fuels

2014 Woodhead Publishing Limited

165

7Advanced fuel additives for modern

internal combustion engines

J. Bennett, Afton Chemical Limited, UK

DOI: 10.1533/9780857097422.1.165

Abstract: Additives and the fuels they are used in continue to evolve. this chapter will discuss the various types of additive, what changes are occurring in market fuels, how these together impact on vehicles, and how changes in vehicles are affecting their appetite for additives.

Key words: additives, injector deposits, biofuels, direct injection, diesel, gasoline.

7.1 Introduction

Additives have a long history of use within automotive fuels. Overall they can be considered to fall into two categories: finished fuel additives and performance fuel additives. Finished fuel additives are generally used to help ensure the fuel is fit for purpose and in compliance with the relevant local standards, whilst performance fuel additives will help to improve the function of the engine and vehicle and are typically used by branded fuel retailers to provide product differentiation. the types and composition of additives is affected by the fuel that they are intended for. Changes to mineral oil derived fuels have been taking place, with a particular focus on sulphur removal and also on minimising components such as benzene. Alternatives to conventional mineral oil derived fuels have at the same time been growing. A significant portion of these alternative fuels are derived from biomass, particularly by conversion into ethanol and biodiesel. the other main source of alternative fuels is by the conversion of other fossil fuels such as natural gas or coal into a liquid form. Whilst all of these fuels are capable of being used as single components or at high concentrations, they are most commonly used as extenders within conventional fuel blends. Doing this has the advantage of allowing them to be used within the normal fuel distribution system and be utilised in conventional vehicles, thereby maximising market uptake. ethanol blends typically incorporate up to 10% ethanol. this level has been widely used in the United States for many years, and has more recently been introduced in europe and other countries. Discussions about increasing this to 15% and beyond are ongoing. Having a high indigenous capacity for

166 Alternative fuels and Advanced vehicle technologies

production of ethanol (from cane sugar), Brazil has for many years used a higher blend known as gasohol with nominally 22% ethanol content. even higher levels of ethanol are also used in various countries, either as e85 (a blend of 85% ethanol and gasoline) or e100, but in both of these cases, specially modified vehicles are necessary to use them. Biodiesel (FAMe) is widely used in europe where up to 7% blends within normal retail fuels are both permitted and encouraged, but moving beyond that is meeting resistance due to concerns about product stability and fuel into engine oil transfer. Blends above 7% are available, subject to local market demands, with up to 20% available in some parts of the United States, and 30% being used in captive fleets, particularly in France. 100% biodiesel enjoyed some success in Germany, but largely disappeared when the focus turned to using biodiesel as a blend extender. the conversion of other fossil fuels such as gas or coal into gasoline and diesel replacements or extenders has a long history, often driven by local shortages or restrictions on the supply of crude oil. the conversion of these molecules is generally undertaken using the Fischertropsch process. The resulting fuels are highly paraffinic and generally of very high purity, lacking many of the residual materials left from distillation of crude oil. these fuels are sometimes referred to as GtL and CtL (gas and coal to liquid, respectively). Biomass can also undergo the same process to make BtL (biomass to liquid) or can be hydrotreated, either on its own or co-processed with fossil fuels within a refinery to produce HVO (hydrogenated vegetable oil), which has a similar paraffinic nature and purity to those fuels produced by the Fischertropsch process. Unlike biodiesel blends, there is no limit on the proportion of these components, the only restrictions being physical constraints such as density. Simultaneously, engine and vehicle technology continues to evolve, with a range of changes to both hardware and mode of use/operation. A short list summarising potential additive influencing factors can seen in Table 9.1. All of these changes have been affecting the appetite of fuels for additives. Feedstocks and production processes other than those mentioned above are under development or even enjoy small scale production. However, it is unlikely that the properties of the fuels they produce will differ significantly from those already mentioned, so they are not discussed in this section.

7.2 Additive types and their impact on conventional and advanced fuels

7.2.1 Antioxidants and stabilisers

With the passing of time, fuel degradation is not uncommon. Degraded fuels can form gums and sediment that plug fuel filters, create deposits on

167Advanced fuel additives for modern internal combustion engines

sensitive fuel system components and interfere with the proper operation of motor vehicles. Due to these common features, fuel specifications often include performance tests, such as oxidative and thermal stability tests, to ensure the fuels are fit for use in the marketplace. Additives can be used to prevent the degradation, and are typically used at treat rates in the 25 to 100 ppm range. the type of degradation that occurs, however, is different between gasoline and diesel fuels. Gasoline can undergo oxidative degradation and form gum deposits on engine components that lead to poor drivability and increased emissions, fuels with high olefin and diene concentrations being typically more susceptible to oxidation. To control gum formation in gasoline, refineries use phenolic and aromatic amine antioxidants. Antioxidants are most effective when they are added to reactive blend streams such as cat cracked (FCC), coker or polymeric gasoline streams as they are produced. Once the degradation process begins it becomes more difficult to control. One of the benefits of the desulphurisation of gasoline is the improvement in its oxidative stability. Many of the reactive species in gasoline are removed during desulphurisation and normally only low concentrations of antioxidants are needed to control oxidation in low sulphur gasoline. Gasoline blended with ethanol has similar stability properties as the hydrocarbon base gasoline. Unlike gasoline, diesel fuel normally degrades by mechanisms other than oxidation. Frequently, heterocyclic and other sulphur and nitrogen containing molecules that result from cracking heavier streams to make diesel and gasoline blend components will react with each other or acidic components found in other diesel fuel blend components. the fuel in some diesel engines passes through the high pressure fuel system where it undergoes considerable heating before being returned to the fuel tank. Under such conditions, the above reactions may occur resulting in the formation of sediments, which can plug fuel filters and produce deposits in critical fueling locations, such as

Table 7.1 Potential engine technologies for improved fuel economy and potential cleanliness needs to be addressed through use of fuel additives

Technology Potential cleanliness from additive use

Cam phasingVariable valve liftTurbochargingCylinder deactivationVariable charge motionVariable compression ratioGasoline direct injectionHomogeneous charge compression ignition (HCCI)Diesel HSDIHybrids

Valve, injector, intakeValve, injector, intakeHotter engine, EGR, intakeValve and InjectorIntake system, charge motion deflectorIntakeInjectorsNew fuel, injectorsIncreased diesel use, injectorsValve, injector, intake


fuel injectors, resulting in power loss, smoke and higher overall emissions. Amine-based additives, sometimes combined with dispersants, are used to control the stability of diesel fuel. Deep desulphurisation or hydro-processing (normally used to make ultra-low sulphur diesel fuel) will eliminate most of the reactive species found in higher sulphur diesel fuel. Ultimately, hydroprocessing produces a very stable diesel fuel requiring the use of little, if any, stability additive. Biodiesel, while normally low in sulphur, degrades through an oxidative process that can be controlled, typically using hindered phenol antioxidants. It is important to treat biodiesel when it is produced, as any degradation will be largely irreversible. Both gasoline and diesel fuel are occasionally contaminated with metals, such as copper, that can catalyse the formation of gums and sediment. Consequently, metal deactivators may be used at low levels to neutralise the catalytic effect of these metals. Most biodiesel antioxidant packages contain a metal deactivator to control catalytic degradation.

