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Tribological Evaluation of low Sulfur Automotive Diesel in the. Presence of ... The California Air Resources Board (CARB) complies ..... College, 1985. 18. Wei ...
Tribological Evaluation of low Sulfur Automotive Diesel in the Presence of Specific Types of Acid Derivatives G. ANASTOPOULOS, D. KARONIS, E. LOIS, S. KALLIGEROS, F. ZANNIKOS Laboratory of Fuels Technology and Lubricants, National Technical University of Athens, Greece

Summary This paper assesses the impact of three acetoacetates and seventeen esters of mono and di-carboxylic acids, on the lubrication properties of low sulfur diesel fuels. All types of acid derivatives were dissolved in the base fuels at five different concentrations of 50, 100, 500, 750 and 1000 ppm. Tribological experiments were carried out on the HFRR test rig, according to the CEC F-06-A-96 method.The obtained wear results showed that the addition of mono and dicarboxylic acid esters at concentrations 500 ppm to 750 ppm resulted in significantly decreasing the wear scar diameter values of the base fuels. Concentration levels of additives that were below 500 ppm had no effect on the fuel lubricity, while those higher than 750 ppm did not lead to any significant increase in lubricity. In the case of acetoacetates only two of the three acetoacetic esters used, provided satisfactory mean wear scar diameter (WS 1.4) of less than 460 μm, at the concentration level of 750 ppm or higher.

density, lower polyaromatic hydrocarbons (PAH), lower 95 % boiling point and higher cetane number (14).

1. INTRODUCTION Governmental regulations mandating lower pollutant emissions from diesel powered vehicles are forcing significant changes both in engine design and in diesel fuel. Major reductions in emissions can be achieved by modifying the engines, or through the use of devices such as oxidation catalysts, particulate filters and by employing techniques such as exhaust gas recirculation. Diesel fuel reformulation, with emphasis on low sulfur content, can also improve the air quality and will be used extensively in the present vehicle population (1-3).

These modifications to diesel fuel quality have been achieved by increasing the use of refining processes such as hydrotreating or hydrocracking. These processes reduce sulfur content and can also reduce aromatic content. They generally improve other fuel quality parameters such as ignition quality, colour and thermal stability. However these processes also tend to reduce the lubricating properties of the fuel. In other words, the desulfurization treatment minimizes polyaromatics (triand above) and polar compounds. Polyaromatics and polar compounds such as oxygen and nitrogen containing compounds were recently known to enhance fuel lubricity (15-18).

The 1993 U.S. EPA regulations had limited the sulfur content of highway diesel fuel to less than 500 ppm[4]. The California Air Resources Board (CARB) complies with EPA regulations and has limited the aromatic content of the diesel fuel to less than 10% by volume, effective since October 1993 (5-9). In 1991, Sweden introduced, even more strict sulfur and aromatics specifications, limiting the sulfur content to less than 10 ppm (by mass), 5% by volume aromatics for Class 1 type fuel and 50 ppm (by mass) sulfur, 20% by volume aromatics for Class 2 type fuel (10-13).

Fuel lubricity can be enhanced by lubricity additives. A moderate dosage of chemically suitable additive is beneficial in most cases but if the dosage is too high, some common diesel – fuel additives can cause fuel injector deposits, water separation problems, or premature filter plugging. These problems are not always identified in the standard fuel specification tests, and result in field problems (13, 14).

All countries which subscribe to the European Diesel Specification EN 590 produce diesel with sulfur content below 350 ppm, while the agendas of fuel specifications for the year 2005 limit the sulfur content to 50ppm. The complete diesel specifications for the year 2005 that were finalized at the end of 1999 will also require lower

This study includes the evaluation of the lubricating properties of low sulfur diesel fuels, additized with three types of acetoacetic esters and seventeen esters of mono and di-carboxylic acids. Data were generated to identify the minimum concentration of the above

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additives, which provide lubricity improvement down to the 460 microns wear scar diameter level. The value of 460 microns was proposed by the European Committee for Standardization (CEN) in February 1997, and generally adopted by the industry, as the minimum required for an acceptable field performance (23).

the mean wear scar diameter normalized to a standard vapor pressure of 1.4 KPa. The repeatability was calculated using the following equation (32): R = 139 - (0.1648X WS 1.4) Seventeen esters of mono and di-carboxylic acids and three acetoacetates were used as lubricating additives in the present work. Table 2 presents nomenclature, chemical structure, molecular weigth and purity of the applied acid derivatives.

