Product Specifications
Contents
All Products
1.0 Background
1.1 Odour
1.2 Appearance
1.3 Density
1.4 Specific Gravity
1.5 Viscosity
1.6 Distillation
1.7 Sulphur
1.8 Aromatics
1.9 Water Content
1.10 Particulate Matter
1.11 Flash Point for Distillates
Gasoline (petrol) Specific
2.0 Background
2.1 Octane
2.2 Vapour Pressure
2.3 Lead Content
2.4 Benzene Content
Diesel Specific
3.0 Background
3.1 Cetane Number
3.2 Cetane Index
3.3 Lubricity
3.4 Cloud Point
3.5 CFPP
Gasoil Specific
4.0 Background
4.1 Cetane Number
4.2 Cetane Index
4.3 Cloud Point
4.4 CFPP
Kerosene Specific
5.0 Background
5.1 Smoke Point
5.2 Char Value
All Products
1.0 Background Everyone dealing with petroleum products is aware of the many
specifications associated with them, but very few understand the importance of the
long lists of properties and how they impact on the end use of a particular product.
Several properties apply to all fuels and the main ones are explained below.
1.1 Odour The odour of a product is invariably the first property that customers
experience. Each fuel type has its own distinctive smell, however pungent odours
caused by sulphur or in severe cases hydrogen sulphide can indicate problems with
the product. If gasolines have been in storage for long periods they can oxidise. This
is where they absorb oxygen and can start to form gummy deposits. Oxidised
gasolines have a distinct odour similar to the smell of varnish.
1.2 Appearance The appearance of a fuel is the second simple indication of the
quality in terms of water content, colour and sediment. A cloudy appearance suggests
the presence of water, sediment or particulate matter, which indicates some form of
contamination. Likewise, a products colour can also suggest contamination.
Excess water in a product can affect its handling and combustion characteristics.
Water can block filters and contribute to the corrosion of fuel systems.
Sediment can also block filters and cause damage to pumps, injectors etc.
1.3 Density The density of a fuel is important for 2 main reasons.
Firstly, because the units are weight over volume, e.g. kg / litre, the number of litres
per tonne can be quickly calculated using the formula 1,000 / density (kg/litre).
Conversely the weight for the quantity of litres can be determined by using the
number of litres x the density kg/litre.
As an example, a gas oil with a density of 0.8650 kg / litre would equate to 1,000 /
0.865 = 1,156 litres per tonne.
1,000 litres of gas oil with a density of 0.865 kg / litre would weigh 1,000 x 0.865 =
865 kg or 0.865 tonnes.
To determine a density from a litres per tonne number there is a similar calculation,
for example kerosene at 1,242 litres per tonne would equate to a density of 1,000 /
1,242 = 0.8051 kg / litre.
The second reason that density is important is in the combustion of the fuel in an
engine. Fuel is fed into the engine as a set volume, this means that in simple terms, the
higher the density of the product the greater the weight of fuel available. As the
energy content increases with weight, more power is generated by a heavier fuel.
However, in practice engines are designed to run on quite a narrow density range. If
the density is too low the engine will not produce sufficient power, if the density is to
high there will be too much fuel leading to incomplete combustion and again power
will be lost.
1.4 Specific Gravity Specific Gravity (or SG) is similar to density and for general
purposes the values are the same.
1.5 Viscosity The viscosity of a product is best described as its consistency, for
example treacle is highly viscous whereas water is less viscous. Knowing the
viscosity of a product across its operational temperature range is important so that
suitable pumps etc can be used to transfer it. If a product is too viscous, pumps can
struggle to pump it but if it is not viscous enough it can leak past the pump
components and seals.
1.6 Distillation The distillation properties of a fuel show what parts boil out and
condense at certain temperatures. This information has different implications for
different fuels.
For gasolines, a small amount of low boiling point material is essential for easy
starting. Mid range boiling point material is required to vaporise sufficiently to give
immediate acceleration and cold weather driveability. Some higher boiling point
material is required to give economical cruising without allowing liquid spray to enter
the cylinder and so dilute the engine oil.
In diesels for road fuel, low boiling point diesel is required for easy starting,
particularly at low operating temperatures. Mid point of the boiling range has to be set
as a compromise to prevent excess smoke, coking and lube oil contamination versus
low energy content and low cetane number. Some high boiling point fuel is required
to provide boundary lubrication but must avoid causing excess smoke and coking.
For kerosene, the distillation is controlled to ensure that the product gives clean
combustion in both vapourising burner and pressure jet applications. Clean
combustion is essential to minimise smoke and soot formation.
1.7 Sulphur The sulphur content of fuels has become much more significant in
recent years, as controls are tightened due to environmental reasons.