7.2.2 Cold flow improvers

Diesel fuel contains naturally occurring paraffinic molecules distilled from crude oil. the source, chemical composition and processing of the crude oil determines the concentration and distribution of these saturated, straight-chained molecules in the fuel. Higher molecular weight paraffins have limited solubility in diesel fuel and precipitate from the bulk fuel in the form of wax crystals when the fuel cools. this separation occurs at the cloud point, the temperature where wax crystals precipitating from diesel fuel become visible to the naked eye. As more wax precipitates, large plate-like crystals begin to develop. These crystals can restrict the flow of diesel fuel from storage tanks and pipes and plug vehicle fuel lines and filters, resulting in an engine losing power or being rendered unable to start. In the winter, the refinery streams used to make road diesel are selected to reduce the higher MW paraffin concentration and lower the cloud point of the fuel. The resulting fuels have lower densities and viscosities than those found in summer. If such streams are not available or their use is uneconomical, cold flow improving (CFI) or wax crystal modifying additives can be used to improve the low temperature properties of diesel fuel. CFIs can be divided into two different categories:

1. pour point depressants2. operability additives.

Pour point depressants are used to keep diesel fuel fluid and transportable. They co-crystallise with paraffins coming out of solution as a fuel cools and disrupt the structure of the wax crystals. Pour point depressants can also reduce the size of wax crystals, but not necessarily to a size where they will


pass through a vehicle fuel filter. Fuels are normally treated with pour point depressant at the refinery. Operability additives are designed to improve the filterability of diesel fuel through diesel fuel filters at low temperatures. Effective operability additives function as wax crystal nucleators, crystal modifiers and crystal dispersants. When a fuel cools, nucleators precipitate from the fuel as wax crystals begin to precipitate and offer many sites for the initiation of wax crystal formation. With many nucleating sites, the crystals remain small and are able to pass through vehicle fuel filters without plugging them, or will have a different shape that that does not block fuel flow so readily. Operability additives normally treat at considerably higher treat rates compared to pour point depressants, usually in the 100 to 500 ppm range, but can be above for extreme requirements. Biodiesel in general has a negative effect on the low temperature properties of diesel fuel and an additives ability to improve the low temperature properties of biodiesel blends. this is also true with renewable fuels derived from refineries processing oils and fats in hydroprocessing units or with Fischertropsch diesel fuel derived from biomass to liquids processes. Renewable diesel fuel and blend components have a high paraffin content. In all cases, greater concentrations of less soluble paraffin and biodiesel components precipitate when a fuel cools and are more difficult to control. Figure 7.1 illustrates the impact of treat rate of CFI additives on the cold filter plugging point (CFPP) of base diesel fuels and the base fuel containing soy methyl ester (SMe) biodiesel.

Base fuel

B(5)

B(110

0 1x 2x 3xAdditive treat rate

CFP

P (

C)

10

12

14

16

18

20

22

24

26

28

30

7.1 Average CFPP response of 5 base fuels and biodiesel blends of the 5 base fuels and soy biodiesel treated with 3 levels of cold flow improver.


When fuel below its cloud point remains undisturbed for a period of time, wax crystals tend to settle to the bottom of the vessel containing the fuel, causing stratification of the wax in the fuel. This results in different cloud points between the top and bottom of the fuel in its storage or fuel tank. to control wax settling to the bottom of a diesel fuel tank, wax anti-settling additives (WASA) are used to disperse the wax throughout the fuel tank. Typically, ethylene vinyl acetates (EVA), copolymers and terpolymers and their derivatives form the core of a todays cold flow packages. Other proprietary components enhance the performance of these polymers.

7.2.3 Lubricity improvers and friction modifiers

the diesel fuel fraction of crude oil contains sulphur and nitrogen compounds that provide natural lubrication to protect vehicle fuel pumps and injectors from wear. Fuel sulphur limits are becoming progressively more stringent and widespread, and the process of desulphurisation of diesel fuel tends to remove these naturally occurring lubricating components from the diesel fuel. Many diesel fuel injection systems rely on this lubricity and the absence of such compounds will result in fuel that gives increased wear and eventual failure of critical fuel-delivery system components. Low sulphur and ultra low sulphur (ULSD) diesel fuels are frequently additised with lubricity additives to protect critical injection system components. Lubricity additives are generally classified as neutral or acidic. neutral additives, esters and amides, may require higher treat rates compared to more cost-effective, mono-acid lubricity additives (see Fig. 7.2), and the mono-acid type has become the most commonly used, often being able to deliver the necessary lubricity improvements at the lowest treat cost and at treat rates of around 100 ppm. Biodiesel fuel has chemistry similar to some lubricity additives and when blended with diesel fuel provides significant improvement to the fuels lubricating properties, in many cases eliminating the need for additional lubricity additives. Figure 7.3 shows that if the biodiesel content exceeds 23% in the finished fuel, a lubricity additive would typically be considered unnecessary. Renewable diesel, either from Fischertropsch or hydrodrogenation of vegetable oils, contains negligible amounts of the compounds that would aid lubricity. therefore, unlike biodiesel, it does not provide any lubricity benefits when incorporated into a mineral diesel fuel. The resulting fuel blend is likely to require either a lubricity additive or to be used in association with biodiesel. Similarly, where renewable diesel is used as a single component fuel, lubricity additives are necessary to protect the fuel injection system. Friction modifiers are similar in chemistry and function to lubricity improvers, but are generally targeted at gasoline fuels. Friction modifiers


are transferred by fuel wetting into the lubricating oil layer on the cylinder walls. the friction between the piston and rings and the cylinder walls is one of the more significant sources of parasitic losses within an engine. A reduction in friction in this area will translate into a measurable improvement in engine efficiency, improving both fuel consumption and power whilst offering enhanced wear protection. The benefits can also spread beyond the cylinder area, with the friction improved oil layer being gradually moved into the bulk oil in the sump and eventually acting on other areas of friction such as the valvetrain. Diesel engines do not have the same level of cylinder wall wetting, and it has been established that the potential efficiency benefits are negligible.

