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Module8:Engine Fuels and Their Effects on Emissions
Lecture 38:Diesel Fuels
The Lecture Contains:
DIESEL FUELS
Ignition Quality
Distillation Range
Viscosity
Chemical Composition
Sulphur Content
Lubricity
Summary of Effect of Diesel Fuel Quality on Emissions
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Module8:Engine Fuels and Their Effects on Emissions
Lecture 38:Diesel Fuels
DIESEL FUELS
Diesel fuel is a mixture of a few hundred hydrocarbons derived from refining of crude petroleum.
When petroleum prices were low, the diesel fuels were produced mostly by blending various refinery
streams from the atmospheric distillation unit of petroleum refineries. To meet the increasing demand
of the diesel fuels, products of secondary refinery processes like thermal and catalytic cracking,
hydro-cracking, vis-breaking etc., also are used as blending components of the current diesel fuels.
The diesel fuel streams evaporate generally in the temperature range of 150- 390º C. The key
properties of the diesel fuel have already been given in Table 8.1. Besides these, other significant
properties include cold flow characteristics at low ambient temperatures, water and sediment content
etc. The important diesel fuel quality parameters are discussed below.
Ignition Quality
Ignition quality is a measure of ease of self-ignition of diesel fuel when the fuel is injected in hot
compressed air in the engine cylinder. Cetane number (CN) is the most widely accepted measure of
ignition quality as it is measured by a test on the engine. The cetane number scale is defined in
terms of blends of two pure hydrocarbons used as reference fuels;
A high ignition quality hydrocarbon: n- Hexadecane or Cetane ( n-C 16 H 34 ) given CN =100
Another hydrocarbon with poor ignition quality: Hepta-methyl nonane (HMN) assigned CN =15.
The cetane number scale is given by:
CN = % n-cetane + 0.15 x % HMN (8.3)
Cetane number is measured in a standard single cylinder, variable compression ratio CFR engine
according to ASTM D613 method. The test engine is a prechamber diesel engine. The test conditions
are:
(i) Intake air temperature = 65.6° C,
(ii) Coolant temperature = 100° C,
(iii) Engine speed = 900 rpm,
(iv) Injection advance = 13° btdc.
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Module8:Engine Fuels and Their Effects on Emissions
Lecture 38:Diesel Fuels
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Engine compression ratio is varied to obtain start of combustion at top dead centre i.e. ignition delay
is maintained equal to 13° CA for the test fuel and two blends of reference fuels that bracket the
compression ratio obtained with the test fuel. The reference fuel blends should not be more than 5
CN units apart. The cetane number of the test fuel is determined by interpolation from the
compression ratio values and the corresponding cetane numbers of the reference fuel blends.
Correlations of ignition quality with the physical properties of the diesel fuels have also been
developed for ease of quality control during refining. These correlations are applicable only to the
neat petroleum derived diesel fuels when no additives are used to improve ignition quality. The
Calculated Cetane Index (CCI) determined by ASTM D 976 or ASTM D 4737 methods are more
commonly used as an alternative to CN for the neat diesel fuels. The calculated cetane index is not a
substitute for ASTM cetane number. It is only a supplementary parameter for predicting cetane
number when used keeping in view its limitations. The CCI calculation methods are not suitable for
pure hydrocarbons, or non-petroleum based fuels derived from coal.
ASTM D 976 uses a two variable equation to determine CCI from the mid-boiling point and density of
the diesel fuel as below,
CCI976 = 454.74 - 1641.416 D + 774.74 D 2 - 0.554 B + 97.803 (log B)2 (8.5)
where:
D = density at 15°C (g/ml) determined by Test Method ASTM D 1298.
B = 50% evaporation (mid-boiling) temperature (°C) determined by Test Method ASTM D 86 and
corrected to standard barometric pressure.
ASTM D 4737 is a newer more exhaustive method in which CCI is correlated to four variables; the
density and temperatures for evaporation of 10% (T 10 ), 50% (T 50 ) and 90% (T 90 ). Details are
available in the relevant standards and other texts.
