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Literature Review on the Effects of Organometallic Fuel Additives in Gasoline and Diesel Fuels.

Introduction

Metal-containing additives, most often in the form of organometallic compounds, have long been used in market fuels to promote various beneficial performance attributes. The best known and most widely used metallic fuel additive (MFA) is tetraethyl lead (TEL), which has been used worldwide as an anti-knock additive in gasoline. Manganese-based additives, particularly methylcyclopentadienyl manganese tricarbonyl (MMT), have also been widely used as gasoline anti-knock agents. In diesel fuel, barium-containing additives have been used for soot suppression [1] and cerium-based additives have been used to reduce particulate matter (PM) emissions [2]. In more recent years, a variety of MFAs have been evaluated in diesel engines to serve as fuel-borne catalysts for diesel particulate filter (DPF) regeneration, while other MFAs have been used in gasoline and diesel fuel as corrosion inhibitors, friction modifiers, anti-static agents, and fuel stabilizers [3-5].

Under sponsorship of the Coordinating Research Council (CRC), a recent literature review study was conducted to investigate the effects of MMT on gasoline vehicles [6, 7]. Subsequently, it was decided to conduct a similar literature review regarding use of other fuel additives in both gasoline and diesel fuel. This paper summarizes the follow-on CRC Project No, E-114-2 [8].

The objectives of this CRC project were to obtain, organize, summarize, and synthesize relevant information regarding the use of MFAs in market gasoline and diesel fuel, and the effects of such usage on vehicle engines and exhaust aftertreatment systems. The focus of literature searches on these effects was on modern vehicles (post-2000), which are equipped with on-board diagnostic (OBD) systems and advanced emissions control systems. For gasoline, this includes vehicles categorized as National Low Emission Vehicles (NLEV) and Tier 2 or beyond in the U.S., and Euro-3 through Euro-6 in the EU. For diesel, this includes engines/vehicles with original equipment manufacturer (OEM)-equipped diesel oxidation catalysts (DOCs) and DPMs. Some supporting historical literature and field experiences prior to 2000 are also included when relevant.

The information obtained through published literature and additional web searching was used to develop a compendium of MFAs and their known effects on vehicle performance. The complete compendium was assembled into an Excel table, which provides information regarding each additive's use, its effects on engine and exhaust aftertreatment systems, its producer, location of use, and recommended concentration range. The complete compendium, which is included as part of the original project final report [8], lists information on MFAs found in the literature or in the marketplace since 2000.

Approach

A starting point for assessing MFA usage and effects is a recent (2013) Bio Intelligence Service report to the European Commission, which included an extensive literature review on metallic additives to provide information for health and environmental risk assessments [5]. MFAs identified in this report included aluminum (A1) powder, boron (B) nanoparticles, cerium (Ce) oxide, iron (Fe) fuel additives, chromium (Cr), copper (Cu) compounds, lead (Pb)-based fuel additives, magnesium (Mg) oxide, MMT, perovskite compounds, potassium (K) compounds, platinum (Pt), rhodium (Rh), palladium (Pd), and zirconium (Zr) salts. Specific additives were further investigated through literature reviews and web/on-line searches, focused on publications with information relevant to vehicular uses. These searches excluded items that focused on health effects, exposure assessments, and measurement or characterization methodologies.

The literature reviews were conducted using the following computer-based search tools and approaches:

1. Web of Science (WOS) - WOS enables key word searches of over 5,300 technical/scientific journals, as well as some patents and conference proceedings. Key word search terms such as "organometallic" and "additives" returned fewer than 30 published papers. Additional specific key word searches included various combinations of the terms: organometallic, fuel additive, metallic, ferrocene, barium, fuel borne catalyst, cerium, diesel, and gasoline. Specific searches were not done for manganese/MMT, as this topic was covered in a recent paper [7]. Approximately 60 abstracts were returned when limiting the time period to 2000 and later.

2. Society of Automotive Engineers (SAE) literature search engine - The SAE literature search tool has much less flexibility than WOS searches, but returned approximately 80 items of interest, from as early as 1945.

3. Department of Energy (DOE) citation database - This literature search returned numerous reports, but very few were relevant to the use of MFAs in market fuels.

4. Trade literature, patents, conference proceedings, and other on-line sources.

Screening of abstracts and articles was done by selecting material relevant to vehicle transportation, focusing on gasoline and diesel additives, but excluding jet fuel and marine application additives. The additives of interest include those containing metals that are deliberately introduced into market fuels. Specific searches were not conducted for aftermarket additives, although some information about these was uncovered. In addition, although some MFAs used for DPF regeneration are considered aftermarket additives, they were included as relevant search items. Metallic materials arising from lubricant additive packages or trace fuel contaminants were not considered.

While literature searches provided considerable information regarding research aspects of MFAs, reports on marketplace usage were scarce. Much of the relevant information regarding MFA usage likely exists in company documents and other grey literature, rather than in the open literature. To get a better understanding of MFA usage in market gasoline, recent fuel survey information from Summer of 2015 and Winter of 2015/2016 was purchased from SGS [9, 10]. Summer 2015 data included 784 gasoline samples, while the Winter 2015/16 dataset included 794 gasoline fuel samples, both collected worldwide from 139 countries. Only the inductively coupled plasma (ICP) analysis results for metals were included in the purchased dataset. This included 19 different metals: Al, B, Ba, Ca, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, P, Si, Sn, Ti, V, and Zn. Equivalent information regarding metals in diesel fuels was not obtained.

The remainder of this paper includes a brief synopsis of fuel specifications and regulations as they relate to use of MFAs. A summary of MFA categories and usages is then provided, along with a discussion about what is known regarding the effects of MFAs on engine and exhaust treatment systems.

Fuel Specifications and Regulations

U.S. gasoline fuel specifications are defined in ASTM D4814, which is also followed in many other countries. These fuel specifications are driven by vehicle performance considerations, and include requirements on vapor pressure, vapor lock protection, distillation, and octane number, as well as several compositional properties. ASTM D4814 also places maximum limits on lead and phosphorous in unleaded gasoline (0.013 g/L), and prohibits intentional addition of these elements [11]. Diesel standards are specified in ASTM D975 [12]. Specific metals are not limited by this standard, but ash content is specified not to exceed 0.01% m/m to avoid injector, fuel pump, piston and ring wear, and engine deposits from abrasive solids. The standards also outline testing methodologies for fuel contaminants.

The World Wide Fuel Charter (WWFC) provides recommendations and specifications of fuels for different market categories, and outlines testing methodologies for each property. These recommendations tend to be slightly more stringent than ASTM specifications. The most recent WWFC is the 5th edition, published in 2013 [13]. Within this version, testing methodology was added to measure trace metals in gasoline and diesel fuel. Also, a 5th specification category was added for markets having highly advanced requirements for emissions control and fuel efficiency [13]. Category 4 market specifications include requirements for emission controls such as US Tier 2 and Tier 3, US 2007/2010 Heavy Duty On-Highway, California LEV II, Euro 4/IV, Euro 5/V, EURO 6/VI, and JP 2009 or equivalent emission standards. Category 4 fuels enable sophisticated NOx and PM after-treatment technologies, while the new 5th Category includes US 2017, California LEV III or equivalent emission control and fuel efficiency standards, in addition to the Category 4 requirements.

Within gasoline fuels, the WWFC recommends that ash-forming additives such as organo-metallic additives be avoided. While the use of Pb has already been phased out, the WWFC also recommends limiting the use of MMT (citing results from a 2008 report by Sierra Research [14]), and ferrocene in gasoline, stating that ferrocene causes iron oxide deposits to form on spark plugs, catalysts and other exhaust system components, which results in premature failure of spark plugs. The WWFC also recommends that P, Si and Cl not be added to gasoline, nor used as components of an additive package. Phosphorous, occasionally used as a valve seat recession additive (VSRA), can foul spark plugs and deactivate catalytic converters. Contamination with Si can cause failure of [O.sub.2] sensors and high levels of deposits in engines and catalytic converters, leading to catastrophic engine failures. Chlorine can form highly corrosive acid during combustion and reduce durability of the engine, fuel system or emissions control system.