Neutral

Mono-acid

0 50 100 150 200 250 300Relative additive treat rate (ppm)

HFR

R M

wS

D (

mic

rom

etre

s)

600

550

500

450

400

350

300

7.2 Mono-acid lubricity additive comparison to non-acidic (neutral) lubricity additives. HFRR is the high frequency reciprocating rig and MwSD is the mean wear scar diameter.

0 5 10 15 20 25Palm oil fatty acid (%)

HFR

R M

wS

D (

mic

rom

etre

s) 500

450

400

350

300

250

200

7.3 Effect of palm oil fatty acid biodiesel on lubricity of diesel fuel as measured by HFRR.


7.2.4 Pipeline corrosion inhibitors

Most diesel fuel and gasoline at some point are transported through a mild carbon steel pipeline and stored in mild carbon steel storage tanks. Fuel commonly contains small quantities of dissolved water that can become free water when the fuel cools. Free water can cause internal corrosion in pipelines and storage tanks, thereby producing fine rust that coats the inside of pipeline and storage tank walls. this rust can increase drag in the pipeline resulting in decreased pumping efficiency and higher energy costs. Fine rust particles can also separate from the pipeline and storage tank walls and create unnecessary wear on critical pipeline metering and pumping equipment, resulting in high-maintenance costs and frequent cleaning. Rust inhibitors, such as dimer, trimer and alkyl succinic acids, are sometimes used to protect the distribution system from corrosion, and can deliver the necessary protection at relatively low treat rates, around 20 ppm. By forming a thin film on the pipeline wall, the rust inhibitors protect the distribution system from oxidative corrosion caused by water and oxygen. Other corrosion inhibitors have also been used, such as sodium nitrate and sodium hydroxide. However, the trend is moving away from the use of corrosion inhibitors and instead controlling corrosion by good housekeeping, making sure that the fuels distribution network is as far as possible kept free of water and other contaminants. this trend has been accelerated by recent issues with diesel injectors suffering from internal deposits linked to the use of the sodium compounds mentioned above and also to some alkyl succinic acid corrosion inhibitors as discussed by Schwab et al. (2010). Where corrosion inhibitor use continues, the selection of product should be done in discussion with the relevant suppliers. Biodiesel blends and gasoline containing ethanol can have corrosion issues if the renewable fuels are not free of contaminates which can come from the manufacture of the fuel or in the handling and distribution of the fuel. For instance, metal salts found in some biodiesels can increase the corrosivity of the fuel. this is best addressed by controlling the quality of the biodiesel being used, but where required, typical pipeline corrosion inhibitors can provide the necessary protection. Unlike biodiesel, ethanol blends are not transported by pipelines, and instead the blend is made at the distribution terminal where delivery tankers are loaded. the ethanol is either blended with gasoline in a storage tank prior to loading, or by adding to the fuel during the tanker loading process. However, the use of corrosion inhibitors is recommended for protection of the ethanol distribution system, and should be added as early as possible after manufacturing the ethanol. After blending ethanol with gasoline, there is a risk of phase separation. If exposed to moderate levels of water, including that absorbed from the


atmosphere, the ethanol will separate from the gasoline producing a separate layer in the storage vessel, either in the filling station or in the vehicle. In these cases, ethanol corrosion inhibitors should be designed to not only protect from corrosion in the hydrocarbon phase but also the ethanol water phase if phase separation should occur. Figure 7.4 shows examples of the protection provided by a corrosion inhibitor when tested according to AStM D665. In this test, polished steel pins are exposed to fuel mixed with water and a remarkable difference can be seen when compared to the fuel without inhibitor. the concerns about ethanol corrosion have led to their use being recommended by a variety of industry bodies including COnCAWe and Cen in europe and the RFA in north America. Some ethanol corrosion inhibitors provide buffering properties designed to control the pHe of the ethanol. Where specifications instead use conductivity limits as a surrogate for measurement and control of pHe and corrosivity, the presence of some inhibitors can be seen to affect conductivity. the ethanol conductivity should therefore be measured before the corrosion inhibitor is introduced. Care should also be taken in selecting ethanol corrosion inhibitors, with DuMont et al. (2007) noting that some can increase engine intake system deposits, especially in high ethanol content fuels such as e-85.

7.2.5 Other corrosion inhibitors

Although iron oxidation is a major cause of fuel system corrosion, other non-ferrous components are also susceptible to corrosion caused by reactive sulphur species found in some diesel fuels and gasoline. As a result, many fuel specifications have copper corrosion limits as determined by a copper strip test. typically thiadiazole chemistry is effective at controlling copper

Base gasoline + 10% ethanol + corrosion inhibitor: 0% rust, A-rating

Base gasoline: 100% rust, E-rating

7.4 Corrosion protection recommended for fuel ethanol and blends (a) base gasoline + 10% ethanol + corrosion inhibitor: 0% rust, A-rating and (b) base gasoline: 100% rust, E-rating.


corrosion. More recently, there have been cases of silver contacts in fuel sending/sensing units suffering damaged from free sulphur and other sulphur species remaining in gasoline after the desulphurisation process. this problem has occurred sporadically in north America and other regions. In the US, the AStM requirements now stipulate that gasoline pass a silver corrosion test, whilst in other markets, fuels distributors regularly check that products meet their own internal specifications. Figure 7.5 illustrates the effect of free sulphur on silver strip ratings. the protection provided by two different fuel additives is shown in Fig. 7.6.

IP22

7 so

lver

str

ip r

atin

g

5

4

3

2

1

00 5 10 15 20 25 30

Elemental sulphur concentration (mg/litre)

7.5 Effect of elemental sulphur on gasoline silver corrosion ratings. Silver strip immersed in unleaded regular gasoline inside glass test tube sealed in stainless steel bomb at 50C for 3 hours.

Additive A

Additive B

0 5 10 15 20 25 30Elemental sulphur (mg/l)

Trea

t ra

te o

f ad

dit

ive

(pp

m)

12

10

8

6

4

2

0

7.6 Treat rate of two different corrosion inhibitor additives needed to achieve zero silver strip rating for a given concentration of elemental sulphur in gasoline.