CCI values are quite close to the cetane number. Several national fuel standards also specify cetane
index values in addition to cetane number.
Cetane or ignition improvers are used to improve ignition quality of the diesel fuels. Nitrates like
isopropyl nitrate, cyclo-hexyl nitrates, ethyl-hexyl nitrate (EHN) and peroxides like di-tertiary-butyl
peroxide are used as cetane improvers. These compounds readily decompose at the engine
compression temperatures and produce free radicals that accelerate precombustion reactions and
thereby reduce ignition delay. The cetane improvers are used typically in dosages of around 500 to
2000 ppm by volume.
With high cetane fuels cold starting is easier and engine warm up is faster. Therefore, use of high CN
fuels results in lower HC emissions during engine warm-up phase and the reduction in HC is more
significant at low ambient temperatures. High cetane number fuels give shorter ignition delay and
thus, reduce the amount of fuel burned in premixed phase which results in lower peak combustion
pressures and temperatures. Therefore, high CN fuels are expected to give lower NOx emissions.
Typical effect of CN on NO x emissions is shown on Fig. 8.5. Ratio of NO x emissions obtained for 58
CN fuels relative to that obtained with 50 CN fuel are shown at different engine loads. Depending
upon the engine load, up to 6 - 8% lower NO x may result with increase of CN from 50 to 58. At full
engine load however, a slight increase in NO x with increase in CN was seen.
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Module8:Engine Fuels and Their Effects on Emissions
Lecture 38:Diesel Fuels
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Effect of cetane number increase from 50 CN to 58 CN on NO x
Figure 8.5 emissions; natural and additive improved cetane number fuels
have slightly different effects..
Effect of CN on PM emissions depends on the engine design. For high speed, light duty engines a
small reduction in PM emissions with high CN has been observed while low speed, heavy duty
engines showed no significant change. With high cetane fuels the ignition delay is shorter and more
fuel burns in diffusion combustion phase, which may increase soot emissions depending upon the
engine operating conditions and design. Current fuel specifications are setting minimum limits of more
than 50 units for the CN and minimum cetane index is also being specified as the natural high
cetane fuels generally give overall better engine performance with respect to PM and HC emissions.
Distillation Range
A typical distillation curve for diesel fuel is shown in Fig. 8.6. The temperature for 50 percent
distillation temperature or mid-boiling point, 90 percent point and the final boiling point are the
important distillation parameters.
Lower the boiling point of the fuel, more readily it vaporizes and mixes with air giving more complete
combustion. The mid boiling volatility is also correlated to the other physico-chemical properties like,
density, viscosity and ignition quality. A higher mid boiling point fuel has higher density and viscosity,
and usually a lower CN. Low mid boiling point fuels give faster cold starting and hence lower HC
emissions. The fuel components boiling above 350º C may not burn completely, forming high soot
concentrations and combustion chamber deposits. Fuels with high T90 and final boiling point are seen
to result in an increased injector coking leading to poor combustion and higher smoke emission.
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Figure 8.6 Typical distillation characteristics of diesel fuel.
For the modern diesel fuels low maximum permissible limits for T90 or T95 around 350 to 360º C are
being stipulated due to their negative long-term effects on emissions
Density
The density of diesel fuel varies generally in the range 810 to 880 kg/m 3 . The density, volatility,
cetane number, viscosity and heat of combustion of petroleum fuels are interrelated. An increase of
10 percent in density increases the volumetric energy density (MJ/ m3 ) of the fuel approximately by 6
percent. The balance 4 percent is accounted for by the decrease in heat of combustion (MJ/kg) for
the 10% heavier (higher molecular weight) fuels is metered volumetrically by injection pumps. The fuel
density affects engine calibration and power as the fuel mass injected/stroke varies with fuel density.
High-density fuels also have a higher viscosity thus, influence injection characteristics. Increase in the
fuel density advances the dynamic injection timing by up to 1 ºCA. Thus, the fuel density affects
engine combustion and emissions.
