SIST-TP CEN/TR 16884:2016
(Main)Automotive fuels - Diesel fuel - Cold operability testing and fuel performance correlation
Automotive fuels - Diesel fuel - Cold operability testing and fuel performance correlation
This document lays down the results of a study on the field correlation of the different cold operability (cold flow and cloud point) test results in relation to actual fuel performance in engines in real world cold conditions. For this work historical data on both manual and automatic tests and on 1988, current and, if possible, future engine concepts shall be used. Real market distillate fuels and FAME, plus common blends thereof, shall be used.
Dieselkraftstoffe für Kraftfahrzeuge - Prüfung der Betriebsfähigkeit bei Kälte und Zusammenhang des Kraftstoffverhaltens
Goriva za motorna vozila - Dizelsko gorivo - Odvisnost med hladnim obratovalnim preskušanjem in učinkovitostjo goriva
Ta dokument obravnava rezultate študije o odvisnosti med različnimi rezultati preskusov hladnega obratovanja (hladni tokokrog in motnišče) v povezavi z dejansko učinkovitostjo goriva v motorjih v realnih nizkotemperaturnih pogojih. Pri tem delu je treba uporabljati zgodovinske podatke o ročnih in avtomatskih preskusih ter konceptih motorjev iz leta 1988, trenutnih konceptih in prihodnjih, če je mogoče. Uporabljati je treba realna tržna destilatna goriva in njihove splošne mešanice.
General Information
Standards Content (Sample)
SLOVENSKI STANDARD
SIST-TP CEN/TR 16884:2016
01-april-2016
*RULYD]DPRWRUQDYR]LOD'L]HOVNRJRULYR2GYLVQRVWPHGKODGQLPREUDWRYDOQLP
SUHVNXãDQMHPLQXþLQNRYLWRVWMRJRULYD
Automotive fuels - Diesel fuel - Cold operability testing and fuel performance correlation
Dieselkraftstoffe für Kraftfahrzeuge - Prüfung der Betriebsfähigkeit bei Kälte und
Zusammenhang des Kraftstoffverhaltens
Ta slovenski standard je istoveten z: CEN/TR 16884:2016
ICS:
75.160.20 7HNRþDJRULYD Liquid fuels
SIST-TP CEN/TR 16884:2016 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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CEN/TR 16884
TECHNICAL REPORT
RAPPORT TECHNIQUE
February 2016
TECHNISCHER BERICHT
ICS 75.160.20
English Version
Automotive fuels - Diesel fuel - Cold operability testing and
fuel performance correlation
This Technical Report was approved by CEN on 17 August 2015. It has been drawn up by the Technical Committee CEN/TC 19.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2016 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR 16884:2016 E
worldwide for CEN national Members.
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Contents Page
European foreword . 4
Introduction . 5
1 Scope . 7
2 Cold flow additives . 7
2.1 Application . 7
2.2 Storage, handling and blending of cold flow additives . 14
3 Cold flow tests . 19
3.1 Vehicle operability . 19
3.2 Assessment of vehicle operability . 20
3.3 Low temperature operability rigs . 20
3.4 Diesel cold flow test methods . 24
3.5 Summary of improvements made to the CFPP test in the past 15 years . 34
4 Fuel Quality Trends . 39
4.1 Changes to fuel production . 39
4.2 Fatty Acid Methyl Esters . 42
4.3 Hydroprocessed Vegetable Oils (HVO) . 45
4.4 Synthetic Paraffinic Diesel (GTL/BTL) . 48
4.5 Distillate Demand Changes . 49
4.6 National cold flow choice . 50
4.7 Winter Diesel Fuel Surveys . 54
5 Vehicle fuelling systems . 59
5.1 Fuel system development . 59
5.2 Vehicle fuel system design considerations in terms of cold operability . 66
5.3 Development of fuel filters . 72
6 Vehicle operability issues experienced in the field . 73
6.1 Description of field issues . 73
6.2 Field issues during recent winters in Germany . 75
6.3 Field issues during recent winters in other European countries . 76
7 Correlation between vehicle operability and laboratory tests . 77
7.1 Introduction . 77
7.2 Summary of DGMK projects . 78
7.3 Daimler studies on German winter diesel fuel samples . 83
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7.4 Opel studies . 89
7.5 Results and analysis of WG34 investigations of winter 2011/2012 samples. 90
8 Conclusions . 97
Bibliography . 100
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European foreword
This document (CEN/TR 16884:2016) has been prepared by Technical Committee CEN/TC 19
“Gaseous and liquid fuels, lubricants and related products of petroleum, synthetic and biological
origin”, the secretariat of which is held by NEN.