7.2.6 Conductivity improvers

In order to eliminate the potential for a spark to ignite fuel vapours, jet fuel, and in some cases, other distillate fuels, have for many years been treated with additives to dissipate static charge build-up. Static charge build-up is fairly common when diesel fuel is discharged into an improperly grounded vessel such as a fuel tanker. As a result, if the tanker switches back-and-forth between hauling diesel fuel and gasoline, the potential for gasoline vapours to remain in the container while loading diesel fuel creates the risk of an explosion and fire. Desulphurisation of diesel fuel significantly lowers diesel fuels natural conductivity resulting in a greater risk of explosion or fire. To reduce the potential for static charge build-up during transfer of fuel from one vessel to another, conductivity additives are now frequently added to ultra-low sulphur diesel fuel. Fuel conductivity is temperature dependent with conductivity decreasing as a fuel cools (Fig. 7.7). Conductivity improvers are used at very low treat rates, often around 1 to 2 ppm, and can be added at either the refinery or at terminals before the fuel is loaded into a tanker. One benefit of biodiesel is its ability to increase the conductivity of diesel fuel. Fig. 7.8 shows that difference sources of biodiesel have different effects on conductivity, even when they are made from the same feed stock.

7.3 Impacts of additives on combustion characteristics

the combustion characteristics of fuel are generally controlled by the fuels chemical composition. One of the key combustion properties is the ability to ignite the fuel and air charge at an optimum time in an engine. Fuel ignition is a radical-driven reaction, and properly designed fuel additives can have a significant impact on ignition, enhancing or retarding the propensity for ignition.

25 20 15 10 5 0 5 10 15 20 25Temperature (C)

Co

nd

uct

ivit

y (p

s/m

)

400

300

200

100

0

7.7 Fuel conductivity decreases with fuel temperature.


7.3.1 Diesel ignition improving additives

Diesel engines function by compressing the air in the combustion chamber to a temperature at which the atomised fuel that has been injected into it will spontaneously ignite. the ease with which a fuel will ignite is measured as the cetane number and directly affects an engines ability to start and efficiently burn fuel, especially whilst the engine is cold. the cetane number itself is determined by measuring the delay between fuel being sprayed into the hot combustion chamber and ignition starting, with a short delay denoting a high cetane. A long delay will lead to the eventual combustion being very rapid, causing an unacceptably high rate of pressure rise in the combustion chamber, producing significant noise, whilst incomplete combustion, loss of power and increased emissions will also occur. Conversely, reducing the ignition delay results in improved startability, less noise and lower overall emissions. numerous studies have demonstrated that increasing the cetane number of a fuel will result in general reduction in nOx emission from a diesel engine. this is a result of the shorter ignition delay time for higher cetane fuels and subsequently lower peak temperatures within the diesel engine. the US ePA (2003) conducted a thorough analysis of cetane quality impact across a broad range of fuels and engine types, and demonstrated the typical levels of nOx reductions obtained using additives to increase cetane. the chemical composition of diesel fuel determines its cetane quality. Fuels with higher paraffin content tend to have a higher cetane quality, while more aromatic and unsaturated components have a lower cetane quality, and this can be seen in the empirical calculation of cetane index, which

SME #1SME #2

0 20 40 60 80 100Volume SME (%)

Ele

ctri

cal

con

du

ctiv

ity

(pS

/m)

900

800

700

600

500

400

300

200

100

0

7.8 Conductivity of biodiesel blends as a function of biodiesel concentration for SME from two different sources.


looks at the fuels density and distillation characteristics to predict cetane. It is possible to increase the cetane number of a fuel by the use of energetic additives that encourage the autoignition process. Figure 7.9 illustrates the delay in ignition observed for a low cetane base fuel versus base fuel to which a cetane improver additive was added to yield a ten cetane number increase. Commonly used additives with demonstrated capability to improve fuel cetane quality include alkyl nitrates and peroxides. Alkyl nitrates, particularly 2-ethyl hexyl nitrate, are the preferred commercial source of cetane improver due to low cost and favourable handling characteristics, and are typically used at up to 1500 ppm, both for base fuel improvements as mentioned below and as part of diesel performance additive packages, where they offer both performance benefits and a reduction in noise that will be readily perceptible to the consumer. the use of cetane improver offers particular opportunites in territories using CEN standards and similar, as there is a significant difference between the minimum cetane index of 46 and the minimum cetane number of 51. It is possible to produce a fuel that meets the 46 index minimum and use cetane improver to meet the 51 number, offering savings for the fuel producer. Depending on the source of FAMe, the use of biodiesel can increase or decrease the cetane number of a diesel fuel. this is dependent on the cetane number of the base fuel to which the biodiesel is added. In areas with low cetane number specifications such as the US, biodiesel normally increases cetane number. In regions where cetane number is high such as europe, some biodiesels will lower cetane number. Biomass to liquids and hydroprocessed

+10 CN

Base

Start of injection

Greater ignition delay with unadditised

base fuel

350 355 360 365 370 375 380

Crank angle ()

Net

hea

t re

leas

e (k

J/)

0.4

0.3

0.2

0.1

0

0.1

7.9 Reduced ignition delay for diesel fuel with 10 cetane number increase (+10 CN) compared to base fuel.


oils provide distillate blend stocks that have a very high cetane number, and coupled to their being mainly paraffinic, make good components for blending to produce high quality, low emissions diesel fuel.

7.3.2 Octane improving additives

Gasoline engines use a spark to initiate combustion, and unlike diesel engines, it is important for the fuel to resist auto ignition. the resistance of a gasoline fuel to auto ignition enables hotter conditions to be tolerated in the combustion chamber, allowing higher, more efficient, compression ratios and advanced ignition timings to be adopted. Resistance to auto-ignition is measured on the octane scale, with higher numbers denoting increased resistance. the chemical composition of a gasoline determines the base gasoline octane level. Molecules such as aromatics and olefins are less susceptible to spontaneous ignition under pressure and have a high octane value. A fuels octane quality can be improved by the use of high-octane blend components such as oxygenates or aromatic components or by the use of octane-improving additives such as mmt (methylcyclopetadienyl manganese tricarbonyl).

7.4 Diesel performance and deposit control additives

the need to maintain diesel injector cleanliness has been recognised for many years, and has generally focused on the effect of nozzle coking, i.e. deposits forming in the hole through which fuel is sprayed into the combustion chamber. However, over recent years, deposits forming inside the injectors have also become a significant issue. Being able to address both of these phenomena has become a necessity in modern diesel performance additives.

