Diesel Fuel Characteristics
Cetane Number
Fuel that has a high cetane number will give a shorter
ignition delay. A high cetane number will produce a
better ignition quality. Cetane numbers are derived
for fuels against proportions of cetane and
heptamethylnonane in the standard CFR engine.
Refer to “ISO 5165” for the test method.
Cetane numbers more than 45 are normally expected
from current diesel fuel. However, a cetane number
of 40 may be experienced in some territories. The
United States of America is one of the territories that
can have a low cetane value. A minimum cetane
value of 40 is required during average starting
conditions. A fuel with higher cetane number is
recommended for operations at high altitudes or in
cold-weather operations.
Fuel with a low cetane number can be the root cause
of problems during a cold start.
Viscosity
Viscosity is the property of a liquid of offering
resistance to shear or flow. Viscosity decreases with
increasing temperature. This decrease in viscosity
follows a logarithmic relationship for normal fossil
fuel. The common reference is to kinematic viscosity.
Kinematic viscosity is the quotient of the dynamic
viscosity that is divided by the density. The
determination of kinematic viscosity is normally by
readings from gravity flow viscometers at standard
temperatures. Refer to “ISO 3104” for the test
method.
The viscosity of the fuel is significant because fuel
serves as a lubricant for the fuel system components.
Fuel must have sufficient viscosity to lubricate the
fuel system in both extremely cold temperatures and
extremely hot temperatures. If the kinematic viscosity
of the fuel is lower than “1.4 cSt” at the fuel injection
pump, damage to the fuel injection pump can occur.
This damage can be excessive scuffing and seizure.
Low viscosity may lead to difficult hot restarting,
stalling, and loss of performance. High viscosity may
result in seizure of the pump.
Perkins recommends kinematic viscosities of 1.4 and
4.5 mm2/sec that is delivered to the fuel injection
pump. If a fuel with a low viscosity is used, cooling of
the fuel may be required to maintain 1.4 cSt or
greater viscosity at the fuel injection pump. Fuels with
a high viscosity might require fuel heaters to lower
the viscosity to 4.5 cSt at the fuel injection pump.
Density
Density is the mass of the fuel per unit volume at a
specific temperature. This parameter has a direct
influence on engine performance and a direct
influence on emissions. This influence determines
from a heat output given injected volume of fuel. This
parameter is quoted in the following kg/m
3
at 15 °C
(59 °F).
Perkins recommends a density of 841 kg/m
3
to
obtain the correct power output. Lighter fuels are
acceptable but these fuels will not produce the rated
power.
Sulfur
The level of sulfur is governed by emissions
legislations. Regional regulation, national
regulations, or international regulations can require a
fuel with a specific sulfur limit. The sulfur content of
the fuel and the fuel quality must comply with all
existing local regulations for emissions.
Perkins 404F-E22F and 403F-E17T diesel engines
have been designed to operate only with ULSD. By
using the test methods “ASTM D5453, ASTM D2622,
or ISO 20846 ISO 20884”, the content of sulfur in
ULSD fuel must be below 15 PPM (mg/kg) or
0.0015% mass.
NOTICE
Use of diesel fuel with higher than 15 PPM sulphur
limit in these engines will harm or permanently dam-
age emissions control systems. The engine service
interval will also be shortened.
Lubricity
Lubricity is the capability of the fuel to prevent pump
wear. The fluids lubricity describes the ability of the
fluid to reduce the friction between surfaces that are
under load. This ability reduces the damage that is
caused by friction. Fuel injection systems rely on the
lubricating properties of the fuel. Until fuel sulfur limits
were mandated, the fuels lubricity was generally
believed to be a function of fuel viscosity.
The lubricity has particular significance to the current
ultra low sulfur fuel, and low aromatic fossil fuels.
These fuels are made to meet stringent exhaust
emissions.
The lubricity of these fuels must not exceed wear
scar diameter of 0.52 mm (0.0205 inch). The fuel
lubricity test must be performed on an HFRR,
operated at 60 °C (140 °F). Refer to “ISO 12156-1”.
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