Attention is drawn to the possibility that some of the elements of this document may be the
subject of patent rights. CEN [and/or CENELEC] shall not be held responsible for identifying any
or all such patent rights.
In 2010, CEN/TC 19 adopted Resolution 2010/11 setting the title and scope of WG 34 which
were defined as follows:
Title: “Diesel fuel cold operability correlation”
Scope: “Develop a study on the field correlation of the different cold operability (cold flow and
cloud point) test results in relation to actual automotive diesel fuel performance in engines in real
world cold conditions. For this work historical data on both manual and automatic tests and on
1988, current and, if possible, future engine concepts shall be used. Real market distillate fuels and
FAME, plus common blends thereof, shall be used. The working group shall advice towards WG 14,
WG 31 and WG 24 on possible improvements towards their test methods and specifications. The
result of the group will be, as a minimum, the development of a Technical Report on "Cold
operability testing and fuel performance correlation".
In view of the parallel ongoing work within DGMK Project 764 “Cold flow properties of diesel
and operability of vehicles in winter”, this study is focusing on an investigation into the field
correlation of cold operability descriptors (e.g. CFPP, Cloud Point) with actual vehicle
performance. In addition the study is evaluating the impacts of fuel properties, cold flow
additives and blending, and vehicle technology on cold operability. Given the close relationship
between the work of WG 34 and the DGMK Project 764, a liaison between these groups has been
established throughout the drafting of this report.
WG 34 would like to acknowledge the significant contributions by members of the working
group who have contributed to the publication of this report.
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Introduction
Low temperature operability of diesel vehicles is a common concern for all the stakeholders including the
vehicle manufacturers and the fuel suppliers. The stakeholders’ shared desire is to ensure that the end
user is able to operate their vehicle regardless of the ambient temperature conditions.
The Cold Filter Plugging Point (CFPP) method is included in EN 590 as a means to ensure vehicle
operability. As all European countries experience different climatic conditions, the limits for cold flow
properties of diesel fuel are decided by the National Standardisation Body of each member state within
the framework allowed by EN 590. For member states with temperate climates, a different grade of diesel
fuel with its corresponding CFPP limit is selected from Table 2 of EN 590:2013 for each season depending
upon the climatic conditions.
For member states with arctic or severe winter climates, a different class of diesel fuel is selected from
Table 3 of EN 590:2013 for each season depending upon the climatic conditions. In addition to a CFPP
limit, Table 3 also includes a maximum Cloud Point limit for each class of diesel, as well as different limits
for several other fuel properties (e.g. density, viscosity, distillation, cetane). Some member states also
select different climatic grades / classes for specific geographic regions within the country (for example
mountainous or colder regions). A lower density can result in diesel fuel with lower volumetric energy
content which can negatively impact vehicle volumetric fuel consumption. Thus a balance between
ensuring vehicle cold operability and fuel cost is needed.
The application of the CFPP test in middle distillate fuel specifications has facilitated a trade-off between
the needs of the market and the costs of the whole system for the customer (i.e. the investment costs in
the vehicle diesel fuel filter system and the recurrent costs of the fuel supply). To meet the CFPP
specifications without significantly decreasing the yield of middle distillate fuels, the use of cold flow
improver additives has been widely adopted by refineries.