In recent years fatty acid methyl esters (FAME), commonly known as biodiesel, have successfully been used as diesel fuel lubricity improvers (24-26). Wei and Spikes considered, that the significant wear reduction was produced by oxygen compounds with phenolic – type or carboxylic acid groups and occurred at a concentration of just a few parts per million (27-29). On the other hand the use of oxygen containing fuels, such as esters, assists in the reduction of particulate matter emissions. More generally, it has been mentioned in the literature that the oxygen – carbon ratio [O/C] of a fuel significantly affects particulate emissions; so to achieve low smoke emissions (lower than 0.5 in the Bosch range), the O/C ratio must be higher than 0.2 (30).

Ethyl acetoacetate and several mono and di-carboxylic acid esters were obtained from Aldrich Chemical Company and they were used as received, without further purification. Hexyl and octyl acetoacetates were prepared through the reaction of the corresponding acetic esters with sodium hydride (Claisen ester condensation) (33), while the esters of mono and dicarboxylic acids that were not commercially available, were prepared by reacting chlorides of carboxylic acids with alcohols. The alcohols and small excesses of triethylamine were dissolved in toluene, followed by the gradual addition of 1.2 mol equivalent of the acid chlorides (when preparing esters of dicarboxylic acids, a 0.7 mol equiv. of the corresponding dichloride was used) with continuous stirring and cooling. The mixtures were stirred at room temperature for 24 h. Afterward, they were washed with ice cold water, HCI 0.01 N and 5% aqueous sodium bicarbonate. The organic phases were dried over anhydrous sodium sulfate and finally the residues were distilled on a vacuum evaporator to receive the final products, whose properties were similar to those reported in the relevant literature.

Although the lubricating efficiency of fatty acid methyl esters has been closely examined, the impact of adding acetoacetaes and esters of mono and di-carboxylic acids has not been examined in detail. A previous study in our laboratory showed that the mono and di-carboxylic acid esters are two interesting categories of diesel fuel extenders, as they incorporate very good cetane rating with satisfactory cold flow performance, and they fulfill the rest of the diesel fuel specifications (31). 2. EXPERIMENTAL PROCEDURE 2.1. Test Fuels In order to assess the impact of the selected acid derivatives on the lubrication properties of low sulfur automotive diesel, three fuels that comprised distillates of the hydrodesulfurization process were obtained by a Greek refinery and were used for all the tribological experiments as the base fuels. The properties of the three test fuels are presented in Table 1. The base fuels have low sulfur content, according to the specification EN - 590. The cetane number of the fuels lay in the range 42 – 60, the density at 15 °C ranged from 841 to 861 kg/m3, the kinematic viscosity varied between 2.9 – 4.5 cSt at 40 °C, and the final boiling points of used fuels lay in the range 344 – 375 °C.

The three acetoacetates were added in the fuel I, while the esters of monocarboxylic acids were added in the fuel II. Finally dicarboxylic esters were added in the fuel III.All types of the acid derivatives were dissolved in the base fuels at five different concentrations of 50, 100, 500, 750 and 1000 ppm. 3. RESULTS AND DISCUSSION The base fuels initially analyzed to determine their lubrication effectiveness. Figure 1 illustrates the corrected WSD values of the 3 base fuels, on the first day of its production. It is evident that all the fuels appear to have wear scar diameter values over the acceptable limit of 460 μm and automatically were characterized as fuels with poor lubricating properties. Repeated tribological measurements at the next day confirmed this conclusion. Consequently, these fuels are well suited to determine the response of nitrogen and oxygen compounds on their lubrication properties.