7.4.1 Injector nozzle coking

Allowing injectors to form nozzle coking deposits will result in flow loss and modify the spray pattern of the fuel. Figures 7.10 and 7.11 show the deposits seen at the tip of a modern diesel injector. If these interfere with the flow of the fuel through the nozzle hole, it may cause incomplete mixing of fuel and air, inefficient combustion and affect the ratio of fuel to the inducted air and eGR gases, leading to degraded driveability, reduced power, increased emissions and even to operational failure. Prior to the widespread use of deposit control additives within fuels, engine manufacturers responded to the coking issue by assuming that cleanliness would not be maintained, and deliberately optimised engine calibrations to account for some deposit formation. Older indirect injection engines with distributor pump injection


systems and their pintle needle injectors are at their most efficient with some, although not excessive, deposits present. this was amply demonstrated in the paper by Reading et al. (1991), which identified that flow retention in the range 50% to 75% was the optimum. In addition to the measured emission improvements shown in that paper, it is well recognised that the formation of some nozzle deposits during the early life of the injector reduces engine noise. to establish the deposit forming tendency of fuels in such applications

7.10 Deposits on the tip of a light duty diesel injector.

7.11 SEM close up of deposits around the injector nozzle hole.


and to encourage the development of additive-based solutions, engine tests were introduced. For the heavy duty diesel engine market in north America, the Cummins L10 test (now defunct) described by Gallant et al. (1991) was available. In europe, the focus was on light duty engines, where the Coordination european Council for the Development of Performance tests for Fuels, Lubricants and Other Fluids (CeC) introduced a test based on the Peugeot XUD-9 engine (CeC F-23-01, 2001). this test uses a 1.9 litre indirect injection distributor pump type engine very much like the one used by Reading et al. (1991), and measures loss of flow through injectors after 10 hours of relatively low load and speed operation following a prescribed cycle. this test is still a fully approved and supported engine test and has, for many years, defined the performance of fuels and their additives in a range of markets. the introduction of biodiesel has had some limited impact on the deposit forming tendency of base fuels and the response of fuels to additives within this test. Response testing in fuels containing up to 10% biodiesel has therefore become a common stage in the validation of additive packages. However, the nature of the response can be unpredictable with both improved and degraded test performance being reported with such blends. Since the approval of the XUD-9 test in 1997, light duty/passenger car engines have changed significantly, with the base engine design moving from indirect to the direct injection and the fuel injection systems also changing to high pressure common rail (HPCR), enabling significant improvements in performance, emissions and fuel consumption. these systems use significantly higher injection pressures than their predecessors, with resulting increases in both fuel temperatures and the heat rejection within the systems. Recognising these changes, the CEC introduced a new test to help define the performance of fuels and additives in resisting injector coking for such engines. Designated CeC F-98-08 (2008) and more commonly known as the DW10, this test uses a 2.0 litre Peugeot engine equipped with deposit sensitive prototype injectors designed to meet Euro V emissions. The test itself measures flow loss indirectly by monitoring power reduction with time, and the test cycle is severe, with Hawthorne et al. (2008) observing that a significant amount of the engine running being done under full load conditions. Vehicle manufacturers have generally indicated that power loss of less than 2% is acceptable in this test. Whilst field experience has shown that high mileage real world vehicles are likely to have developed injector nozzle deposits, most mineral based fuels do not form coking deposits within the DW10 test. to encourage their formation and reduce the duration of the test, a zinc based compound is added to the fuel. However, as shown by Ikemoto et al. (2011), the resulting deposits are substantially derived from the zinc addition, a source of some contention within the industry as the relationship to real world high mileage deposits


has not been demonstrated. Conversely, some biodiesels have also shown they can also lead to coking deposits in the DW10. Generally attributed to questionable biodiesel quality, where such power loss does occur, the precise source of the problem within the biodiesel has not so far been successfully identified. Due to the biologically derived nature of biodiesel, reproduction of the problem fuels has not proved possible. Conversely, the presence of the biodiesel may even cause the zinc compound addition to have unpredictable effects regarding deposit formation. Figure 7.12 compares the power loss generated by a zinc treated mineral fuel with the same fuel plus addition of a deposit control additive, whilst Fig. 7.13 shows similar data, but in this

with additive

No additive

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33Test time (h)

Po

wer

lo

ss (

%)

1

0

1

2

3

4

5

6

7.12 Power loss due to injector coking in high speed direct injection engine test (Dw10) for mineral base fuel with zinc and for same fuel containing diesel additive.

with additive

No additive

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33Test time (h)

Po

wer

lo

ss (

%)

1

0

1

2

3

4

5

6

17.13 Power loss due to injector coking in high speed direct injection engine test (Dw10) for B10 (SME) base fuel and for same fuel containing diesel additive.


case with a B10 biodiesel blend as the base fuel. In both cases, the use of a suitable deposit control additive is able to prevent power loss. Given that the operating envelope and conditions for the DW10 test are significantly different to the XUD9, and that the injector tip temperatures have increased greatly from circa 160 C to nearer 270 C, it is not surprising that the additive response has also changed. Additives that had been previously shown to give good performance in the XUD-9 were found to require significantly higher treat rates to provide good DW10 performance. The response of the additive industry has been to develop new deposit control additives that, if correctly designed, will give the correct performance in the XUD9, good cleanliness in the DW10 and also address coking and hence performance in real world vehicles.

7.4.2 Diesel injector internal deposits

Starting around 2005, there were sporadic reports in europe of problems with common rail fuel injector needles sticking. However, by 2009, the problems had become much more widespread and were also occurring in North America. The issue was identified as being mainly due to soaps and lacquers forming on the moving parts of high pressure common rail injectors. Where those components had a tight clearance relative to the injector body or other parts, the deposits could lead to those elements moving in a sluggish manner and affecting the injection timing, or becoming stuck in either a closed or open position. the effect of these deposits was to cause a range of problems including power loss, starting issues, rough running, fuel filter plugging, increased emissions and sluggish acceleration. It was identified that there were two main types of the deposits, one being a whitetan crystalline deposit, as shown in Fig. 7.14, whilst the other was an orange lacquer. A paper by Ullmann et al. (2008) postulated several mechanisms, whilst sodium carboxylate deposits and their likely source were specifically identified by Schwab et al. (2010), using a proprietory engine test for verification. This paper established that the root causes included the reaction between certain corrosion inhibitors being used by pipeline operators and the sodium contamination that is almost ubiquitous when handling fuel. A second mechanism was identified by Galante-Fox and Bennett (2012) as being based on organic amides with certain deposit control additives being implicated. In both cases, solubility of the material being deposited is seen as a key factor. It is instructive to note that the chemical components held responsible for the deposits have a long history of use without such issues. However, it is believed that the operating environment has changed, with the move to lower sulphur diesel having affected the ability of the fuel to solubilise these materials, whilst the increasingly sophisticated common rail injection


systems so critical to reducing emissions may be less robust against such deposits. to address the issues of internal deposits, advanced additive systems are being developed that are capable of either preventing or removing the deposits. In Fig. 7.15, the individual exhaust port temperatures of an engine are being monitored immediately after cold start. the fuel in this case has been deliberately treated to produce sodium carboxylate deposits. In (a) at the start of test, all four cylinders warm up at the same rate. In (b) after only 3 hours of testing, two of the cylinders are malfunctioning due to insufficient fuel being injected, and the other two try to compensate by overfuelling. In (c) it can be seen that an attempt to clean using a conventional additive has no impact, whereas in (d) an advanced additive restores the failing injectors.