Since the CFPP method was developed in the 1960s, several studies have been performed to develop
other laboratory methods in an attempt to improve upon the correlation with vehicle cold operability.
However this has proved difficult due to constantly changing diesel engine technologies which have
necessitated changes in vehicle fuel system design driven by ever more stringent emissions legislation
(e.g. the move to direct injection and common rail systems with high pressure pumps requiring changes
to fuel filter materials and efficiency). At the same time, middle distillate fuel production has changed
significantly over the years as refineries have had to meet increasingly tight fuel quality requirements (e.g.
reductions in sulfur content, density and polyaromatic hydrocarbons as well as the introduction of
biofuels and higher cetane requirements). Despite all these changes to the vehicles and the fuels, and the
development of alternative lab tests, the CFPP remains the foremost test used to protect the end user
from cold operability vehicle failures.
At the 37th meeting of CEN/TC 19/WG 24 (November 2009, Brussels) questions were again asked
regarding the correlation between cold flow tests and actual vehicle operability at low temperature. Some
participants thought that the situation had worsened due to the introduction of finer fuel filters and
FAME blending.
At the 38th meeting of CEN/TC 19/WG 24 (March 2010, Teddington) the WG 24 convenor and secretary
suggested a scope for a new working group to be formed. This was accepted by WG 24 and the proposal
was then forwarded to CEN/TC 19 members (CEN document N1451). The proposal was accepted by
CEN/TC 19 on 10 May 2010 (resolution 2010/11).
Following a number of vehicle operability issues experienced, for example during a cold period in the first
half of February 2012, in Germany and Austria in particular, a DIN-FAM “mirror” working group was set-
up in Germany as a taskforce to investigate the issue. A key outcome was the creation of a new DGMK
project 764 “Determination of Cold Operability of Diesel vehicles” to develop and execute a joint industry
project. Phase 1 was intended to evaluate several current vehicles from different OEMs to select a new
reference vehicle for operability testing. It was also proposed that Phase 2 would investigate the
development of a rig test and evaluate a wider range of different fuels in the selected reference vehicle.
CEN/TC 19/WG 34 is maintaining close contact with the DGMK project group.
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This Technical Report covers operability and tests to assess diesel fuel performance below the
fuel cloud point. Although a high filter blocking tendency above the cloud point can have an
impact on vehicle operability at low temperature, the development of a lab test to identify this
specific issue is being pursued by CEN/TC 19/WG 31 rather than by WG 34.
Finally, it should be borne in mind that the refiners and vehicle manufacturers are not the only
stakeholders when it comes to ensuring low temperature vehicle operability. There are a number of other
stakeholders involved, including fuel blenders, fuel retailers, biofuel suppliers, cold flow additive
suppliers, vehicle fuel system manufacturers, motorists and standardisation bodies like CEN. With this in
mind, it is important that each stakeholder shares the responsibility for ensuring low temperature vehicle
operability.
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1 Scope
This Technical Report lays down the results of a study on the field correlation of the different
cold operability (cold flow and cloud point) test results in relation to actual fuel performance in
engines in real world cold conditions (below the fuel's cloud point). For this work historical data
on both manual and automatic tests and on old (1988), current and future engine concepts have
been used. Real market distillate fuels and Fatty Acid Methyl Esters (FAME), plus common
blends thereof, have been investigated.
2 Cold flow additives
2.1 Application
2.1.1 Diesel fuel characteristics
Middle distillate fuels are primarily complex mixtures of hydrocarbon molecules. Depending on
the source of the petroleum crude and on the level of refinery processing, some 15 % to 30 % of
these are n-alkanes (also referred to as n-paraffins). The carbon number chain length of these
alkanes is typically in the region of C8 to C28/C32. As middle distillates are cooled, the heavier
n-alkanes start to precipitate from the fuel. These are in the form of wax crystals which can be
2
as large as 1mm and are typically in the form of flat, thin rhomboid plates. As the cooling
continues, the wax crystals grow very quickly with n-alkanes as low as C18 involved in the
precipitation (Figure 1).