The only WWFC specifications directly related to metal content of the fuels pertain to trace metals in both gasoline and diesel. In each fuel, trace metals, including Cu, Fe, Mn, Na, P, Pb, Si, Zn and Cl are not to exceed 1 mg/kg, and the intentional addition of MFAs is not allowed. The ash content of diesel fuels is also specified not to exceed 0.001% m/m (10 ppm), with additional considerations under review for DPF related issues. Testing related to metals in gasoline include methodologies for K, P, Si, Cl, Pb and other trace metals listed above. Within diesel fuel, recommended methodologies include testing for trace metals and ash content. In general, WWFC recommendations (as well as regulatory specifications within the U.S. and Europe) have become more stringent over time, as advances in engine and aftertreatment technologies demand increasingly low levels of ash and other fuel contaminants.

European Regulations

The EU Fuel Quality Directive 98/70/EC sets legal parameters for petrol and diesel that have an effect on the environment and health, and specifies greenhouse gas (GHG) reduction targets. Metallic additives may be approved for use following satisfactory evaluation of short term and long term vehicle effects [15].

The United Nations Economic Commission for Europe (UNECE) promotes development of international transport through harmonization of rules and requirements within its Working Party on Road Transport (SC.1) and Working Forum for Harmonization of Vehicle Regulations (WP.29) [16]. A subsidiary of this group, the Working Party on Pollution and Energy (GRPE) prepares regulatory proposals to develop emissions and energy requirements for vehicles. In 2014, this group provided a market fuel quality guideline to recommend minimum fuel quality of gasoline and diesel [17]. The guidelines apply to fuel quality parameters that directly affect the performance and durability of engines and exhaust emissions control equipment, and recommend "no intentional addition" of lead and no permitted additions of other metallic additives to gasoline, including Mn, Fe, and P. In addition, maximum sulfur limits are recommended, beginning at 500 ppm for Euro 2, and decreasing to 150 ppm and 50 ppm for Euro 3 and Euro 4 gasoline. Similarly, allowable sulfur content of diesel fuels decreases from 500 ppm for Euro II to 350 ppm for Euro III and 50 ppm for Euro IV. Other metals contents are not specified for diesel fuels, although total ash must be less than 0.01% m/m and total particulate contamination less than 24 ppm.

These recommendations are based on evidence that organo-metallic components create ash that adversely affects operating systems and increases emissions, providing examples of effects of MMT and ferrocene. The GRPE report also describes negative effects of Fe as ferrocene, including deposition of iron oxide onto spark plugs, catalysts and other exhaust system parts, resulting in premature spark plug failure and 90% reduction in spark plug life [17]. In addition, iron oxides were reported to act as a physical barrier between the catalyst/oxygen sensor and exhaust gases, which can lead to erosion, plugging, and poor functioning of the catalyst.

Although these parties recommend against the use of MFAs, a 2008 fuel survey showed the presence of several metals in fuel pools. As part of a 2008 study to evaluate emission factors of heavy metals from on-road vehicles, CONCAWE collected 65 petrol samples and 110 diesel fuel samples from service stations in nine EU countries [18]. Concentrations of metals (As, Cd, Cr, Cu, Hg, Ni, Pb, Se and Zn) showed high variability among the samples. Cd, Se, As, and Ni were not detected (or were minimally detected) in diesel fuel. Relative to the other metals, high concentrations of Zn were seen in both gasoline and diesel. As, Hg, and Se exhaust emissions were dominated by fuel combustion, while Cd, Cr, Cu, Ni, Pb and Zn emissions were dominated by lubricant oil combustion. From this study, emission factors of heavy metals from on-road European vehicles were generated, which were much lower than those reported previously.

U.S. Regulations

Within the U.S., the EPA manages and monitors fuels and fuel additives as defined in CFR Part 80. While the regulations define sulfur limits, other metallic components are not individually defined. The EPA requires that all fuel additives be registered, and the list of 10,020 registered fuel additives is available to the public [19]. However, details about specific inclusion of each additive are unavailable through this website, as the information is proprietary to individual companies [20].

Some gasoline and diesel fuel additives are allowed waivers based on "substantially similar" interpretations. Section 211(f) of the Clean Air Act prohibits any fuel or fuel additive to be manufactured that is not substantially similar to any fuel or fuel additive utilized in the certification of vehicles or engines under Section 206 [20, 21]. Based on a "substantially similar" interpretation, MMT was allowed a waiver in 1995. However, recent fuel survey information indicates that metallic additives are not currently used to a significant degree in any gasoline fuels within the U.S.

Current Use of Metallic Additives in Gasoline

SGS fuel survey data were examined to assess usage levels of MFAs in marketplace fuels around the world in 2015 and 2016 [9, 10]. Although a complete set of analyses was conducted on each sample, only the ICP analysis of 19 metals was purchased for this work. The metal concentrations are reported in units of mg metal/kg of fuel. Concentrations below detection limits are presented as <0.1 mg/kg. Selected metals (Cu, P, Fe, and Mn) are also reported in mg/L. Eight of the 19 metals measured (Al, Ba, Cr, Mo, Ni, Sn, Ti, and V) were not detected in amounts greater than 0.1 mg/kg in any fuel sample. Mg, B, and Ca were detected in less than 1% of the samples. The range of concentrations of the remaining elements found in the samples are shown in Table 1. The three most frequently detected are Fe, Mn, and Si. Zn is also seen relatively frequently, albeit in very small quantities. No metals were detected in any samples collected from the U.S. or Canada.

Mn was detected more frequently and in higher concentrations than any other element in both summer and winter, reaching as high as 91 mg/kg (66 mg/L) in the winter season, although the average concentration was much lower, at 9.8 mg/kg. In total, Mn was seen in 267 fuel samples collected from 48 different countries, with some seasonal variation for certain locations. A regional summary of Mn content, reported as mg/L, is shown in Figure 1. The bars in this figure represent the mean of all measurements above detection limits for a given country; the error bars represent minimum and maximum concentrations within each country. In 113 cases (42%), the concentrations measured were less than 2 mg/L, which is the maximum specification in China and the EU. Another 72 (27%) of the samples had concentrations less than 8.3 mg/L (corresponding to 1/32 g Mn/gal, the most recently revised ASTM specification). In 9 instances, however, the concentration exceeded 33 mg Mn/L. These samples were collected in the countries of Mauritania, Pakistan, Algeria, Senegal and Tanzania. In general, the locations of Mn use were primarily in Latin American, African, Middle Eastern, and Asian countries. No Mn was detected in any samples collected in Canada or the U.S.

Iron was detected relatively frequently, and was measured in 75 samples collected from 32 countries. When identified, however, it was most frequently measured at low concentrations: 44 of the 75 samples had Fe concentrations below 1 mg/kg (~0.73 mg/L), the recommended limit by the WWFC, while another 13 samples were below 5 mg/kg (3.6 mg/L). The remaining 18 samples had concentrations ranging from 5.0 to 25 mg/kg and were found in multiple fuel samples from Myanmar, Cameroon, Kenya, Mauritania, and Tanzania. A regional summary of iron content, reported as mg/L, is shown in Figure 2, with the bars representing the average of all non-zero recordings, and the error bars representing the minimum and maximum concentrations within a country.

Silicon is a contaminant that was detected in a relatively large number of fuel samples from around the world; it was found in 111 samples collected from 35 countries. The highest frequency of detection was in samples from China, where 29 of the samples collected showed some level of Si contamination, with one sample reaching 59 mg/kg. The Russian Federation, Panama, and Paraguay also had multiple instances of Si in fuel samples, as shown in Figure 3. In 22 countries, Si was detected in only one or two fuel samples, and in relatively low concentrations.

