fuel heater is the most practical way to
prevent the problem. A fuel heater will keep the fuel above the
cloud point as the fuel flows through the fuel system. The fuel
heater will permit the wax to flow through the filters with the
fuel.
Modifying the Cloud Point
You can lower the cloud point of a diesel fuel by mixing the
diesel fuel with a different fuel that has a lower cloud point.
No. 1 diesel fuel or kerosene may be used to lower the cloud
point of a diesel fuel. The efficiency of this method is not good,
because the ratio of the mixture does not have a direct relation
to the improvement in cloud point. The amount of fuel with low
cloud point that is required makes the process less preferable
to use.
The following illustration contains a table that can be used to
find the necessary mixture for two fuels with different cloud
points. In order to use the table, you must know the exact fuel
cloud point of each fuel. This specification can change from
one purchase of fuel to the next purchase of fuel. This
specification is normally available from personnel at the source
of the fuel supply. When fuels that have a lower cloud point
are not available, this method cannot be used.
The manufacturer of the fuel can add cold flow improvers to
the fuel. Cold flow improvers modify the wax crystals in the
fuels. The cold flow improvers do not change the fuel's cloud
point. However, the cold flow improvers keep the wax crystals
small enough to pass through standard fuel filters. For mixing
precautions, see the topic "Pour Point".
Maintenance Section
Fuel Specifications
1-35