Figure 1 — Wax crystals in untreated diesel fuel (source: Infineum)
The plate-like crystals also exhibit strong edge-edge attractive forces between individual
crystals which results in the formation of a gel structure where the majority of the fuel
remaining in the liquid phase is trapped in the interlocking crystal lattice. As a consequence of
this, a very small amount of precipitated wax may be sufficient to cause solidification of the fuel.
Without the use of external heaters or cold flow additives, this phenomenon rapidly causes the
fuel filters found within car and heating fuel systems to block resulting in fuel starvation to the
engine, loss of power and eventually engine stalling.
In recent years diesel fuels have become more complex as fatty acid methyl ester (FAME),
hydrotreated vegetable oils (HVO), Gas-To-Liquid (GTL), etc. have been increasingly introduced
into diesel blends. HVO and GTL are paraffinic fuels that are composed of molecules already
present in petroleum fuels. FAMEs on the other hand are chemical species that are not present
in fossil fuels. However – like n-alkanes – saturated FAMEs can crystallise upon cooling and
form part of the precipitated wax together with n-alkanes.
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2.1.2 Reasons for using cold flow additives
The use of cold flow additives (or MDFI – Middle Distillate Flow Improvers) is not actually a
prerequisite when it comes to producing middle distillate fuels. Their use is primarily
determined by three factors:
1. Refinery economics
2. Supply and demand balance
3. Regional climate conditions
At all petroleum refineries there are price differentials between the different grades of products.
Refiners may therefore improve their blending economics and reduce diesel production cost by
either ‘backing out’ kerosene from diesel blends to produce more jet fuel, or by upgrading heavy
gasoil components to produce more diesel fuel. Producing more diesel fuel is becoming
increasingly important as diesel demand is increasing faster than for all other petroleum
products. The same trend is expected to continue for the foreseeable future.
However both mechanisms result in higher fuel cloud point and degraded low temperature
operability. By use of an appropriate MDFI additive, the cold temperature operability
performance of the cloud point elevated diesel fuel can be maintained at that of the original,
non-upgraded fuel. This is illustrated in Figure 2 where diesel production is increased by 15 %.
Although this results in an 8 °C increase in the cloud point of the fuel, its operability
temperature as measured by Cold Filter Plugging Point (CFPP) is maintained at that of the
original fuel. For a detailed description of the cold flow properties and their related test
methods mentioned in this section, please refer to 3.4.
4 % Higher Yield
Kerosene Kerosene
on Crude (~15 %
increased diesel
production)
Diesel Diesel
Fuel Oil
Fuel Oil
Flow
Improver
Diesel Cloud Point: -6 °C +2 °C
Diesel CFPP: -8 °C -8 °C
Figure 2 — Heavy gasoil upgrade (illustrative example) (source: Infineum)
2.1.3 Wax crystal growth
The crystallisation of waxes from diesel fuels, not treated with cold flow additives, normally
results in the formation of large, flat plates. Although the overall growth rate of the crystals is
slow, the fastest rate occurs at the edge of the crystals where the long axes of the paraffin waxes
stack up next to each other perpendicular to the plane of the crystal. The plates then begin to
slowly thicken to form diamond type shapes. This can be explained by two theories:
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- Gibb’s theory of discontinuous layer addition
- Frank’s theory of continuous growth via dislocations
Gibb’s/Frank’s theories are graphically illustrated in Figure 3. In step 1, the wax crystal
molecule (W) diffuses towards the crystal edge and may be adsorbed at any point (step 2).
However, molecule binding onto the existing crystal is stronger at a ‘kink’ site and this is where
the paraffin molecule will in the end be incorporated (step 3). The new kink/dislocation site
formed by (W) acts in a similar manner to the original by attracting other paraffinic molecules
(V) and continuing the crystal growth (steps 4, 5 and 6).