When fuels with high sulphur content are burned, oxides of sulphur are released into
the atmosphere where they can accumulate and contribute to acid rain. This is the case
with gas oils, diesels, kerosenes and gasolines.
Sulphur in kerosene has to be controlled for applications where it is used for heating
agricultural greenhouses etc, where high sulphur levels poison the plants.
Sulphur in gasoline has a detrimental effect on exhaust catalyst systems in that it
poisons the catalyst and reduces its efficiency. For this reason sulphur in petrol has
been significantly reduced over the last few years to todays level of 50 parts per m??
(ppm) in the UK with a further reduction to 10 ppm being legislated for 2008. Direct
Injection gasoline engines promise reductions in exhaust emissions and fuel economy,
but are highly sensitive to sulphur poisoning. As a result, to allow their widespread
introduction, 10ppm sulphur fuel is needed.
1.8 Aromatics Aromatic compounds occur in finished products depending on the
manufacturing processes involved. Some of the aromatic compounds found in
gasolines are benzene, toluene and xylene. All these have good high-octane
properties. In diesel fuels it has been found that fuels with higher aromatic contents
tend to have higher densities and lower cetane numbers resulting in increased smoke
and unburned hydrocarbon emissions. In kerosene, the fuels with higher aromatic
contents tend to burn with a smoky flame.
1.9 Water Content All products contain an amount of water. In clear and bright
product the water is dissolved within the fuel matrix. As the amount of water
increases the product becomes more cloudy or hazy. There is a subjective visual
method for appearance that works by comparing the sample under review with
photographs of samples of known water content in front of a series of black lines on a
white card. The ratings go from 1 (very slight haze / almost clear) to 7 where even the
thickest black line cannot be seen. There are several test methods to actually quantify
the water content of a sample to compare it with the allowable value in a
specification.
1.10 Particulate Matter Also referred to as sediment, this is a measure of the
amount of solids in a product. Usually visible to the naked eye, the actual quantity can
be determined by filtration or some other means to compare to the allowable value in
a specification. A typical particulate matter is rust.
1.11 Flash Point (for distillates) Diesel, gas oil and kerosene all have a minimum
flash point specification. The flash point is defined as the temperature at which the
fuel ignites when exposed to source of ignition. This property is important in defining
the transport and storage requirements for the fuel.
As gasoline ignites at very low temperatures the flash test is not required and the
product is classified separately from distillates.
Gasoline (Petrol)
2.0 Background Gasoline is a complex mixture of hydrocarbons (hydrogen and
carbon) forming a readily vapourised, normally clear liquid with a distinctive odour. It
is highly flammable with boiling ranges between 0 210 oC and will ignite at all
typical ambient temperatures within the UK. It is intended for spark ignition engines
and may contain a number of additives.
2.1 Octane Octane is a measure of a gasoline s resistance to igniting during
compression. The higher the octane rating the more it can be compressed without preigniting.
This means that higher octane fuels are suitable for high compression
engines offering greater engine efficiency.
It is a common misunderstanding that a higher octane fuel is more powerful. As
explained above it is actually because it resists pre-ignition and can be used in higher
compression engines that it appears to offer more power.
There are 2 main measures for octane, RON and MON. RON (research octane
number) is measured on a special single cylinder engine known as a CFR engine,
running at low speed and temperature to represent an engine running under light load.
MON (motor octane number) is measured on the same type of engine at higher speed
and temperature to represent an engine running under load.
The measured octane number is derived by comparing the test fuel to samples of a
known octane value. The octane number represents the percentage of iso-octane (100
octane number) mixed with n-heptane (0 octane number). For example 95 octane
represents a mixture of 95 % iso-octane with 5 % n-heptane.
2.2 Vapour Pressure Gasoline is a highly volatile liquid. This means that it gives
off vapours even at low ambient temperatures. This property is controlled within tight
tolerances to ensure good operability of engines to suit the prevailing atmospheric
conditions. The vapour pressure range is set to give sufficient low temperature
vapourisation to ensure easy starting, sufficient mid range vapourisation to promote
smooth acceleration and sufficient high end vapourisation for cruising economy. All
this is achieved whilst at the same time minimising the natural release of vapours into
the atmosphere.
2.3 Lead Content Lead used to be used as an octane-boosting additive. However
with the introduction of catalytic converters it had to be removed because like
sulphur, it acts as a severe catalyst poison. Lead was also a toxic element in exhaust
emissions so simply removing it from gasolines gave an immediate environmental
benefit. Lead content is now controlled to the extent that none is added during
manufacture, however checks are still carried out because of its detrimental effects.
2.4 Benzene Content Benzene is one of the most toxic aromatic compounds in
gasoline and controls are in place to limit the amount present to less than 1% of the
total fuel volume.
