Reduction of Lubricant Oil Soot Loading

FIELD OF THE INVENTION

The invention relates to the reduction of soot loading in engine lubricant.

BACKGROUND OF THE INVENTION

In contrast with the performance observed in modern gasoline engines, the lubricating oil used in Compression Ignition (Cl) engines might be severely affected by soot loading in the lubricating oil originating from the combustion of the fuel in the cylinder chamber. This negatively affects the lubricity characteristics of the lubricant oil and, consequently, is indeed a restriction on the length of the lubricating oil drain interval. Moreover, while the drained oil can be recycled, there is an ecological incentive to maximise the use of a product like lubricating oil which is made up of base oils and complex and expensive chemical products that work as additives.

Therefore, there is a clear incentive to investigate means and ways to extend the lubricating oil drain interval in Cl engines both because of economic and ecological considerations.

DESCRIPTION OF THE INVENTION

In this specification, the term "soot loading" in the context of lubricating oil is the mass percentage of soot in the lubricating oil.

A "Soot Index" has been developed by Wearcheck Africa and available from their web site (www.wearcheck.co.za) as Technical Bulletin No.26 "We are ready for more soot" by J. S. Evans and Neil Robinson (2003) It uses Fourier Transform Infrared (FTIR) technology to determine an absorption value. The soot percentage is then derived through an empirical correlation.

While the invention is supported from extensive experimental results obtained using heavy duty engines, the inventors believe that it should also be of benefit with smaller Cl engines like those used in passenger vehicles and stationary applications.

FT products cover a broad range of hydrocarbons from methane to species with molecular masses above 1400; including mainly paraffinic hydrocarbons and much smaller quantities of other species such as olefins and oxygenates. Such a diesel fuel could be used on its own or in blends to improve the quality of other diesel fuels not meeting the current and/or proposed, more stringent fuel quality and environmental specifications.

The Low Temperature FT (LTFT) process has been described extensively in the technical literature, for example in Fischer Tropsch Technology, edited by AP Steynberg and M Dry and published in the series Studies in Surface Science and Catalysis (v.152) by Elsevier (2004). Some of its process features had been disclosed in, for example: US 5,599,849, US 5,844,006, US 6,201 ,031 , US 6,265,452 and US 6,462,098, all teaching on a "Process for producing liquid and, optionally, gaseous products from gaseous reactants".

Surprisingly the inventors found that it is possible to reduce the soot loading in the lubricating oil of Cl engines by using a highly paraffinic synthetic fuel derived from a LTFT process.

This invention is achievable using neat LTFT diesel fuel, or blends of LTFT and conventional diesel fuels.

Thus, according to a first aspect of the invention, there is provided for use of a low aromatics synthetic LTFT fuel in a Cl engine, said use resulting in a reduction of the soot loading of a Cl engine lubricating oil by at least 30% mass relative to crude derived Cl engine fuels when the engine is operated over a travelled distance of from 4 000 km and up to at least 15 000 km under normal working conditions.

Typically, the reduction in soot loading of the lubricating oil is at least 35 mass%, or even at least 40 mass%.

According to a second aspect of the invention, there is provided a method for reducing the soot loading of lubricating oil of a Cl engine relative to a Cl engine operated using a crude derived fuel, said method including combusting a low aromatics LTFT fuel under Cl engine operating conditions thereby to obtain a soot loading of at least 30 mass% less over a distance of at least 4000 km than that expected under similar operating conditions when combusting a crude derived fuel.

LTFT fuels usable in Cl engines have a density at 15°C below 0.79 kg/I, a distillation T10 temperature below 225°C (method ASTM D86), a cetane number above 74 (method ASTM D613), and a non detectable total aromatics content as measured with method IP 391/95, say below 0.01% (mass).

A typical set of physical properties for a LTFT fuel is presented in Table 1.

Conventional petroleum diesel fuels usable in Cl engines have a density at 15 ⁰ C above 0.81 kg/I, a distillation T10 temperature below 225°C (method ASTM D86), a cetane number below 60 (method ASTM D613), and a total aromatics content above 10% (mass) or even higher when measured with method IP 391/95. These conventional fuels typically report a di-aromatics contents above 3% (mass) or even higher when measured with method IP 391/95.

The crude derived diesel fuel may be one that conforms with an advanced fuel specification like EN590.

A typical set of physical properties for a EN590 compliant diesel fuel is presented in Table !

Table 1 Fuel Properties

DESCRIPTION OF EXAMPLES OF THE INVENTION

EXAMPLES

All examples refer to a detailed test program carried out under the following guidelines:

• Twenty commercial bus units. • Test length was 15 000 km in normal commercial bus routes in between lubricant oil changes. These tests were at least duplicated. The estimated total distance travelled during the test program was about 750 000 km.

• Samples of lubricant oil were extracted and analysed for soot levels using the following intervals: 3 750, 7 500, 9 375, 11 250, 13 125 and 15 000 km. Obviously the real distance intervals was slightly different but within experimental reproducibility.