2.2. Applied Test Procedures All tribological measurements were carried out by using the HFR2 test procedure, according to CEC F-06-A-96. The test temperature was 60 °C, and the volume of fuel sample used was 2 ml. The relative humidity was kept between 50 – 55 %, while the mean ambient temperature in the laboratory was approximately constant at 23 °C. The lubricating efficiency of the fuels was estimated by measuring the average wear scar diameter (WSD) of the spherical segment by using a photomicroscope. The wear scars quoted are corrected to give 1.4 WS values. The HFRR WS 1.4 parameter is

Figure 2 gives, graphically, the impact of the addition of the thee acetoacetates on the lubrication properties of fuel I. On the basis of the HFRR test results, acetoacetic ester A1, did not appear to be suited to increase the lubricity of the fuel to an acceptable level. Although the wear scar diameter decreased from 530 to 469 microns

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at the concentration range 50 – 750 ppm, it did not reach the desired limit value of 460 microns. A higher dosage of the same ester, caused the increase of the wear scar diameter value. In contrast, the addition of acetoacetates A2 and A3, improved the fuel lubricity. At a concentration of 750 ppm, the maximum permissible HFRR mean wear scar diameter of 460 microns, required for commercial diesel fuels was satisfied.

acetoacetic esters concentration, leads to an insignificant increase of the lubrication effectiveness. c.

Figure 3 gives the impact of the ten fatty acid esters on the lubrication properties of fuel II. It is obvious that, all mono - carboxylic esters improve the lubricity of the base fuel, since a treat rate of 500 ppm or more, led to a mean wear scar diameter (WS1.4) of less than 460 microns. Small concentrations of the fatty acid esters, between 50 – 100 ppm, were not sufficient to set the (WS1.4) value well within the required limit. Figure 4 shows the effect of dicarboxylic acid esters on the lubrication properties of base fuel III. It is evident that, in order to improve the lubrication properties of low sulfur diesel fuels, small concentration levels of dicarboxylic esters ranging from 500-750 ppm, were necessary to bring the wear scar diameter value within the required limit, and any extra addition of dicarboxylic acid esters, did not give any significant improvement in the lubricity of the fuels.

5. REFERENCES

Although, all types of the di-carboxylic esters tested in this series of experiments, had a beneficial impact on the lubricity of conventional low sulfur automotive diesel, an interesting conclusion is derived from the comparison of diesters produced from the same di-carboxylic acid. Among the diesters derived from the same di-carboxylic acid, i.e. the adipate and azelate series, an increase of the chain length of the alcohol involved in the esterification reaction, namely from diethyl to dibutyl and dioctyl alcohol, leads to a higher lubrication performance; conversely, if the chain length of the alcohol is kept constant, an increase in dicarboxylic acid chain length does not cause significant improvement in lubricity. 4.

CONCLUSIONS

In an effort to investigate the impact of acetoacetic esters and di-carboxylic acid esters on the tribological properties of low sulfur diesel fuels, three alkyl acetoacetates and seven dicarboxylic esters were added to low sulfur fuels. The following conclusions can be drawn from the work performed in the present study: a.

b.

In the case of the dicarboxylic acid esters, small concentration levels of the esters ranging from 500-750 ppm, were necessary to bring the wear scar diameter value within the required limit of 460 microns, and any extra addition of di-carboxylic acid esters, did not give any significant improvement in the lubricity of the fuels. Among the diesters derived from the same di-carboxylic acid, an increase of the chain length of the alcohol involved in the esterification reaction, leads to a higher lubrication performance; conversely, if the chain length of the alcohol is kept constant, an increase in dicarboxylic acid chain length does not cause significant improvement in lubricity.