7.4.3 Diesel performance additive packages

Besides controlling deposit formation, diesel performance additives are frequently formulated with a number of other components to provide additional functionality to the package. Packages frequently contain cetane improvers, wax crystal modifiers or CFIs, combustion improvers, lubricity additives, demulsifiers and antifoaming additives. Since diesel fuel has a tendency to foam when fueling a vehicle, antifoams are frequently added to diesel fuel to control spillage and increase the rate of fueling diesel vehicles. the dispersant

7.14 Sodium carboxylate internal deposits on the needle of a diesel injector.


used in diesel fuel also tends to emulsify water, which can lead to plugged fuel filters. Therefore, incorporating adequate demulsifiers into the package to control water pickup by the fuel is highly important. Depending on the performance claims being targeted, treat rates for the additive packages can range from around 100 ppm, which is the minimum injection rate for many of the systems that dispense additive into the bulk fuel, up to and beyond 1000 ppm for the packages used in some premium fuels.

7.5 Gasoline performance and deposit control additives

7.5.1 Gasoline engine deposits

During the normal operation of a gasoline-powered vehicle, fuel related deposits tend to form on various fuel and induction system components.

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0 100 200 300 400Time (sec)

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0 200 400Time (sec)

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0 200 400 600Time (sec)

(d)

Start of test 2 injectors stuck

Conventional additive Advanced additive

7.15 Effect of internal deposits on injector cold start function individual exhaust port temperatures (a) start of test, (b) two injectors stuck, (c) conventional additive and (d) advanced additive.


When these deposits occur, they are likely to cause poor fuel delivery and volatilisation leading to reduced engine efficiency, increased emissions, drivability problems, and loss of power. Deposits also form in the combustion chamber leading to increased octane requirements, loss of performance, and in some cases, engine knock. In each of the above scenarios, deposit formation can also lead to increased fuel consumption. Performance additives to control fuel system deposit problems have been around for over 50 years. Many early versions of gasoline additives were low molecular weight amine-based chemistries designed to keep carburettors clean and avoid carburettor icing. With the introduction of fuel-injected vehicles in the 1980s, the low concentration of additive needed to keep carburettors functioning was insufficient to keep injectors from fouling. While higher concentrations of these additives were effective in keeping fuel injectors clean, they also contributed to a growing problem caused by intake valve deposits (IVD). In Europe, several vehicle manufacturers were badly affected by those deposits, the primary symptom seen by the end user being degraded driveability. In this case, driveability is affected by the IVD making the fuelair mixture become lean when the driver tries to accelerate and causing the engine to initially stumble and misfire. This behaviour is uncomfortable for the driver, leads to increased emissions and can even cause exhaust catalyst failure. to address this, the CeC introduced an engine test, CeC-F05-93 (1993), also known as the M102e, which allowed additive companies to design suitable packages to counter this issue. to control IVD, higher MW amine-based chemistry blended with mineral oil carriers became the predominate method for controlling fuel injector and intake valve deposits. In some cases, additive treat rates to effectively control intake valve deposits approached 1000 ppm. these high concentrations soon led to significant increases in combustion chamber deposits and increases in engine octane requirements. As a result of the additives inefficiencies, in the early 1990s, mineral oil carriers were phased out while synthetic carriers started becoming widely used in formulating gasoline performance additives (GPA) as we know them today.

7.5.2 Gasoline performance additive (GPA) packages

A typical GPA will consist of, at a minimum, an active deposit control component and a synthetic carrier. the carrier serves several functions in an additive package:

1. to provide a fluid medium through which the deposit control agent can disperse deposits precursors

2. to provide valve stick protection, and 3. to not contribute to combustion chamber deposits.


More usually, these packages also contain demulsifiers, corrosion inhibitors, antioxidants, and friction modifiers. Many of these components are surface active and may not readily coexist in their concentrated form, so to provide storage stability, solvents will to generally be incorporated in the package to address this. Similar to diesel performance additive packages, treat rates can range from around 200 ppm up to 1000 ppm and beyond.

7.5.3 Cleanliness and performance of port fuel injected gasoline engines

Port fuel injected gasoline engines have been in the market for many years and represent the engine technology utilised in the large majority of the gasoline vehicles. To significantly reduce emissions and improve fuel economy, manufactures have worked to improve design and control of the engine, specifically to control the airfuel ratio. Deposits in the fuel intake system can have detrimental impact on preparation of the airfuel charge leading to sub- optimal engine performance. Deposit control additives added to gasoline can work to remove previously formed fuel system deposits and restore vehicle performance, lowering emissions and improving fuel economy. the performance of the gasoline deposit control additive and its ability to clean-up fuel-intake system deposits in the standard M102e test is illustrated in Fig. 7.16 for a number of different fuels including a market fuel containing ethanol (e10). the severity of the three fuels differed, building different levels of deposits, but in all cases, subsequent operation on gasoline containing a deposit control additive provided significant clean-up of the valve deposits.

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EN228 Gasoline Reference fuel Market E-10

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7.16 Intake valve deposit clean-up using base fuels containing gasoline additive in M102E IVD test. Clean-up is of deposit built from base fuel. Examples for three different base fuels.