Slow Growth
Rapid
Growth
Rapid Growth
Figure 3 — Wax crystal growth (source: Infineum)
Using this model, it is relatively easy to visualise how a cold flow additive would cause crystal
growth inhibition. One side of the molecule resembles a wax molecule and could readily be
absorbed into a kink-site on the wax crystal surface. The other side of the molecule is not wax-
like and contains a non-binding or blocking group that inhibits further n-alkane adsorption and
slows down crystal growth. This allows other wax crystals to form with the result that there are
more, smaller crystals
Such cold flow additives are primarily n-paraffinic in nature with bulky, blocking groups and
have a significant effect on the growth and size of the wax crystals (Figure 4).
Figure 4 — Chemistry and binding nature of MDFIs (source: Infineum)
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One of the most commonly used MDFIs is ethylene vinyl acetate (EVA) which consists of a long
polyethylene acting like the n-paraffin and the vinyl acetate side chains acting to sterically
hinder the laying-down of further n-paraffins in that plane (Figure 5).
O
O
EVA copolymer
Figure 5 — EVA copolymer (source: Infineum)
Cold flow additives cause growth in the A-B planes (but not in the C plane) to slow down
sufficiently such that growth in all three directions is comparatively even. This causes the
growth of ‘tall’ needle-like crystals which compared to the rhombic plates of an untreated fuel
are more compact. Although each individual crystal will have a smaller surface area, the total
surface area of all the modified wax crystals combined vastly increases. Small crystals would
normally ‘pack’ closely on a filter thus rapidly reducing porosity, however the needle shaped
nature of the crystals allows them to form a relatively thick porous cake on a filter before it
blocks sufficiently to restrict fuel flow (Figures 6 and 7).
Increased
Reduced
Growth
Growth
Reduced
Growth
Figure 6 — Wax crystal growth modified by MDFI (source: Infineum)
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Thin, flat wax crystals of up to 5 mm in Cold flow additives modify the wax crystals to
diameter can form in untreated diesel fuels at compact needle shapes, which allow the fuel
low temperatures to pass through the wax layer on the filter
Figure 7 — Comparison of wax crystals in untreated and treated fuels (source: Infineum)
2.1.4 The stages of Wax Crystal Modification
The different stages of wax crystal growth, both with and without the use of MDFIs, are shown
in Figure 8.
Figure 8 — Temperature dependence on wax crystal growth (source: Infineum)
Above the Cloud Point, all n-alkanes (and additives) are in solution. As the temperature
decreases to the Cloud Point, MDFI treated fuel contains nucleators that produces more seed
crystals than in untreated fuel. Between the Cloud Point and the Pour Point, the fewer nuclei
crystals in the untreated fuel grow into visibly larger crystals, whereas the treated fuel contains
many small crystals. At the Pour Point, the now much larger crystals in the untreated fuel begin
to adhere. The MDFI treated fuel also contains growth arrestor which slows growth, alters
shape and reduces adhesion. Below the Pour Point, the large crystals in the untreated fuel
adhere and form a gel thus resulting in performance failure. Action of the MDFIs in the treated
fuel maintains crystal separation and allows the Pour Point to be lowered.
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The wax formation steps listed above are for diesel fuels that do not contain Fatty Acid Methyl
Ester (FAME). The saturated esters present in FAME will behave in a similar fashion to that
described. However, trace contaminants that are sometimes present in FAME (e.g. saturated
monoglycerides and sterol glycosides) do not behave in this way. They are described in a later
section.
2.1.5 Vehicles and fuel systems
The use of MDFI in fuels sufficiently modifies the wax crystals formed on slow cooling such as to
require a relatively ‘thick wax cake’ to block the filter. In comparison, an untreated fuel will
rapidly block a fuel filter with a thin, almost invisible wax layer (Figure 9). That a greater
amount of precipitated wax is required in MDFI treated fuels to block a filter is not the only
parameter associated with fuel system failure. The cloud point and wax content of the fuel will
give an indication of its operability at low temperatures as will the system flow rate and
temperature (especially at the filter). The latter is considerably influenced by the design of the
vehicle fuel system. It is convenient to describe a vehicle fuel system as being made up of high
and low pressure sections (HP and LP) where the cold flow performance is most critical in the
LP areas.