• Twenty-sets of results were produced using LTFT diesel fuel - fuel properties listed in table 1. This fuel had a density at 15°C of 0.7732 kg/I, a distillation T10 temperature of 225°C (method ASTM D86), a 81.0 cetane number (method

ASTM D613), and a non detectable total aromatics content as measured with method IP 391/95.

Thirty-one sets of results were produced using EN590 diesel fuel - fuel properties listed in table 1. This fuel had a density at 15°C of 0.8311 kg/I, a distillation T10 temperature of 195°C (method ASTM D86), a 55.5 cetane number (method

ASTM D613), and a 25.31% (mass) total aromatics content as measured with method IP 391/95.

The lubricating oil used in all tests was a high performance commercial diesel multigrade lubricating oil 1 15W40.

The reported soot loading in the lubricating oil is the mass percentage of soot in the lubricating oil and is based on a "Soot Index" developed by Wearcheck Africa and available from their web site ( www.wearcheck.co.za ) as Technical Bulletin

No.26 "We are ready for more soot" by J. S. Evans and Neil Robinson (2003) It uses Fourier Transform Infrared (FTIR) technology to determine an absorption value. The soot percentage is then derived through an empirical correlation.

The results obtained using this methodology are summarised in table 2 for those tests completed using the LTFT fuel, and in table 3 for those done using the

EN590 diesel.

Example 1 Average Results for the tests completed using LTFT fuel

The test results shown in table 2 were statistically grouped and analysed. The results of the linear regression model show an excellent fit as indicated by the linear regression coefficients R ² , always being greater than 0.999.

These weighted results are also presented in the figure 1 below, showing the results from the linear regression analysis.

3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.00

0 2000 4000 6000 8000 10000 12000 14000 16000

Figure 1 - Linear Regression Plot for the Weighted test results for LTFT fuel

Example 2 Average Results for the tests completed using EN590 fuel The test results shown in table 3 were statistically grouped and analysed. The results of the linear regression model show an excellent fit as indicated by the linear regression coefficients R ² , always being greater than 0.989.

These weighted results are also presented in the figure 2 below, showing the results from the linear regression analysis.

5.00 4.50 4.00 3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.00

2000 4000 6000 8000 10000 12000 14000 16000

Figure 2 - Linear Regression Plot for the Weighted test results for conventional petroleum diesel

Example 3 Comparison of Weighted Results The Weighted Results for the Soot Load developed for examples 1 and 2 were compared for similar Nominal Distances. The results presented below show a

consistent difference in favour of the LTFT fuel when compared with the performance of the EN590 diesel. This calculated difference is always above 40%.

The weighted averaged results for the soot loading, as measured from the engine tests are compared in the following figure 3. It is evident that the use of the synthetic fuel resulted in significantly lower loading results and, consequently, better engine performance.

2000 4000 6000 8000 10000 12000 14000 16000

Distance (km)

LTFT Diesel EN590 Diesel

Figure 3 - Comparison of Weighted Test Results for LTFT fuel and conventional petroleum diesel (EN590)

The same results can also be compared in a relative way as shown in the figure 4 below where the performance of the LTFT fuel is always resulting in at least 40% mass lower soot loads compared with the same measured performance of similar Cl engines using EN590 conventional diesel.

3750 7500 9375 11250 13125 15000

Distance (km)

Figure 4 - Difference in Soot Load in Lubricating Oil - Weighted Average Results

Table 2 Test Results using LTFT Diesel Test Series A

Table 2 (continuation)

Test Series B

Table 3 Test results using EN590 Diesel Test Series A

Table 3 (continuation)

Test series B

Bus# 513 514 515 517 520 522 ¹ 524 525

Nom km km soot % km Soot % km Soot % km Soot % km Soot % km Soot % km Soot % km Soot %

0.2 159 0.2 217 0.3 226 0.2 256 0.2 14 0.3 10 0.1 791 0.6

3750 3610 1.6 4261 1.5 4364 1.3 3944 1.0 3941 1.2 3663 1.3 3988 1.1 3918 2.3

7500 7558 3.0 7636 2.4 7619 1.8 7579 2.2 7549 2.0 7621 2.3 7496 17 7558 43

9375 9375 3.6 9468 3.0 9516 2.2 9614 2.7 9398 2.1 9403 2.2 9348 2.2 9202 4.9

11250 11368 4.2 11145 3.3 11215 2.5 11228 3.1 11317 2.7 11289 2.6 11096 5.1

13125 13052 4.9 13274 3.4 13079 3.2 13179 3.9 13117 2.9 13246 3.1 13168 6.2

15000 15436 5.6 14953 4.2 15242 3.6 15361 5.3 15090 33 15245 3.6 14711 6.5

Bus# 526 527 528

Nom km km Soot % km soot % km Soot %

0 80 0.5 56 0.3 80 0.0

3750 3747 2.1 3632 1.6 3748 0.9

7500 7559 4.2 7382 2.9 7497 1.6

9375 9426 4.4 9294 3.6 9307 1.8

11250 11473 5.5 11229 4.4 11469 2.3

13125 12313 59 13034 44 13292 28

15000