Traces of other metals were seen occasionally. P, K, and Na were rarely detected in any fuel samples. K was found in relatively high concentrations in South Africa, where it has been used as an anti-valve seat recession (AVSR) additive [22]. Cu was seen in a relatively high number of fuels from the European Union. Zn was also seen frequently, but at very low concentrations. Summaries of the combined seasonal results for Cu, K, Na, P, and Zn are shown in Figure 4.

Summary of MFAs in Gasoline

There is a long history of additive use in gasoline, beginning with anti-knock additives [23]. More recently, the focus of additive usage has been on maintaining clean and well-functioning engine and exhaust system components. In addition, increased use of biofuels, including ethanol, has heightened concerns of deposit formation, requiring higher levels of additives to be used [24]. The worldwide additive market continues to grow, despite reductions in fuel demand [25]. The primary gasoline additives include detergents and deposit control additives (DCA), which prevent the formation of deposits. These are usually composed of polyether amines (PEA) or polyisobutyl amines (PIBA), and do not include organometallics. Other non-metallic additives include fluidizers, friction modifiers, antioxidants, demulsifiers, and corrosion inhibitors [24]. Additives that sometimes contain organometallic materials include anti-knock or octane number (ON) boosters, anti-valve seat recession (AVSR) additives, and conductivity improvers. Each of these is described further below.

Anti-Knock Additives/Octane Boosters

The primary traditional use of organometallic additives in gasoline has been to provide ON enhancement. Such octane boosters, or anti-knock additives, are blended at the refinery to meet fuel ON requirements. Organometallic additives functioning as octane boosters include tetraethyl lead (TEL), ferrocene, and (MMT) [26]. Iron pentacarbonyl was investigated as a Pb replacement antiknock agent in ethanol as early as 1945 [27]. However, engine studies conducted at that time found that iron oxide deposits readily formed during the combustion process. While Fe was an effective antiknock agent, these deposits could seriously interfere with engine operation.

Lead, primarily in the form of TEL, was widely used as an antiknock prior to its phase-out after the U.S. Clean Air Act of 1970. Since then, it has been eliminated from gasoline in most countries worldwide. According to the United Nations Environment Program (UNEP), as of January 2016, only three countries still allowed the use of TEL - Algeria, Iraq, and Yemen [28]. Innospec Ltd, a UK-based company, claims to be the only major producer of TEL in the world [29], yet faces scrutiny for doing so [30]. TDS Chem, a company in China, also claims to manufacture and sell TEL additives [31].

The effects of Pb on engine and vehicle performance were well studied prior to its phase-out. Since then, few evaluations have been conducted to identify Pb in gasoline fuels, or evaluate its effects at the low levels currently allowed in fuels (0.013 g/L in the U.S.) Literature searches returned no results related to low level lead impacts on vehicle components. However, several aftermarket Pb additives are available, primarily marketed to high performance specialty vehicles, race vehicles, and classic cars. Octane 130[TM] is one such example in the U.S.; [32] Tetraboost[TM] is a similar product available in the UK [33].

After the phase-out of Pb in the U.S., alcohols and ethers became the primary additives for gasoline ON enhancement [34], although organometallics continued to be used in rare cases. In particular, Mn and ferrocene have been used in some instances, and patents on such additives have been filed [35, 36]. Several companies continue to produce additives containing Mn, Fe, and Pb that are marketed for blending with gasoline. Cestoil, Innospec, Afton Chemical, and TDS, are examples of chemical producers that market antiknock additives containing metallic additives.

Mn in the form of MMT has been widely used, and a large body of literature exists regarding its effects on engine and exhaust components. This literature was recently summarized in a CRC report and an SAE publication [6, 7]. Along with Afton Chemical, which produces MMT under the trade name Hi-Tec 3000[TM], MMT is included in products from Innospec under the trade name Octaburn[TM], Cestoil under the trade name CESTOBURN 8000[TM], and TDS Chemical.

Recommended dosing rates range from 8 to 36 mg/L, although the WWFC and UNECE both recommend against the use of MMT, and ASTM is undergoing a revision process for ASTM D4814 with respect to Mn content [11, 13, 16].

Ferrocene has also been used as an ON booster in gasoline, with other benefits such as AVSR [37]. However, literature searches for the use of ferrocene in gasoline returned a very limited number of items. An investigation conducted in the early 1990's of the effects of ferrocene dosed at 15 ppm found lower pollutant emissions and improved fuel economy [38, 39]. One recent evaluation of ferrocene and MMT on gasoline properties found that these additives have little effect on the distillation properties of the fuel [40]. Ferrocene is offered for sale by several fuel additive producers, and can be found in products listed by Innospec (PLUTOcen G[R]), Cestoil (CESTOBURN 9000) and TDS Chem. As of 2001, it was used as an anti-knock in Russia [41], and has been evaluated for use in Bulgaria [42]. (Note that recent fuel survey data described above indicates that there is very little iron in gasoline from either of these locations.) A few after-market ferrocene additives may still exist, although some, such as Ferox Fuel Tabs, which was produced by Parish Chemicals, are no longer available.

An evaluation of the effects of ferrocene on engine and vehicle performance has shown that gasoline containing ferrocene results in iron oxide deposits that adhere to the combustion chamber, spark plugs, and exhaust system. Spark plug deposits have been shown to have high conductivity, which contributes to abnormal electrical discharge. In addition, reddish-brown iron oxide deposits have been observed at the inlet of the catalytic converter. These deposits are thought to contribute to increased fuel consumption, elevated exhaust temperatures, and higher exhaust emissions [43].

Conductivity Improvers

Static electricity can accumulate during fuel pumping operations at refineries, terminals, or filling stations, which presents an obvious fire or explosion hazard. Additives are often used to enhance conductivity of blended fuels and prevent static discharge. Fuel-soluble Cr-based materials or other non-metallic additives are used for this purpose, with typical treat rates ranging from 1-40 mg/kg [24]. The use of Cr has been seen in fuel surveys conducted in the past [18], although the recent SGS surveys discussed above did not find significant levels of Cr in any 2015-2016 gasoline samples.

Anti-Valve Seat Recession (AVSR) Additives

Exhaust valve seat recession (VSR) occurs during prolonged high temperature or high speed operation during which the like-metal components of the valve seat and valve are repeatedly welded together and torn apart. This process can generate hard wear particles that grind or abrade valve seats during repeated opening and closing contact exposures to the hot exhaust valves. This valve seat erosion can result in loss of compression, which leads to power loss, rough engine operation and increased emissions, and may eventually lead to engine failure [24].

Engines designed for use with leaded fuels had relatively soft valve seats that were more susceptible to recession. Pb in the fuel produced lead oxides during combustion, which formed a protective layer on valve and seat surfaces to prevent metal-to-metal contact and the formation of hard particles. The lower limit of Pb's effectiveness as an AVSR was estimated to be 0.026 g Pb/L [44]. The phase-out of Pb resulted in the onset of VSR problems in existing vehicles at that time, and necessitated the use of other AVSR additives [45].

The combustion of P, K, Na, and Mn organometallics also produces ash that deposits a similar protective layer on valve seats. Typical treat rates of these additives range from below 50 mg/kg to 200 mg/kg of the active element [24]. However, because there is no standard test methodology to evaluate the effectiveness of AVSR additives, in-use dosages were often based on recommendations from additive suppliers, that were meant to avoid harmful side effects [44].

Problems related to VSR were significantly reduced after the early 1970's, when induction hardened exhaust valve seats were implemented [46]. A report on VSR issued by the United Nations Environment Programme (UNEP) Partnership for Clean Fuels and Vehicles in 2004 reported that no significant VSR issues had been experienced in countries that had phased-out lead [47]. At that time, France was the only country reporting the use of an AVSR additive (potassium-based), but had plans to phase it out. The report also noted that some European countries that had used Pb replacement additives did not observe valve wear problems, but had experienced other engine problems, such as destructive corrosion of turbo-chargers, valve sticking, and valve burnout.