Diesel
3.0 Background Diesel is a complex mixture of petroleum distillates, normally clear
or pale yellow in colour. It is flammable with boiling ranges between 150 380 oC
and can give off flammable vapours above 56 oC.
Diesel is used as a road fuel and sometimes referred to as Derv (an acronym for diesel
engined road vehicle) or ULSD (ultra low sulphur diesel).
3.1 Cetane Number The cetane number of diesel is a measure of its ignition quality.
In typical fuels the higher the cetane number the smoother the combustion process.
3.2 Cetane Index The cetane index is an estimation of the cetane number calculated
from known fuel parameters including distillation points.
3.3 Lubricity With the introduction of low sulphur diesels it was found that the
manufacturing processes used to remove fuel sulphur also removed other components
from the fuel that gave it natural lubricating properties. This resulted in accelerated
fuel pump wear and necessitated the development of lubricity improving additives
and specific tests to determine the lubricity performance of a diesel. The European
standard test for lubricity is the High Frequency Reciprocating Rig (HFRR). Thisdevice rubs a small steel ball over a small steel sample piece using the fuel as alubricant, the resultant wear scar is measured and converted into a lubricity reading.
The current standard is a maximum wear scar diameter of 460 Pm (microns or 460
millionths of a meter) at 60 oC.
3.4 Cloud Point The cloud point of diesel designates the temperature at which wax
crystals begin to appear as the sample is cooled down at a controlled rate. This is an
important parameter as it would be an early indication to a user that the product was
approaching the limit of it s operating temperature. For UK diesel the maximum
winter cloud point is 5 oC.
3.5 CFPP The CFPP of a diesel is it s cold filter plugging point. This test goes
beyond the cloud point in continuing to cool the sample whilst simultaneously
drawing the sample under vacuum through a filter. The CFPP is noted as the
temperature at which filtration fails and represents an approximation of the minimum
operating temperature of the fuel. For UK diesel the maximum allowable winter
CFPP is 15 oC.
Gas Oil (also known as Red Diesel or 35 Second Oil)
4.0 Background Gas oil is a complex mixture of petroleum distillates, normally
clear or pale yellow in colour, however it may also be dyed red. It is flammable with
boiling ranges between 150 380 oC and can give off flammable vapours above 56
oC.
Gas oil has two main uses, the majority goes into slow revving industrial, agricultural
and marine engines with the remainder used as heating oil instead of kerosene.
4.1 Cetane Number The cetane number of gas oil is a measure of its ignition
quality. In typical fuels the higher the cetane number the smoother the combustion
process. The cetane number of gas oil is typically 5 or 6 numbers below that of diesel.
4.2 Cetane Index The cetane index is an estimation of the cetane number calculated
from known fuel parameters including distillation points.
4.3 Cloud Point The cloud point of gas oil designates the temperature at which wax
crystals begin to appear as the sample is cooled down at a controlled rate. This is an
important parameter as it would be an early indication to a user that the product was
approaching the limit of it s operating temperature. For UK gas oil the maximum
winter cloud point is 2 oC.
4.5 CFPP The CFPP of gas oil is it s cold filter plugging point. This test goes
beyond the cloud point in continuing to cool the sample whilst simultaneously
drawing the sample under vacuum through a filter, the CFPP is noted as the
temperature at which filtration fails and represents an approximation of the
minimum operating temperature of the fuel. For UK gas oil the maximum
allowable winter CFPP is 12 oC.
Kerosene (also known as Regular Burning Oil [RBO] or 28 Second Oil)
5.0 Background Kerosenes are mixtures of liquid hydrocarbons, clear (but may
contain marker dyes) with a paraffinic odour. They are flammable with boiling ranges
between 150 300 oC and can give off flammable vapours above 38 o C.
Most refiners in the UK produce a dual grade product for use as kerosene and
aviation turbine fuel (avtur) for jet aircraft. Although the manufacturing process is the
same the avtur is very carefully controlled throughout the distribution system to
ensure that any particular batch is fully traceable from production through to end use.
5.1 Smoke Point The smoke point measures the maximum flame height (in
millimetres) at which kerosene will burn in a lamp under prescribed conditions
without producing smoke.
5.2 Char Value The mass of carbon material formed on a wick after a specific
amount of kerosene has been burned in a special lamp under standard conditions.
Cetane Boosters
Cetane boosters only improve cold starting on diesel engines due to increase of flamability of the mixture at lower pressure and temperature. It has been proven that no effect on the power of the engine is attained once the engine is at normal operating conditions.