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8. All mono - carboxylic esters improve the lubricity of the base fuel, since a treat rate of 500 ppm or more, leads to a mean wear scar diameter (WS1.4) of less than 460 microns.

Nikanjam, M.: Development of the First CARB Certified California Alternative Diesel Fuel. SAE Technical Paper Series 930728.

9.

Two of the three acetoacetic esters used, i.e the ones derived from the heavier hexyl and octyl alcohols, provide satisfactory HFRR mean wear scar diameter (WS 1.4) of less than 460 microns, at the concentration level of 750 ppm. The concentration levels below 750 ppm had no effect on the fuel lubricity. Any increase in

Nikanjam, M.; Henderson, P.T.: Lubricity of Low Aromatics Diesel Fuels. SAE Technical Paper Series 920825.

10. Rantanen, L.; Mikkonen, S.; Nylund, L.; Kociba, P.; Lappi, M.; Nylund, N.O.: Effect of Fuel on the Regulated, Unregulated and Mutagenic Emissions of DI Diesel Engines. SAE Technical Paper Series 932686.

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22. Nikanjam, M.; Diesel Fuel Lubricity Additive Study. SAE Technical Paper Series 942014.

11. Mikkonen, S.; Rantanen, L.; Alve, V.M.; Nylund, L.; Kociba, P.; Korhonen, K.; Lindroos, L.: Effect of Diesel Fuel Composition on Fork – Lift truck Emissions. SAE Technical Paper Series 952365.

23. European Commitie for Standardization (CEN): Specification Automotive Diesel. 14th Meeting, 1997, Vienna Austria.

12. Rantanen, L.; Mikkonen, S.; Niemi, M.; Nylund, L.; Kociba, P.; Suhonen; S.: Effect of Fuel and Oxidation Catalyst on Diesel Vehicle Emissions. 10th World Clean Air Congress, Espoo, May 28 – June 2, 1995, Proceedings, vol. 1, section 015.

24. Hertz, P.B.; Winter Engine Wear Comparisons with a Canola Biodiesel Fuel Blend. Saskatchewan Canola Commission Report, 1995. 25. Galbraith, R.M.; Hertz, P.B.: The Rocle Test for Diesel and Biodiesel Fuel Lubricity. SAE Technical Paper Series 972862.

13. Tucker, R.F.; Stradling, R.J.; Wolveridge, P.E.; Rivers, K.J.; Unbbens, A.: The Lubricity of Deeply Hydrogenated Diesel Fuels – The Swedish Experience. SAE Technical Paper Series 942016.

26. Karonis, D.; Anastopoulos; G.; Lois; E.; Stournas, S.; Zannikos, F.; Serdari, A.: Assessment of the Lubricity of Greek Road Diesel and the Effect of the Addition of Specific Types of Biodiesel. SAE Technical Paper Series 1999-01-1471.

14. Octane Week: XII (15), April 14 1997. 15. Meyer, K.; Stolz, U.; Rehbein, P.A.: Tribological Approach to Determine the Friction and Wear Properties of New Environmentally Benign Diesel Fuels in Conjunction with Wear Mechanisms in Critical Parts of Diesel Injection Equipment. 21st Leeds-Lyon Symposium on Tribology ‘Lubricants and Lubrication, 1994, Leeds, UK.

27. Wie, D.P.: The Lubricity of fuels I. Wear Studies on Diesel Fuel Components. Acta Petrolei Sinica, 2 (1986), 79-87. 28. Wie, D.P.: The Lubricity of fuels II. Wear Studies Using Model Compounds. Acta Petrolei Sinica, 4 (1990), 90-99.

16. Spikes, H.A.; Wei, D.P.: Fuel Lubricity – Fundamentals and Review. 1st International Colloquium, Fuels, January 16 – 17, 1997, Esslingen.