Hutcheson et al. evaluated other metallic additives for their effectiveness as AVSR agents, including P, Mn, K, and Na [44]. They determined that a treat rate of 8 mg/L generally provided sufficient protection for the majority of service conditions, and 36 mg/L offered complete protection under all possible in-service conditions. However, they also found engine durability issues associated with the use of these additives. For example, use of Na was correlated with hot salt corrosion of turbochargers, while some alkali metals promoted valve sticking.

The Associated Octel Co. (Octel) investigated use of ferrocene as an AVSR additive. They found that while beneficial as an ON booster, a treat rate of 25 mg Fe/kg offered very little AVSR protection [37]. However, when combining ferrocene with phosphorous, a high level of AVSR protection was seen as well as an ON boost. In 2006, Octel became Innospec, Inc., which currently markets ON boosting additives using the organometallics MMT and ferrocene, although no products are marketed specifically as AVSR additives.

Potassium was frequently used in lead replacement petrol (LRP) in European markets and other countries, including South Africa [22, 44]. However, the use of LRP was also phased-out beginning in the early 2000's in Europe and other locations. Little information is available regarding on-going use of AVSR additives at the refinery level in other markets. Due to decreasing problems with VSR, as older vehicles have been phased out, AVSR additives are not frequently blended into fuels in most markets. It is possible, however, that some markets that have a large fleet of older vehicles may still use AVSR additives. The high level of potassium seen in South African gasoline (see Figure 4) may be evidence of this. In addition, AVSR additives are still available in after-market products at auto parts shops in select markets (e.g. Penrite Valveshield[TM]).

Effects of MFAs on Engine and Exhaust Components

Within the body of literature reviewed, very few reports investigated the durability or emissions effects of specific MFAs. Much of the literature on this topic is related to use of MMT, which was summarized and reported previously [6, 7]. The effects of Pb have also been studied with older vehicles, but no literature on its effects in model year (MY) 2000 and newer vehicles was found, even at low levels of use.

Other reports discuss the effects of metal-containing lube oil additives and contaminants. While these topics were not specifically included in the literature search, any information that surfaced in related searches is summarized here. However, it should be noted that this information is incomplete, as targeted searches were not performed for other contaminants or oil additives. In one such study, engine oil additives containing Ca (as calcium salicylate), Zn (as zinc dithiophosphate) and Mo (as molybdenum dithiocarbamate) were diluted directly into combustion fuels [48]. Ca and Zn were found to increase deposit formation in the combustion chamber and facilitate auto-ignition and knock intensity. Mo compounds showed no significant effects.

An evaluation of various combustion chamber and valve deposits showed that Ca, P, S, Zn, and Mo were the most significant inorganic deposit constituents [49]. These elements are believed to originate primarily from engine oil additive packages. In addition, valve deposit formation has been attributed to Si, Fe, and Cr [50, 51]. Another investigation of valve deposits showed that an additive containing calcium phenate (CaPh) resulted in higher inlet valve deposit formation than other metallic detergent additives studied, including calcium sulfonate (CaSu), magnesium sulfonate (MgSu), or neutralized calcium sulfonate (Neut. CaSu) [52].

A summary of pre MY 2000 vehicle effects can be found in a recent review of abnormal ignition by fuel and lubricant derivatives [53]. Phosphorous and boron as deposit control additives (DCAs) have been shown to suppress abnormal ignition. However, these metals have been removed from DCAs as they act as catalyst poisons. (1) The use of P, Ba, Ca and Zn as engine oil additives has also been shown to reduce surface ignition, although there are some contraindications of these effects [53].

Ferrocene has been shown to improve exhaust emissions and increase fuel economy [38], although some field problems have also been experienced. In 2003, the Canadian Vehicle Manufacturers Association (CVMA) investigated a ferrocene additive field trial that was launched the previous year by two fuel suppliers in Vancouver Island, BC [54]. In this trial, gasoline was treated with sufficient ferrocene to provide approximately a one number boost in anti-knock index (AKI; approximately 7 mg/L Fe) [55]. (At that time, MMT was widely used as an additive in Canada, typically at about 10 mg Mn/L.) Within a few months of the field trial, complaints of spark plug fouling, manifested as poor driveability and mis-firing, were reported by dealers when customers used gasoline containing both ferrocene and MMT. Total failures were found in a few hundred vehicles and affected several common, late model vehicles, although reports of malfunction indicator light (MIL) illumination were very low. Replacing spark plugs alleviated the complaints [55].

Two redacted reports on analyses of returned parts during this Canadian field trial were reviewed for the current study [56, 57]. These reports documented the appearance of reddish-brown deposits and black glazed areas on the spark plug insulator core noses. The glazed areas exhibited spider-web-like tracking marks where the spark energy traveled across the deposits rather than firing the gap. X-ray fluorescence (XRF) analyses showed the deposits were comprised mainly of Mn, Fe, and P, which were attributed to the use of MMT, ferrocene, and lube oil, respectively. It was concluded that the spark plugs misfired due to the conductive Mn and Fe deposits produced from the fuel additives. Other literature also supports the hypothesis that iron oxide particle deposits in the combustion chamber and on spark plugs result in abnormal function [17, 43]. These deposits increase conductivity on the surface, and result in spark plug current leakage.

Catalytic converter performance is typically reduced by thermal degradation and chemical poisoning. P, Zn, S, and Pb are known contaminants of catalytic converters [58]. Other contaminants include Ca, Cu, and Fe. These metals irreversibly deposit onto the washcoat and reduce the catalyst's active surface area. With sufficient contaminant accumulation, catalyst plugging can occur. Plugging can cause greatly reduced conversion efficiencies, as well as increased back pressure. Long-term thermal degradation can also cause reduced catalytic converter performance.

Gasoline contaminated with silicon has also been found to have harmful effects on catalysts and oxygen sensors. Experimental tests of fuel containing 20 ppm of Si resulted in significantly decreased efficiencies of three-way catalysts after only 1500 miles of use, with continued decline to less than 40% efficiency after 15,000 simulated miles [59]. In 2007, widespread reports of vehicle damage in the UK were linked to traces of silicon found in batches of suspect fuel [60]. As many as 4,000 motorists complained of juddering, misfiring, and loss of power.

A survey of early to mid-1990's MY in-use catalytic converters was performed in 2001 to evaluate catalyst degradation over various mileage intervals [61]. Low levels of contamination by Zn, Ca, K, Cu, Na, Fe, Mg and Ba were found on most catalysts. Low levels of Pb contamination were found in catalysts taken from low odometer vehicles, but higher contamination was seen in the high odometer vehicle group. In most cases, the highest contamination levels were from phosphorous. However, thermal deterioration was found to have the strongest influence on loss of catalytic performance, not the presence of metallic contaminants. Other effects of contamination and plugging from use of MMT have been reported [7, 62, 63].

Summary of MFAs in Diesel Fuel

Diesel fuel additives are used for a variety of purposes, including improvement of engine and fuel delivery system performance, fuel handling, fuel stability, and contaminant control. However, very few (if any) of the additives that are blended at the refinery level contain organometallics, although some MFAs have seen limited use as smoke suppressants around the world. Because advanced diesel technologies, such as common rail injection systems, are particularly prone to problems from deposit formation, deposit control additives (DCAs) are frequently used.

A recent review paper summarizes information about some metallic-based additives in diesel applications [64]. The bulk of recent literature regarding organometallic diesel fuel additives involves their use in conjunction with DPFs for reduction of PM emissions. Aftertreatment systems in diesel vehicles consist of multiple components, as no single device can simultaneously reduce N[O.sub.x], PM, and HC. Besides DPF, advanced diesel emissions control technologies include De-N[O.sub.x] systems, such as lean N[O.sub.x] traps (LNT), selective catalytic reduction (SCR) to remove N[O.sub.x], diesel oxidation catalysts (DOCs) to reduce HC and CO, and exhaust gas recirculation (EGR). The use of MFAs to aid in DPF regeneration has been a focus of recent evaluation. This, along with other uses of organometallic additives in diesel fuel is described further.