PM emissions generally increase with increase in fuel density. As the fuel injection system is
calibrated for a particular fuel density, the current fuel specifications set narrow limits acceptable fuel
specific gravity e.g. from 0.82 to 0.85.
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Module8:Engine Fuels and Their Effects on Emissions
Lecture 38:Diesel Fuels
Viscosity
The viscosity of diesel has a strong influence on fuel atomization, a high viscosity fuel resulting
in larger fuel droplets.
An increase in viscosity reduces spray cone angle and increases spray penetration.
Low viscosity on the other hand, results in an increase in leakage of fuel past the pumping
elements and loss of fuel system calibration.
High viscosity of fuel is necessary for lubrication and protection of the injection equipment from
wear.
Most fuel specifications limit kinematic viscosity of diesel fuel in the range 2.0 to 5.0 centistokes.
Chemical Composition
Olefins adversely affect oxidation and storage stability of diesel fuels. Poor fuel oxidation
stability results in formation of gummy substances which cause plugging of fine fuel filters and
sticking of injector needles.
As the aromatic content of fuel increases, the particulate and PAH emissions increase.
Fuels with high aromatic content have a lower natural cetane number and its consequent effect
on combustion and emissions.
Aromatics have higher flame temperatures and hence increase in aromatic content of fuel is
expected to result in higher NO x emissions.
An increase in polycyclic aromatic content generally results in higher PM emissions. .
The diesel fuel specifications in the USA, Europe and several other countries now limit aromatic
content to 10 percent maximum. Also, limits on the poly-aromatic hydrocarbons are being specified.
One side effect of reduction in aromatic content is reduction in lubricity characteristics of the diesel
fuels causing durability problems of injection pump and injectors.
Sulphur Content
Sulphur on combustion produces sulphur dioxide (SO 2 ), of which about 1 to 3% is oxidized to
sulphur trioxide (SO 3 ) and forms sulphates found in particulate emissions. The balance of SO2
is exhausted as gas.
Typically, increase of 500 ppm in sulphur content contributes to about 0.01 g/kWh increase in
diesel PM emissions.
The sulphur trioxide on combining with water forms sulphuric acid that causes wear of engine
cylinder liner and piston rings.
Sulphur increases deposit formation in the combustion chamber and the deposits become
harder in presence of sulphur.
Fuel sulphur has deleterious effect on functioning of advanced after-treatment devices such as
NO x storage- reduction (NSR) catalysts, continuously regenerating diesel particulate traps
(CRT) and catalyzed diesel particulate filters (CDPF). The sulphur dioxide and trioxide poison
the catalyst. NSR catalysts require practically sulphur free (< 5 ppm) fuel.
In CRT, conversion of NO to NO 2 ahead of CRT is reduced by sulphur as it poisons the
catalyst. Sulphur lower than 30 ppm is necessary for functioning of CRT
In most countries during early 1990s, sulphur content of diesel fuel was in the range from 0.2 to 0.5%
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(2000 to 5000 ppm) by mass. After the year 2000, a number of European countries made available
the diesel fuels with less than 0.005% (50 ppm) sulphur. A large number of countries around the
world have diesel fuels with sulphur below 0.05% for road vehicle application. Trends in diesel fuel
sulphur content in some countries are given in Table 8.10.