Deposit control additives also deliver clean-up in vehicle operation, improving fuel efficiency and lower vehicle emissions. The effect of a gasoline deposit control additive on valve cleanliness for three vehicles designed to meet stringent Euro IV emission standards is illustrated in Fig. 7.17. An additive package at a fairly low commercial treat rate, reduced valve deposits by an average of almost 30%. the emission performance and fuel consumption of these vehicles was also measured before operation on the additised fuel and after the clean-up phase. After operation of the additised fuel and subsequent IVD clean-up, average hydrocarbon, carbon monoxide and nitrogen oxide emissions from the vehicles were all reduced. the vehicles also showed a reduction in fuel consumption. Controlling fuel system deposits can also help maintain a vehicles power and acceleration closer to new vehicle design. Figure 7.18 shows the benefit of cleaning up intake system deposits on vehicle power and acceleration. Compared to a newly built engine, the power loss before and subsequent recovery of that loss after clean-up, is significant. In this case, the power recovery led to a 5% reduction in the average time required for the vehicles to accelerate from 100 km/hr to 145 km/hr. these examples illustrate the benefits of using gasoline fuel additives to keep clean or clean-up the engine intake systems.

7.5.4 Direct injection gasoline (DIG) engines and injector plugging

the direct injection gasoline (DIG) engine operates by injecting fuel directly into the combustion chamber and is capable of providing significantly improved fuel efficiency. The gains in fuel efficiency and ability to produce more power

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7.17 Average emission and IVD reduction achieved after clean-up for three vehicles.


are achieved through the higher compression ratios associated with charge cooling, coupled to precise control over amount of fuel and injection timings which are varied according to the load conditions. the proper presentation of the fuelair mixture into the cylinder is important to ensure robust operation of engine overall operating conditions. engine designers utilise wall guided and more recently, spray guided systems to ensure proper formation of a combustible fuelair mixture and robust engine performance. Both of these approaches rely on proper operation of the fuel injection system. Formation of fuel injector deposits can adversely impact fuel spray formation and reduce levels of fuel injected into the engine impacting engine operation. Fuel additives play an important role in maintaining the operation of DIG engines at optimal levels and preventing loss in performance, and their selection is discussed at length in the paper by DuMont et al. (2009). the injectors in the DIG engine are exposed to higher temperature than those used in the PFI vehicles discussed above, and are potentially prone to the formation of deposits that restrict flow and adversely impact performance. Injector manufacturers design the injectors with the goal of reducing the propensity for deposit formation. However, injector deposits will often form, reaching an acceptable equilibrium but nonetheless at a degraded condition compared to the vehicle design intent. the ability to return the injector to a clean, more efficient condition is the key to recovering lost performance and the vehicle operating at its optimum. Figures 7.19 and 7.20 show results of

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Dirty: distance = 117.7 metres/4.87 secs

After clean-up: distance = 114.5 metres/4.65 secs

7.18 Improvement in power and acceleration on cleaning up fuel system deposits. Average of four different vehicles. Dirty-up for 8000 km on commercial gasoline and clean-up for 2400 km on premium level of additive in regular unleaded gasoline.


testing where injectors from an engine that has used a fuel with a tendency to form deposits is compared with injectors exposed to the same fuel, but dosed with a gasoline performance additive. A clear visible difference can be seen inside the injector nozzle holes. Using a vehicle equipped with a four cylinder direct injection gasoline engine, the impact of deposit and the role of additives was demonstrated as shown in Fig. 7.21. to give good discrimination between additives, a bespoke fuel was used that was intended to give a known, high level of injector deposits. the vehicle was operated over a period of 48 hours at highway speeds and underwent periodic hot soaks. After this period of time the base fuel displayed about 9.4% loss in fuel flow through the injectors due to deposit build up. The test was then repeated, firstly with a historical deposit control additive known to give good PFI IVD control performance, and then with a more advanced additive that gives the same level of PFI IVD performance, but which had also been designed to provide DIG injector performance. the difference is obvious, demonstrating that providing good

7.19 Direct injection nozzle deposits with additive use.

7.20 Direct injection nozzle deposits base fuel.


DIG performance should be a fundamental consideration when developing additives for the modern vehicle fleet.

7.5.5 Effects of ethanol on deposit formation

Low levels of ethanol blended with gasoline can affect the amount of deposit formed, but typically have only limited impact on the additive performance, and should be accounted for by additive producers during product development. Figure 7.22 shows IVD performance of two commercially sourced E5s, with a significant difference being seen in base fuel deposition. However,

Ethanol A Ethanol B Commercial E-85

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7.21 Differences in intake valve deposit forming tendencies of E-85 fuel made from two fuel grade ethanol sources and commercial E-85. Tests conducted in a flexible fuel vehicle operating for 5000 miles.

Reference European E5 #1 European E5 #2 E5 #2 + additive gasoline

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7.22 Intake valve deposit formation can vary significantly with ethanol blends. Additives can be very effective even with severe base fuel.


even the most severe of these can be seen to respond to the use of a deposit control additive. Higher level ethanol blends such as 85%, have been shown to increase injector fouling and can also add to intake valve deposits. Figure 7.23 compares the IVD performance of E85 fuels where different ethanol sources have been used. The variation in deposits formed is significant and was identified as being likely to have been due to the type of corrosion inhibiter used. In addition to the need to ensure that gasoline deposit control additives give the correct performance in the presence of alcohol, high alcohol blends such as e85 require consideration of the solubility of the additive used. Where a product is not fully soluble, suspended matter will appear, obstructing light and producing a haze in the fuel. this can be measured as opacity and table 7.2 compares this property for two different additive package chemistries. Opacity below 5 ntU (nephelometric turbidity units) would be considered

Base fuel

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7.23 Cleanliness performance of additives in direct injection gasoline engine.

Table 7.2 Gasoline detergent solubility in E85 based on detergent type

Finished package treat rate (ppm)

Gasoline detergent type

PIBA* PEA**

0120240480960

0.824.316.66

13.2838.00

0.820.850.870.961.02

* PIBA fully soluble at all treat rates up to E50** Low haze numbers indicate that PEA based package has complete solubility in E85


as clear. As can be seen, the polyether amine (PeA) type of additive has effectively complete solubility in e85 even at unusually high treat rates, unlike the more conventional PIBA. Conversely, at lower ethanol concentrations, PIBA is fully soluble and offers performance advantages that make it the preferred option.