There are a number of engine performance characteristics that are generally recognized as important. Their relative importance depends on engine type and duty cycle (truck, passenger car, stationary generator, marine vessel, etc.).
starting ease low wear (lubricity)
sufficient power low temperature operability
low noise long filter life (stability)
good fuel economy low emissions
Engine design, by far and away, has the greatest impact on most of these characteristics. But since the focus of this publication is fuel, this chapter will discuss how they are affected by fuel properties.
STARTING
When a cold diesel engine is started (cold start), the heat of compression is the only energy source available to heat the gas in the combustion chamber to a temperature that will initiate the spontaneous combustion of the fuel (about 750°F [400°C]). Since the walls of the combustion chamber are initially at ambient temperature rather than operating temperature, they are a significant heat sink rather than a heat source. And since cranking speed is slower than operating speed, compression is also slower, which allows more time for the compressed air to lose heat to the chamber walls. (A glow plug provides an additional source of heat in indirect-injection diesel engines.) A fuel that combusts more readily will require less cranking to start an engine. Thus, if other conditions are equal, a higher cetane number fuel makes starting easier. As the compression temperature is reduced by variables like lower compression pressure, lower ambient temperature, and lower coolant temperature, an engine requires an increasingly higher cetane number fuel to start easily. Research indicates that fuels meeting the ASTM Standard Specification D 975 cetane number requirement of a minimum of 40 provide adequate cold starting performance in modern diesel engines. At temperatures below freezing, starting aids may be necessary regardless of the cetane number of the fuel. Even after the engine has started, the temperatures in the combustion chamber may still be too low to induce complete combustion of the injected fuel. The resulting unburned and partially burned fuel is exhausted as a mist of small droplets that is seen as white smoke (cold smoke). This situation normally lasts for less than a minute, but the exhaust is irritating to the eyes, and can be objectionable if a number of vehicles are started together in an enclosed space. A fuel with a higher cetane number can ameliorate the problem by shortening the time during which unburned fuel is emitted to the atmosphere.
POWER
Power is determined by engine design. Diesel engines are rated at the brake horsepower developed at the smoke limit.1 For a given engine, varying fuel properties within the ASTM D 975 specification range does not alter power significantly. For example, in one study seven fuels with varying distillation profiles and aromatics contents were tested in three engines. In each engine, power at peak torque and at rated speed (at full load) for the seven fuels was relatively constant. However, if fuel viscosity is outside of the D 975 specification range, combustion may be poor, resulting in loss of power and fuel economy.
NOISE
The noise produced by a diesel engine is a combination of combustion noise and mechanical noise. Fuel properties can affect only combustion noise. In a diesel engine, the fuel ignites spontaneously shortly after injection begins. During this delay, the fuel is vaporizing and mixing with the air in the combustion chamber. Combustion causes a rapid heat release and a rapid rise of combustion chamber pressure. The rapid pressure rise is responsible for the diesel knock that is very audible for some diesel engines. Increasing the cetane number of the fuel can decrease the amount of knock by shortening the ignition delay. Less fuel has been injected by the time combustion begins and it has had less time to mix with air. As a result, the rapid pressure rise, along with the resulting sound wave, is smaller. One design approach to reducing combustion noise is to shape the injection-setting the rate slow at first and then faster - to reduce the amount of fuel entering the cylinder during the ignition delay period. Another is to use indirect-injection
FUEL ECONOMY
Here again, engine design is more important than fuel properties. However, for a given engine used for a particular duty, fuel economy is related to the heating value of the fuel. Since diesel fuel is sold by volume, fuel economy is customarily expressed as output per unit volume e.g., miles per gallon. Therefore, the relevant units for heating value are heat per gallon (Btu per gallon). Heating value per gallon is directly proportional to density when other fuel properties are unchanged. ASTM specifications limit how much the heating value of a particular fuel can be increased. Increasing density involves changing the fuel's chemistry - by changing aromatics content - or changing its distillation profile by raising the initial boiling point, the end point, or both. Increasing aromatics is limited by the cetane number requirement (aromatics have lower cetane numbers [see Figure 4-7]); changing the distillation profile is limited by the 90% distillation temperature requirement. Combustion catalysts may be the most vigorously promoted diesel fuel aftermarket additive (see Chapter 7). However, the Southwest Research Institute, under the auspices of the U.S. Transportation Research Board, ran back-to-back tests of fuels with and without a variety of combustion catalysts. These tests showed that a catalyst usually made "almost no change in either fuel economy or exhaust soot levels."2 While some combustion catalysts can reduce emissions, it is not surprising that they don't have a measurable impact on fuel economy. To be effective in improving fuel economy, a catalyst must cause the engine to burn fuel more completely. But there is not much room for improvement. With unadditized3 fuel, diesel engine combustion efficiency is typically greater than 98%. Ongoing design improvements to reduce emissions are likely to make diesel engines even more efficient.