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Table 1: Fuel Properties Fuel code 3

Density (kg/m , 15 °C) Viscosity (cSt at 40 °C) Pour point (°C) Cloud point (°C) CFFP (°C) Flash point (°C) Cetane number Cetane index Sulfur % m/m Distillation (°C) IBP 10% 50% 90% FBP

code A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 A13 A14 A15 A16 A17 A18 A19 A20

I

II

III

Test method

861 2.77 -16 -14 -15 71 42 42.9 0.025

841 2.9 -20 -14 -16 93 52.2 51.6 0.045

843 4.5 -5 4 -1 73 60.3 59.8 0.028

ASTM D-1298 ASTM D-445 ASTM D-97 ASTM D-2500 IP 309 ASTM D93 DIN 51773 ASTM D 4337 IP 336 ASTM D-86

198 227 265 313 344

208 235 263 327 350

226 260 299 352 375

Table 2: Characteristics of the esters Chemical Molecular Nomenclature stucture weight ehtyl acetoacetate C6H10O3 130 hexyl acetoacetate C10H18O3 186 octyl acetoacetate C12H22O3 214 Ethyl myristate C16H32O2 256 Ethyl palmitate C18H36O2 284 Ethyl oleate C20H38O2 310 butyl caprate C14H28O2 228 hexyl laurate C15H36O2 248 octyl myristate C22H44O2 340 propyl palmitate C19H38O2 298 octyl palmitate C24H44O2 364 ethyl Stearate C20H40O2 312 hexyl oleate C24H46O2 366 dibutyl adipate C14H26O4 258 dioctyl adipate C22H42O4 370 diethyl azelate C13H24O4 244 dibutyl azelate C17H32O4 300 dioctyl azelate C25H48O4 412 diethyl sebacate C14H26O4 258 bis(2-ethyl-hexyl) sebacate C26H50O4 426

389

Purity (%) 98 98 97 97 98 99 98 99 99

Initial measurement Repeated measurement 1 day after production

580 566

565

559 550

WS 1.4 (μm)

550 534

535

530 522

520 505 490 I

II

III

Fuel code

Figure 1: Tribological properties of the base fuels.

580 540

WS 1.4 (μm)

500 460 420 380

A1-ethyl acetoacetate A2-hexyl acetoacetate A3-octyl acetoacetate

340 300

0

50

100

500

750

1000

A1

530

511

492

479

469

497

A2

530

503

485

471

451

449

A3

530

515

497

476

426

415

Concentration (ppm) Figure 2: Impact of the acetoacetates on the lubrication properties of fuel I.

390

550

500

450

WS 1.4 (μm)

400

350 A4-ethyl myristate A5-ethyl palmitate A6-ethyl oleate A7-butyl caprate A8-hexyl laurate A9-octyl myristate A10-propyl palmitate A11-octyl palmitate A12-ethyl stearate A13-hexyl oleate

300

250

200

150

100

0

50

100

500

750

1000

A4

534

520

480

411

367

349

A5

534

507

476

345

292

224

A6

534

485

472

308

258

221

A7

534

519

483

402

375

357

A8

534

507

472

388

344

318

A9

534

480

465

291

246

203

A10

534

496

475

342

281

217

A11

534

482

462

301

259

212

A12

534

491

470

333

274

228

A13

534

479

463

279

215

198

Concentration (ppm) Figure 3: Impact of monocarboxylic acid esters on the lubrication properties of fuel I.

391

580

540

WS 1.4 (μm)

500

460

420

A14-dibutyl adipate A15-dioctyl adipate A16-diethyl azelate A17-dibutyl azelate A18-dioctyl azelate A19-diethyl sebacate A20-bis(2-ethyl-hexyl) sebacate

380

340

300

0

50

100

500

750

1000

A14

566

524

492

467

428

391

A15

566

494

467

392

353

330

A16

566

540

522

486

457

442

A17

566

527

496

466

423

386

A18

566

490

465

395

351

339

A19

566

536

509

480

452

437

A20

566

495

472

443

416

381

Concentration (ppm) Figure 4: Impact of dicarboxylic acid esters on the lubrication properties of fuel III.

392