Fuel Borne Catalysts for DPF Regeneration

A common method to reduce PM emissions involves use of a DPF, which functions by trapping soot and other particulates from the exhaust. A DPF typically consists of a filter material that is designed to collect solid and liquid particulate emissions, while allowing exhaust gases to pass through. A variety of filter materials and designs have been evaluated with PM collection efficiencies varying from 50-90%. These materials include ceramic monoliths, woven silica fiber coils, ceramic foam, wire mesh, and sintered metal filters [65]. The trapped particulate matter builds up over time, however, and must be removed periodically for continued operation. Apart from removing the filter for manual cleaning, there are several methods of DPF regeneration, during which the collected PM is oxidized or combusted into C[O.sub.2]. For auto-ignition of the PM to occur, temperatures of 600-650 [degrees]C must be achieved. Normal engine exhaust temperatures are typically not sufficient, ranging between 200 [degrees]C to 500 [degrees]C, so either exhaust temperatures must be raised to achieve auto-ignition, or the oxidation temperature of the collected PM must be lowered through use of catalysts.

There are several methods to achieve regeneration, including both use of a continuously regenerating DPF (CR-DPF) and a catalyzed DPF (CDPF). The CR-DPF utilizes a diesel oxidation catalyst (DOC), typically containing platinum, upstream of the DPF to generate N[O.sub.2], which functions as an effective low-temperature oxidizing agent for PM [66]. The CDPF regenerates via a catalyst coating on the DPF that promotes low-temperature oxidation.

A third method of regeneration occurs through use of a fuel borne catalyst (FBC). Catalytic materials such as transition metals including Ce, Fe, Cu, Mn, Na, Sr and Ca act at a late stage of the soot formation process and help to catalyze the complete burning of soot in a DPF. These FBCs can reduce the temperature of activation to 300-350 [degrees]C, which reduces the interval between DPF regeneration events [65, 67-69]. Regeneration events occur as particulate is collected on the filter, causing back pressure to increase and temperatures to rise. During regeneration, the organic fraction of the collected PM is combusted, along with the soot, leaving the inorganic portion of the FBC as part of the ash, along with other products of lubricant combustion and normal engine wear. This ash accumulates within the filter, which periodically must be physically removed and cleaned as part of a filter maintenance program to prevent excessive increase in back pressure across the filter. The frequency of these filter cleaning events depends on engine oil consumption rates, total ash content of lubricant formulations, vehicle duty cycles, filter designs, and dosing levels of the FBC.

Widespread introduction of DPFs on passenger cars was initially limited by difficulties including safe, reliable regeneration under all driving conditions, a fuel penalty with their use, and the development of a low cost system [70]. In Europe, DPFs have been in commercial use in light-duty vehicles since 2000 [71, 72]. Their use in the U.S. has been increasing since 2006 [73]. A survey by the Manufacturers of Emission Controls Association (MECA) in 2014 reported a 65% increase in DPFs sold in California in the first six months of 2014 compared to the same period in 2013, including both OEM and retrofit devices. MECA and the California Air Resources Board (CARB) each maintain a list of manufacturers and verified providers of retrofit devices [72, 74], and a global market survey of DPF filters is available for licensing. [75]. However, DPF with FBC technologies do not appear to be as widely adopted as other methods of continuous regeneration [73]. The majority of heavy-duty diesel manufacturers have selected DPF in combination with SCR and EGR to meet emissions standards [76].

FBC Additives

FBCs used for DPF regeneration can be mixed directly into the fuel tank (on-board or off-board) or introduced via an on-board dosing system. Typical dosing levels are in the range of a few ppm [77]. Initially, most FBCs contained fuel-soluble organic forms of metals, while more recently, nanoparticle forms of the metals themselves are being investigated [78, 79]. FBCs are not added to diesel fuel at the refinery level, as their use may cause adverse effects in vehicles that are not equipped with DPF technologies. However, since they are required for use in specific systems, FBCs are considered to be MFAs for the purposes of this study.

Since the early 1980's, various metallic additives have been evaluated for use in DPF regeneration [80, 81]. The effects of metals including Zn, Ca, Fe, Ce, Cu, Ba, and Mn have been evaluated by universities, automobile manufacturers, and additive producers [82-84]. One of the first commercial introductions of an active DPF system with an FBC was the Peugeot Citroen in Europe. This vehicle used a common rail direct injection (DI) engine with an on-board dosing system of Rhodia's Eolys[TM] cerium-based FBC. Cerium oxide has been shown to reduce soot emissions and lower the oxidation temperature of regeneration [85]. However, while highly effective in suppressing black smoke and reducing PM mass emissions, early introduction of this technology resulted in a 5% increase in fuel consumption, and a 2% reduction in maximum engine output [70]. Other evaluations of Rhodia's cerium-based additives in conjunction with various DPF filter systems confirmed large reductions in PM and small reductions in HC emissions, typically accompanied by increased N[O.sub.x] and a fuel penalty [86, 87]. Many of these studies also focused on the FBC's effects on particle size and number, showing that the reduction of particle number (PN) was relatively insensitive to dosing level [2, 88].

An evaluation of cerium by the Health Effects Institute (HEI) showed that a DPF used in conjunction with Eolys[TM] decreased particle mass by greater than 90% and PN by as much as 99% in diesel exhaust [89]. A small fraction of the added cerium was found in the emissions, totaling 3-18% of the total particulate mass, with the remainder being captured on the DPF itself or deposited on surfaces throughout the engine and exhaust components. Another study found that an unexpectedly high fraction of the FBC was retained in the engine and exhaust system components, with only 10% being found in the DPF [90].

CARB and the Joint Research Council (JRC) also conducted a study to evaluate PM emissions during DPF regeneration events using a cerium-based FBC. Testing was completed during 5 unique regeneration events. Significant increases in PM mass emissions were observed during each event, although PM emission rates generally remained below relevant standards during these events [77].

Octel also investigated the effects of MFAs on DPF regeneration. In early work, they examined use of a fuel-soluble sodium-based FBC containing traces of strontium (Sr) with a ceramic DPF filter. At a dosage of 20 ppm, this package provided effective regeneration, with no adverse effects on the engine or the size distribution of the PM [91]. However, due to other problems with high temperature degradation of the ceramic filter, they turned their focus to combinations of iron and strontium, which gave improved performance [92]. Octel continued to investigate iron-based additives in comparison with cerium in support of their product Octimax 4804[TM], which contains Fe and Sr in a 4:1 ratio [93]. The iron-based additives were shown to perform better than cerium-based additives at the same dosage, achieving lower temperature regeneration, as indicated by lower back pressure increases [71, 94]. Dosage rates as low as 10 ppm were found to achieve similar regeneration temperatures as 30 ppm cerium-based additives. Also, the Fe-Sr blends produced lower temperature oxidation than iron alone. (Lower dosing rates are desirable as this can reduce ash accumulation and extend service intervals.) "No harm" emissions testing of the same additive showed no adverse effects on PN [95]. An independent evaluation of Octimax showed that it gave large reductions of soot emissions at low load, when the FBC-doped soot is more likely to be enriched by metal oxide on the outer periphery of the particle, thereby increasing oxidative reactivity as compared to high load conditions [96].

Octel also performed durability testing of Octimax 4804 over 80,000 km on the road and on a dynamometer. No drivability issues were observed, despite significant backpressure increases before regeneration, and no signs of clogging or failure were seen [97]. However, fuel consumption increased by 5.5% and an increase in CO emissions accompanied the significant PM reduction.