7.6 Conclusions and future trends

the use of additives to enhance the performance of fuels and protect against the consequences of combustion has a long history. Fuels continue to evolve, with increasingly tight controls on the quality of the mineral base fuel being coupled to the incentivised introduction of new, renewable fuel streams, whilst in the search for lower emissions and greater efficiency, the appetite of engines and vehicles for fuels is changing. Biofuels in particular have introduced a range of opportunities for the additive producers to respond to with their different properties compared to mineral oil based fuels, ranging from solubilising performance to the types of contaminants present. As engines increase their specific power and duty cycles change in response to regulatory drivers, the stresses on fuel are changing. engines in hybrid vehicles operate for relatively short periods, but at higher load conditions than conventional powertrains. this will lead to more extreme thermal conditions that the fuel and additives will be exposed to, and to changing requirements of additives to support these engines. Concurrently, in the drive to further differentiate their products and give themselves a marketing advantage, fuel suppliers are becoming more demanding. Having products that address identified consumer needs is critical to the long term sustainability of the fuel additives business.

7.7 Sources of further information and advice

Chevron Diesel Fuels Technical Review (FTR-2). Chevron Products Co, USA, 1998.

Gairing, M., Marriott, J.M., Reders, K.H., Reglitzky, A.A. and Wolveridge, P.e. the effect of Modern Additive technology on Diesel Fuel Performance. SAe technical Paper 950252, 1995.

Marie, e., Chevalier, Y., eydoux, F, Germanaud, L. and Flores, P. Control of n-alkanes cystallization by ethylene-vinyl acetate copolymers. Journal of Colloid and Interface Science, 290, 406418 2005.

Nikanjam, M., Diesel Fuel Lubricity: On the Path to Specifications. SAE technical Paper 1999-01-1479, 1999.

national BioDiesel Board, Electrical Conductivity of Biodiesel. 2009. http://www.biodiesel.org/pdf_files/fuelfactsheets/electrical_conductivity.

pdf.


Stein, Y., Yetter, R.A., Dryer, F.L. and Aradi, A., Autoignition Behavior of Surrogate Diesel Fuel Mixtures and the Chemical effects of 2-ethylhexyl nitrate (2-eHn) Cetane Improver. SAe technical Paper 1999-01-1504, 1999.

Leedham, A., Caprotti, R., Graupner, O. and Klaua, t. Impact of Fuel Additives on Diesel Injector Deposits. SAe technical Paper 2004-01-2935,2004.

Udelhofen, J.H. and Zahalka, t.L. Gasoline Additive Requirements for todays Small engines. SAe technical Paper 881644, 1988.

Lenane, D.L. and Stocky, t.P. Gasoline Additives Solve Injector Deposit Problems. SAe technical Paper 861537, 1986.

Bitting, W.H., Gschwendtner, F., Kohlepp, W., Kothe, M., testroet, C.J. and Ziwica, K.H. Intake Valve Deposits Fuel Detergency Requirements Revisited. SAe technical Paper 872117, 1987.

noma, K., noda, t., Ashida, t., Kamioka, R., Hosono, K., nishida, t., Kameoka, A., Koseki, K., Watanabe, M., takahashi, K., Koide, S., Suzuki, t., Fukui, H., Hirose, M., Ohta, S., notsuki, Y. and tsuboi, K. A Study of Injector Deposits, Combustion Chamber Deposits (CCD) and Intake Valve Deposits (IVD) in Direct Injection Spark Ignition (DISIS) Engines. SAe technical Paper 2002-01-2659, 2002.

Aradi, A., Imoehl, B, Avery, L.n., Wells, P.P. and Grosser, R.W., the effect of Fuel Composition and engine Operating Parameters on Injector Deposits in a High-Pressure Direct Injection Gasoline (DIG) Research engine. SAe technical Paper 1999-01-3690, 1999.

Aradi, A.A., Colucci, W.J., Scull, H.M. and Openshaw, M.J. A study of Fuel Additives for Direct Injection Gasoline (DIG)) Injector Deposit Control. SAe technical Paper 2000-01-2020, 2000.

Aatola, H., Larmi, M., Sarjovaara, t. and Mikkonen, S. Hydrotreated Vegetable Oil (HVO) as a Renewable Fuel: Trade-off between NOx, Particulate emission, and Fuel Consumption of a Heavy Duty engine. SAe technical Paper 2008-01-2500

7.8 ReferencesCEC F-05-93 (1993). Inlet Valve Cleanliness in the MB M102E EngineCeC F-23-01 (2001). Procedure for Diesel engine Injector nozzle Coking test (PSA

XUD9A/L 1.9 Litre 4 Cylinder indirect injection diesel engine)CeC F-98-08 (2008). Direct Injection, Common Rail Diesel engine nozzle Coking

test. DuMont, R.J., Cunningham, L.J., Oliver, M.K., Studzinski, W.M. and Galante-Fox, J.M.

(2007). Controlling Induction System Deposits in Flexible Fuel Vehicles Operating on e85. SAe technical Paper 2007-01-4071.

DuMont, R.J., evans, J.A., Feist, D.P., Studzinsky, W.M. Cushing, t.J. (2009). test and Control of Fuel Injector Deposits in Direct Injected Spark Ignition engines, SAe technical Paper 2009-01-2641.


Gallant, t., Cusano, C.M., Gray, J.t. and Strete, n.M. (1991). Cummins L10 Injector Depositing test to evaluate Diesel Fuel Quality. SAe technical Paper 912331

Galante-Fox, J. and Bennett, J. (2012). Diesel Injector Internal Deposits in High Pressure Common Rail Diesel engines, Proceedings of the Institution of Mechanical Engineers, Fuel Systems for IC Engines, 1415 March 2012, pp 157166.

Hawthorne, M., Roos, J.W. and Openshaw, M.J. (2008). Use of Fuel Additives to Maintain Modern Diesel engine Performance with Severe test Conditions. SAe technical Paper 2008-01-1806.

Ikemoto, M., Omae, K., nakai, K., Ueda, R. et al. (2011). Injection nozzle Coking Mechanism in Common-rail Diesel engine, SAE Int. J. Fuels Lubr. 2011-01-1818.

Reading, K., Roberts, D.D. and evans, t.M. (1991). the effects of Fuel Detergents on nozzle Fouling and emissions in IDI Diesel engines. SAe technical Paper 912328.

Schwab, S.D., Bennett, J.J., Dell, S.J., Galante-Fox, J.M., Kulinowski, A.M. and Miller, K.t. (2010). Internal Injector Deposits in High-Pressure Common Rail Diesel engines. SAe technical Paper 2010-01-2242.

Ullmann, J., Geduldig, M., Stutzenberger, H. and Caprotti, R. (2008). Investigation into the Formation and Prevention of Internal Injector Deposits, SAe technical Paper 2008-01-0926.

US ePA (2003). the effect of Cetane number Increase Due to Additive on nOx emissions from Heavy-Duty Highway engines. Final technical Report, ePA420-R-03-002, 2003.

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