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Module8:Engine Fuels and Their Effects on Emissions
Lecture 38:Diesel Fuels
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Table 8.10
Trends in Diesel Fuel Sulphur in Some
Countries
Country/Region Effective date/Fuel Grade Maximum allowable sulphur
content, % m/m
European Union 1994 0.2
1996 0.05
2000 0.035
Sweden 1990 0.20
1993 EC1 0.001
EC2 0.005
EC3 As per the EN 590 standards
UK 1996 0.05
1998 (Ultra low sulphur grade, 0.005
ULSD)
Japan 1997 0.05
India 1995 0.5
2000 0.2
2005 0.05
2010 0.005
Lubricity
The injection pump and injectors are lubricated by the diesel fuel itself. The heavier, high viscosity
hydrocarbons and polar compounds provide natural lubricity to the diesel fuel. Hydro-treating of diesel
fuel to remove sulphur also converts and removes polar compounds. Consequently, as the sulphur
content of diesel fuel decreases, the lubricity of diesel fuel goes down resulting in excessive injection
pump wear. A high frequency reciprocating rig (HFRR) test in which wear scar size on a standard
test piece with the test fuel is measured, provides good correlation to the injection pump wear in real
life. The HFRR test limit of 460µm wear scar diameter has been accepted in European diesel fuel
specifications to provide adequate protection against injection pump wear.
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Module8:Engine Fuels and Their Effects on Emissions
Lecture 38:Diesel Fuels
Summary of Effect of Diesel Fuel Quality on Emissions
Effect of changes in important diesel fuel properties on CO, HC, NO x and PM emissions has been
investigated in several programmes in Europe and the USA and also by other investigators. The
overall effects of fuel properties on emissions are qualitatively summarized in Table 8.11.
Table 8.11
Summary of Overall Effects of Diesel Fuel Property Changes on
Emissions
Vehicle Fuel Property Property Change CO HC NOx PM
type
Light-duty Increase CN 50 → 55 ↓↓ ↓↓ ±0 ↓
vehicles Reduce density 850 → 820 kg/m3 ↓ ↓ 0 ↓↓↓
Reduce T95 370 → 330 C -0 -0 -0 -0
Reduce PAH 6 → 3 % v/v ? ? ? ?
Reduce sulphur 2000 →500 ppm 0 0 0 ↓
Heavy-duty Increase CN 50 → 55 ↓ ↓ ↓ ↓
vehicles Reduce density 850 → 820 kg/m3 0 0 ↓ ↓
Reduce T95 370 → 330 C -0 -0 -0 -0
Reduce PAH 6 → 3 % v/v ? ? ? ?
Reduce sulphur 2000 →500 ppm 0 0 0 ↓↓
Key: 0 = No effect; ± 0 = -2 to 2 %; ↓ or ↑ = 2 to 10%; ↑↑ or ↓↓ = 10 to 20%;
↑↑↑ or ↓↓↓ = >20%; ? = Insufficient data
Diesel Fuel Specifications
Engine technology is being continuously modified and upgraded to meet the newer and more stringent
emission regulations. Accordingly, the diesel fuel quality also has undergone significant improvements
particularly related to CN, sulphur content, tail end volatility and wear protection of injection system. In
some countries, bio-diesel is also being blended with diesel fuel. To keep injection pump calibration at
acceptable level with diesel-fuel and biodiesel blends, a maximum oxygen content limit has been
specified in the Indian fuel specifications. The key diesel fuel properties in Europe and India are
given in Table 8.12.
Table 8.12
Diesel Fuel Specifications in Some
Countries
Property Europe (EN590: India (IS 1460) (BS IV -
1999) 2010)
Cetane number, min. 51 51
Cetane index, min 46 46
Density @ 15º C, kg/m3 820-845 820-845
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Viscosity @40º C, mm2/s 2.0 -4.5 2.0-5.0
Sulphur, %m/m, max. 0.035 BS III: 0.035
BS IV: 0.005
Total aromatics, % m/m, max. -
Polyaromatics, (di+tri++), max. 11 11
Distillation:
% recovered at 250º C, max. 65
T 85, ºC, max. 350
T90, ºC, max./range -
T95, ºC, max. 360 360
FBP, ºC /range - -
Flash Point, ºC, min. 55 35
Carbon residue, % m/m, max. 0.30 0.30
Water content, mg/kg, max. 200 200
Oxidation stability g/m3, max. 25 25
Particulates, mg/l 24 -
Lubricity(HFRR scar dia. @ 60º C), µm 460 460
Oxygen content, % by mass max. - 0.6
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