In 2006, Octel was rebranded into Innospec, Inc. Octimax 4804 no longer appears as a product on the Innospec web page. Currently, their list of FBC additives includes a product line called Satacen[R], a ferrocene-based product that does not appear to include strontium [98]. Satacen[R] is listed as the tradename of the product sold in Germany, Switzerland and Austria, although the product is called Octel Octimax in other countries [99].

Other evaluations of ferrocene have been published, indicating that it behaves as an effective oxidation catalyst in conjunction with a DPF. DPF retrofits using ferrocene as an FBC have been shown to reduce unregulated emissions, including 16% reduction in total carbonyls and 66% reduction of total PAH, although benzo[a]pyrene equivalent emissions increased from 0.016 to 0.030 mg/kWh, and brake specific N[O.sub.x] increased 4.3% [100]. Evaluation of a range of ferrocene dosage rates (0 to 200 ppm) indicated that higher dosage decreased particle mass and black carbon emissions, but increased PN concentrations. In addition, the Fe concentrations in the particles increased from 0.1% to 7.5% with higher dosage rates [101].

The US Bureau of Mines evaluated the effects of ferrocene on fuel consumption in the early 1990's, finding that its use in diesel fuel contributed to increased C[O.sub.2] and N[O.sub.x] emissions, indicating an increase in fuel consumption [102]. In addition, a deposit layer of ferric oxide was found on the combustion chamber components after only 250 hours of operation.

Several other publications discuss comparative evaluations of FBC additives. The Oil and Gas Institute in Poland recently evaluated a range of FBC additives containing Fe, Ce, Zn, K, and Co to determine the most effective material for DPF regeneration. In total, seven additives with different blends of metals were synthesized and bench tested, along with two commercially available Fe-based additives. While the test results were blinded in the report, the two most effective additives were not the commercially-produced materials. One of the most effective additives contained 12.1% Fe; the other contained 7.2% Fe and 1.8% K [67].

In a study conducted by Ethyl Corporation, it was shown that MMT can also function as a soot suppressant additive. Use of MMT in conjunction with a DPF was found to have beneficial effects both with respect to the rate of soot accumulation within the DPF and the DPF balance point temperature [103].

Infineum UK also evaluated various metallic additives with respect to regeneration performance [104]. Initially, 15 additives containing various combinations of metals (including Ca, Na, and Fe) were screened to determine regeneration capabilities. An iron-based additive was selected for further testing, which included evaluating various treat rates in combination with other ashless components and in comparison with a commercially available cerium additive. It was concluded that the novel, iron-based additive had superior performance at a dosage of 3 ppm metal in the fuel. VERT (Verification of Emissions Reduction Technologies) testing confirmed that the additive had no adverse impact on unregulated emissions when used in combination with a DPF at treat rates up to 25 ppm.

Several investigations, including reports under the VERT program, have indicated that while high metallic additive dosage rates in conjunction with a DPF filter may reduce PM mass emissions, engine-out PN counts may increase [90, 104, 105]. In addition, it was noted that increasing dosage rates beyond the onset of particle formation provides no additional decrease in soot emissions, but contributes to an increase in numbers of both large and small particles [105]. As particles are effectively trapped by the DPF, it is recommended that metallic additives be used only in conjunction with a DPF [104]. Another study showed that low dosage rates (4-8 ppm) of a bimetallic Pt/Ce FBC did not increase ultra-fine PM emissions but provided several benefits, including reducing total PM, HC, and CO emissions while increasing fuel economy [106]. This study also found that 94% of the additive was retained in the engine and exhaust components.

An evaluation of particulates was conducted by the European Commission to investigate the relationships and trade-offs between PM and PN during DPF regeneration. This work showed that PN emissions were two orders of magnitude higher during regeneration than during non-regeneration events [107]. Although gaseous and PM emissions during non-regenerating cycles were compliant with Euro 4 standards, much higher emissions were observed during regeneration events, with N[O.sub.x] emissions exceeding Euro 4 limits by 100%.

FBC Effects on Engine and Exhaust Components

The durability and other effects of FBCs on engine and exhaust components have not been widely published in recent literature, although limited investigations were conducted by Volkswagen in the 1980's to assess the effects of a manganese-based FBC used with a prototype exhaust treatment filter [108, 109]. Results showed that the Mn-FBC system had the potential to reduce PM emissions with no significant effect on gaseous emissions, fuel economy, or exhaust gas treatment systems.

Iron oxides have been shown to build up in combustion chambers over time, and deposit onto spark plugs in gasoline engines [13, 102]. However, an investigation by Solvay Rare Earth Systems showed that an advanced FBC consisting of nano-particles of Fe, formulated with a DCA could prevent nozzle coking deposits in indirect injection engines [110, 111]. Long-term usage of the FBC at treat rates of 4-7 ppm in a DPF application showed no increased engine deposits. In addition, this advanced Fe-FBC (2) was reported to be more stable than a conventional FBC in typical fuels. Other investigations of an iron-based additive, ferrous picrate, without a DPF showed that fuel economy and N[O.sub.x] emissions were decreased, while HC and CO emissions increased [113].

Some metallic additives have been shown to contribute to fuel injector fouling. Trace amounts of Zn can contribute significantly, while Pb contributes to a lesser extent [114]. Other metals, such as Ca, Cu, Na, and Fe were not found to contribute significantly to injector deposit formation.

Much of the literature evaluating various DPF-FBC combinations has focused on reductions of PM emissions, with some indicating effects on other pollutants or fuel consumption. Emissions have been shown to increase during regeneration events of fully loaded traps, particularly at high space velocities, although they generally remain below standards [90]. The use of metallic FBCs without a DPF has also been shown to increase particle emissions of metals [101, 115].

Smoke/Soot Suppressant

A literature search for additives used to suppress soot emissions in diesel exhaust returned a limited number of recent items. Metals including Ba, Ca, Fe, Mg, Mn, and Ni have been evaluated for soot suppression for similar reasons as their use in DPF regeneration, namely, their ability to reduce soot oxidation temperatures and enhance soot oxidation rates during combustion [116-118]. Additives including ferrocene, Ce, Mn and Ba have all seen use as a soot suppressant [119], although they do not appear to be widely available today. One barium-containing product, Lubrizol 565, may still be available [120]. Cerium-based soot suppressants were recently marketed by the OMB Group, but may no longer be available. In general, smoke suppressant additives do not appear to be widely utilized, as superior alternative technologies, such as DPF filters, are now commonly used [121].

More recent research and literature on these FBC additives in diesel are focused primarily on the relationship between N[O.sub.x] and soot formation for the purpose of investigating the "biodiesel N[O.sub.x] penalty" [120,122, 123]. It has been theorized that higher PM concentrations reduce combustion chamber temperatures due to radiative heat transfer, thus reducing the formation of thermal N[O.sub.x] [124]. Accordingly, it has been argued that the use of biodiesel or soot suppressant additives, both of which reduce PM, may result in increased N[O.sub.x]. Soot suppressants such as barium and ferrocene have been used in attempts to demonstrate this relationship, but with no clear effects on N[O.sub.x] formation [119, 120, 122, 123]. One study that used ferric chloride with biodiesel showed a decrease in N[O.sub.x] emissions accompanied by an increase in brake specific fuel consumption and C[O.sub.2] [125].

Effects of Additives in Diesel Systems

Literature on durability testing and specific effects of particular MFAs on diesel engine or exhaust components is limited. One study involving durability testing with Octimax 4808, an additive containing both strontium and ferrocene, reported no negative effects on vehicle components, although an increase in fuel consumption over the life ofthe vehicle was measured [97].