Filter exposed to non-treated fuel Filter exposed to MDFI treated fuel
NOTE The wax appears red in colour due to the fuel containing red dye.
Figure 9 — A vehicle main fuel filter at blocking point (source: Infineum)
In particular, a common feature of most fuel systems is the Low Pressure (LP) return where
excess fuel (from the fuel pump/injector feed) is returned to the fuel tank and/or the filter inlet.
This excess of warm fuel is essential to the operation of diesel fuel systems at low temperature.
As soon as the engine is started, wax starts to accumulate on the main filter. If wax crystal
deposition is quicker than the rate of temperature rise, the filter will block and the vehicle will
fail through fuel starvation. On the contrary, if the rate of temperature rise is quicker than the
rate of wax crystal deposition, the filter will remain sufficiently clear of wax to allow continuous
operation. This balancing effect is illustrated in Figure 10.
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Figure 10 — The operability balance (source: Infineum)
2.1.6 How cold flow additives improve operability
As explained above, it is critical for cold operability that the fuel temperature during driving
raises above its cloud point. If not, the vehicle will stop sooner or later due to filter plugging by
wax. Cold flow additives reduce wax crystal size and make the wax cake accumulating on the
surface of the filter more porous. Filterability is improved as a result, which gives extra time for
warming of the fuel system to the point where the wax that has accumulated on the filter begins
to dissolve. The example in Figure 11 illustrates this point.
Figure 11 — Example of benefits of smaller wax crystals on vehicle operability (source:
Infineum)
2.1.7 The different types of cold flow additive
There are a number of different types of cold flow additive. The original flow improver was the
Pour Point Depressant (PPD). They were developed to lower the pour point of diesel fuels and
heating oils, that is, lower the temperature at which the gel structure forms and filters are
readily blocked. PPDs modify the morphology of the crystal limiting the edge-to-edge
interaction of the wax crystals, preventing them from forming gel structures. When used, they
are often added to improve fuel handling characteristics in fuel logistics. However they exhibit
limited ability in controlling the size of individual crystals and do not always provide a
significant improvement in filterability.
Such modification is achieved using Middle Distillate Flow Improver (MDFI). This type of
additive is the most commonly used today and it can readily control the size of individual
crystals and effectively improve filterability. Further reduction of the wax crystal size can also
be achieved using Wax Anti-Settling Additive (WASA) (see Figure 12).
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MDFI treated fuel WASA treated fuel
50 µm 5 µm
Figure 12 — Wax crystals in WASA vs. MDFI treated fuels (source: Infineum)
The benefits of smaller wax crystals with a more compact shape are twofold:
1) Reduced rate of wax settling in fuels stored at a temperature below their cloud point. Wax
settling results in a top layer which is depleted in wax and a bottom layer that is enriched
with wax. This settling can obviously lead to inconsistencies in the quality of the fuel that is
drawn from the tank, either in vehicles or in distribution systems.
2) Improved vehicle operability due to:
o Improved porosity of the wax cake that builds up on the fuel filter allows the liquid fuel
to get through more easily.
o Small crystals re-dissolve more readily as the liquid fuel begins to warm up when the
engine is running and warm fuel is recycled back to the main filter or to the tank. This
further helps to extend operability to a lower temperature than achievable with
conventional MDFI.
Improved vehicle operability is indicated by better CFPP performance and good wax anti-
settling behaviour.
Cold flow additives are normally a mixture of polymers which have been matched to the
individual characteristics of the fuel to be treated. It is imperative that some of the additive co-
crystallises with the first wax crystals and then the remainder of the addit
...
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