Another common operability problem with diesel engines arises from formation of injector deposits. In particular, diesel common rail fuel injectors are prone to coking or fouling, which can have a negative impact on engine performance, emissions and fuel consumption. Today's modern HPCR (high pressure common rail) fuel injection systems have very tight clearances and cannot tolerate even trace amounts of dirt, particulate or corrosion related particles [126]. Sophisticated injector designs and technologies, along with increasing levels of biodiesel blending can exacerbate coking severity. Deposits on diesel fuel injectors have been attributed to a variety of components, including sodium carboxylic salts, possibly resulting from degradation of polyisobutylene succinimide (PIBSI) detergent additives or from pipeline corrosion inhibitors [127-129]. Some MFAs have been shown to promote injector fouling. Trace amounts of zinc can contribute significantly to formation of deposits, which have a tendency to accumulate in spray-holes and contribute to nozzle coking. Other metals, including Ca, Cu, Na, and Fe do not appear to contribute significantly to deposit formation [114].

The WWFC reports that fuel- and lubricant-derived ash can contribute to coking on injector nozzles and may reduce the life of DPFs. This ash can be present in different forms (e.g. as suspended solids or fuel-soluble metallic soaps) and can originate from different sources (e.g. fuel and lubricant additives, metal wear, and water entrainment in the fuel) [130]. To address these problems, industry standards have been recommended to limit ash content in diesel fuel to less than 0.001%.

In 2008, the Coordinating European Council (CEC) approved a new test procedure based on the Peugot DW10B engine, which has a high pressure common rail (P = 1600 bar) and 6-hole piezoelectric prototype Siemens injectors of 110 microns [110, 131]. During the CEC-98-08 DW10B test, a trace amount of zinc salt (1 ppm zinc as zinc neodecanoate) is added to the test fuel to simulate high-fouling fuels, and engine power is measured over a 60-hour test cycle to determine the level of injector fouling. The test is widely accepted as a measure of base fuel and additive performance in modern direct injection common rail equipped vehicles. Some MFA producers have evaluated their products using this test cycle. An advanced Fe-based diesel FBC for DPF regeneration, along with a proprietary deposit control additive, was evaluated by Solvay Rare Earth Systems and Lubrizol to show that the additive (likely Lubrizol 9040 ZerO[TM]) did not contribute to injector fouling, and could even prevent nozzle coking deposits in direct injection common rail engines [110]. Other analysis of failed fuel injectors by Lubrizol showed that Ca and/or Na were always present in failed or sticking injectors, along with high levels of sulfur or chlorine in some cases [132].

The effects of ferrocene on heat release rates, N[O.sub.x] emissions, and PM emissions have been investigated using single-cylinder test engines of various configurations [133]. Testing incorporated a run-in period with a high additive treat rate of 250 ppm, followed by testing at 25 ppm treat rate. While changes in heat release patterns, N[O.sub.x] and PM emissions were observed, they were inconsistent between tests and instruments, and in some cases, too close to the detection limits to make strong conclusions.

The use of biodiesel has also been shown to increase the propensity for injector fouling, due primarily to contaminants and salts that remain from biodiesel production [134]. Use of sodium hydroxide or potassium hydroxide as a catalyst in the transesterification process to produce biodiesel can result in residual amounts of sodium or potassium in the finished biodiesel fuel [135]. In addition, calcium and magnesium may be used in the fuel purification process. These metallic fuel contaminants are converted to oxides, sulfates, hydroxides or carbonates in the combustion process to form inorganic ash that can be deposited onto the exhaust and emission control devices. These alkali and alkali earth metals have been shown to penetrate into heavy-duty diesel DOCs and SCR catalysts. Effects of this include reduced N[O.sub.x] conversion and degraded catalytic activity for HC and NO oxidation after 150,000 miles, as well as decreased thermal shock resistance of cordierite DPFs [136].

Sodium contamination can also occur from diesel tank water bottoms, where levels exceed 0.1% [126]. Other fuel contamination can occur during handling and storage of diesel fuels. Contamination by Na, Ca, and other metal cations can occur from sources such as sea water, refinery caustic neutralization, insufficient catalyst removal during biodiesel production, use of alkali metals for hydrogen removal during desulfurization, sodium-based corrosion inhibitors for pipeline protection, de-icing compounds such as sodium chloride and calcium chloride, used lubricating oils, or engine oil lubricated fuel pumps [126].

Fuel degradation during storage is promoted by the presence of certain metals such as Cu and Zn, which can lead to additional problems during use. Many fuel system contaminants including P, Ca, and Zn, can be found in lubricating oils [137]. Other metals can also lead to problems. For example, Pb is attacked by fuel acids and forms soap precipitates. Copper may catalytically accelerate fuel oxidation and promote deposition of solids. As a matter of good fuel handling practice, non-ferrous metals should be excluded from use in fuel pipes, storage tanks, and vehicle fuel systems [126].

Summary

A comprehensive literature search was conducted to investigate the use and effects of organometallic additives in transportation fuels. The objectives were to obtain, organize, summarize, and synthesize relevant information regarding the use of MFAs in market gasoline and diesel fuel, and the effects of such usage on vehicle engines and exhaust aftertreatment systems. The focus was on modern vehicles (post-2000), which are equipped with on-board diagnostic (OBD) systems and advanced emission control systems. This literature review was supported and expanded through recent worldwide fuel survey data purchased from SGS.

Information obtained from the literature review and fuel surveys do not provide significant evidence of widespread use of MFAs in either gasoline or diesel fuel. There is some indication that metals are occasionally incorporated into additive packages, however, the highly proprietary nature of the additive industry makes it challenging to determine the extent to which they are used, particularly at the refinery level. Currently, the most widely used MFAs incorporate manganese and iron compounds. The fuel surveys found significant levels of manganese in gasolines from 48 countries, with a single highest concentration of 66 mg Mn/L. Elevated levels of iron (5-25 mg Fe/L) were observed in gasolines from only 5 countries. Table 2 provides a summary of information regarding MFA usage in both gasoline and diesel, based upon literature review and fuel surveys.

Conclusions

MFAs do not appear to be widely used in current gasolines or diesel fuels. Web searches of specific additives indicate that MFAs are available in some locations, but it is not clear whether they are marketed to refiners or fuel blenders on a large scale. Although gasoline fuel survey information indicates that several metals are found in fuel samples around the world, none of the gasoline fuel samples collected in the U.S. or Canada contained measurable amounts of metals.

Historically, gasoline MFAs have included Pb, Fe, and Mn, all of which were used primarily as antiknock agents or octane enhancers. Other metallic compounds frequently found in gasoline (Ca, Zn, and Mo) are believed to originate mainly from lube oil packages and trace contaminants.

In recent years, Mn has been the most widely used metallic additive in both gasoline and diesel. Fuel survey data indicate that Mn is still being used in gasoline around the world. It is most frequently seen in Latin American, African, Middle Eastern and Asian countries, and is usually observed at levels that are permitted in these locations. In a few instances, Mn was seen at levels that exceed the maximum recommended dose of 33 mg Mn/L. The effects of Mn as MMT on vehicle systems and exhaust emissions have recently been thoroughly investigated and summarized. Mn in gasoline has been shown to deposit onto catalysts, reducing their exhaust conversion efficiencies and contributing to drivability problems [7].

Iron as ferrocene has been utilized broadly, both in gasoline (as an antiknock) and in diesel (as a fuel borne catalyst and/or soot suppressant). Its use can lead to iron oxide deposits, which have been shown to build up in combustion chambers and on emission control system components, resulting in field problems, particularly when combined with Mn. In SI engines, spark plug deposits can result in increased electrical conductivity and abnormal spark discharge, thereby contributing to catalyst plugging [17, 43]. However, in diesel engines, the use of ferrocene has been shown to reduce the occurrence of injector plugging and fouling under certain conditions [110]. Fuel survey information does not indicate widespread use of iron. Although its presence was detected in 75 gasoline samples collected within 32 countries, it was most frequently seen at concentrations below 1 mg/kg.

Lead in gasoline has been phased-out throughout most of the world, although it is still permissible in gasoline at concentrations up to 13 mg/kg. The effects of such low levels on engine and exhaust components have not been reported in recent literature. However, older literature has clearly documented the adverse effects of lead contamination on three way catalytic converters. Several additive producers list products containing TEL, and it remains available as an aftermarket additive in some areas for specialty vehicles and racing applications.

Potassium has been used as a lead-replacement in gasoline, and can act to protect valve seats. It is still recommended as an AVSR additive by the WWFC. However, VSR problems occur in a shrinking pool of older vehicles that were designed to use leaded fuels, and generally do not occur in modern vehicles with hardened valves. Therefore, nearly all AVSR additives are distributed as aftermarket products. Fuel survey data showed only 4 gasoline samples worldwide that contained any measurable amount of potassium, although it was seen in fairly high quantities in several samples collected in South Africa, where it may still be used as an AVSR additive.

Most ofthe literature regarding MFAs was found on the topic of fuel borne catalysts (FBC) for DPF regeneration. These FBCs are primarily considered aftermarket additives, as their broad use in vehicles without DPF technologies could result in increased emissions. FBCs most commonly include Ce and Fe, although other metals (including nanoparticles) have been evaluated. Several commercial products exist, although none appears to be widely used, as other methods of DPF regeneration now dominate.

The World Wide Fuel Charter recommends against the use of any ash-forming additives, including MMT and ferrocene [13]. However, fuel survey data provide evidence of contamination of silicon and small amounts of Zn in a significant fraction of fuel samples.

Disclaimer

Specific names of companies and products are mentioned in this paper. These names are taken from published literature, company websites, and other publically-available information. Use of these names does not constitute endorsement of any company or product.

Contact Information

S. Kent Hoekman

Desert Research Institute

2215 Raggio Parkway

Reno, NV 89512, USA

kent.hoekman@dri.edu

Acknowledgments

This work was funded by the Coordinating Research Council (CRC), as part of CRC Project No. E-114-2. We gratefully acknowledge the assistance of the CRC Technical Panel for their guidance and review.

Definitions/Abbreviations

AAM - Alliance of Automobile Manufacturers

AKI - Anti-Knock Index

ASTM - American Society for Testing and Materials

ATC - Technical Committee of Petroleum Additive Manufacturers in Europe

AVSR - Anti-Valve Seat Recession

CARB - California Air Resources Board

CEC - Coordinating European Council

CFR - Code of Federal Regulations

CONCAWE - CONservation of Clean Air and Water in Europe

CRC - Coordinating Research Council

CVMA - Canadian Vehicle Manufacturers Association

DCA - Deposit Control Additive

DI - Direct Injection

DOC - Diesel Oxidation Catalyst

DOE - Department of Energy

DPF - Diesel Particulate Filter

EGR - Exhaust Gas Recirculation

EPA - Environmental Protection Agency

FAME - Fatty Acid Methyl Esters

FBC - Fuel-Borne Catalyst

GRPE - Working Party on Pollution and Energy

HEI - Health Effects Institute

HPCR - High Pressure Common Rail

ICP - Inductively Coupled Plasma

JRC - Joint Research Council

LEV - Low Emission Vehicle

LNT - Lean NOx Trap

LRP - Lead Replacement Petrol

MECA - Manufacturers of Emission Controls Association

MFA - Metallic Fuel Additive

MIL - Malfunction Indicator Light

MMT - Methylcyclopentadienyl Manganese Tricarbonyl

NCWM - National Council on Weights and Measures

NEDC - New European Drive Cycle

NLEV - National Low Emission Vehicle

NM - Not Measured

OEM - Original Equipment Manufacturer

ON - Octane Number

PEA - Polyether Amine

PIBA - Poly Iso-Butyl Amine

PIBSI - Poly Iso-Butylene Succinimide

PM - Particulate Matter

PN - Particle Number

SAE - Society of Automotive Engineers

SCR - Selective Catalytic Reduction

SI - Spark Ignition

TEL - Tetraethyl Lead

UNECE - United Nations Economic Commission for Europe

UNEP - United Nations Environment Program

VERT - Verification of Emissions Reductions Technologies

VSR - Valve Seat Recession

WOS - Web of Science

WWFC - World Wide Fuel Charter

XRF - X-Ray Fluorescence

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S.Kent Hoekman, Desert Research Institute

Amber Leland, Coordinating Research Council

History

Received: 12 Oct 2017

Revised: 12 Dec 2017

Accepted: 14 Dec 2017

e-Available: 18 Apr 2018

Keywords

Metallic fuel additives (MFA), organometallics, gasoline, diesel fuel, fuel surveys, regenerative DPF

Citation

Hoekman, S.K. and Leland, A., "Literature Review on the Effects of Organometallic Fuel Additives in Gasoline and Diesel Fuels," SAE Int. J. Fuels Lubr. 11(1):2018, doi:10.4271/04-11-01-0005.

(1) EPA implemented 40 CFR 80.161 - the Detergent Additive Program, in 1997, which mandates that additives must be certified for use. To become certified, the program requires a fuel injector deposit control test and an intake valve deposit control test. A list of EPA-certified additives can be found on their website, although supporting information is not available and is maintained as confidential. Based on personal communication with an EPA representative, no metallic additives are allowed in certified additives.

(2) The advanced Fe-FBC in the article was referenced as Eolys Powerflex, a Rhodia product. Solvay Rare Earth Systems, the primary author's affiliation, purchased Rhodia in 2011 [112].
TABLE 1 Summary of metals content from SGS fuel survey.

Element  Summer 2015(784 samples)     Winter 2015/2016 (794 samples)
         Min,   Max,   Mean,  Sample  Min,   Max,  Mean,  Sample
         ppm    ppm    ppm    count   ppm    ppm   ppm    count

Cu       0.1     2.8    0.5     15     0.1    0.8   0.4     11
Fe       0.1    25      3.3     55     0.1   16     3.2     20
K        0.4     5.5    2.2      3     8.2    8.2   8.2      1
Mn       0.1    72     11.7    129     0.1   91     9.8    138
Na       0.1     0.6    0.4      2     0.9    1.3   1.1      6
P        1.1     1.1    1.1      1     0.5    1.1   0.8      2
Si       0.1    58.5    2.8     41     0.1   12     1.2     70
Zn       0.2     1.7    0.4     47     0.2    2.2   0.6     24

TABLE 2 Use of organometallic additives in gasoline and diesel.

                         Gasoline
                                           Fuel survey
     Anti-knock   AVSR   Other             Sample count   Range of conc.
                                                          (mg/kg) (a,b)

Al                                          0              --
B                                           6             0.1, 0.1, 0.2
Ba                                          0             --
Ce                                          -             NM
Cr                       Antistatic         0             --
Cu                                         26             0.1, 0.5, 2.8
Fe   X            X                        75             0.1, 3.3, 25
K                 X      Fuel stabilizer    4             0.4, 3.7, 8.2
Mg                X                         6             0.1, 0.2, 0.4
Mn   X                                    267             0.1, 11, 91
Na                X                         8             0.1, 0.9, 1.3
Pb   X            X      Lubricant          -             NM
Pd                                          -             NM
Zr                                          -             NM

     Diesel
     Soot          FBC for   Other
     suppressant   DPF

Al                           Lubricant/catalyst
B                            Lubricant
Ba   X
Ce   X             X         Antioxidant
Cr
Cu                 X
Fe   X             X         Demulsifer, corrosion
                             inhibitor
K                            Stabilizer
Mg                           Lubricant
Mn                 X         Lubricant
Na
Pb
Pd                           Antifoam
Zr                           Lubricant

(a) NM = not measured.
(b) Min., mean, and max. Concentrations reported
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Article Details
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Author:Hoekman, S. Kent; Leland, Amber
Publication:SAE International Journal of Fuels and Lubricants
Article Type:Technical report
Date:Feb 1, 2018
Words:15377
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