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Tribology of Carbon Black Soot Particles in Engine Oil

May, 2021

Soot or carbon black (CB) is a carbon-based material that can contaminate the motor oils present in gasoline and diesel engines up to 0.1 wt. % and 8 wt.%, respectively. Soot in engine oil can increase the fuel consumption because of sludge formation and high viscosity of oil. Increase in fuel consumption will increase the carbon emission, which is not favorable to current environmental condition and emission standards.

Here we have used Ducom High Frequency Reciprocating Rig - HFRR 4.2 (see Figure 1) and scanning electron microscopy (SEM) to investigate the soot wear mechanism on fresh steel surfaces and preconditioned steel surfaces (see Figure 2). 

HFRR 4.2

Figure 1. Description of the Ducom HFRR 4.2 used in this study. 

MATERIALS. 0W20 fully formulated synthetic multigrade engine oil and carbon black (CB) as a surrogate for soot. The 0W20 with 5 wt. % of CB was prepared by stirring the CB particles in engine oil at 70°C for 30 minutes. The tribopair were standard EN52100 balls and disks for HFRR instrument (6 mm and 10 mm in diameter, respectively).

Table 1. Test parameters used in Ducom HFRR 4.2.

Table 1-2

TEST CASES.  The first case was when the steel disk and ball were heated in engine oil 0W20 before a wear test (see Fig. 2A). In the second case, 0W20 engine oil was contaminated with 5 wt. % CB, and it was labeled as “0W20 with CB” (see Fig. 2B). In the third case, the steel disk and ball were heated in engine oil (0W20) before a wear test in 0W20 with CB, and it was labeled as CB (0W20) (see Fig. 2C). Every wear test was scheduled for 180 min, and the temperature was fixed at 100°C. There were five different test cases in total. Here, we reported the first three cases. You can read the full article here.

Soot_Fig. 2-1

Figure 2. Illustration of the protocols for the following test cases: (A) engine oil 0W20, (B) 0W20 with CB: engine oil with 5 wt. % CB, (C) CB (0W20): 0W20 with CB tested on steel surface with chemisorption of ingredients from 0W20.


The ball wear scar image showed a flattened and polished surface after testing in 0W20 with CB (see Fig 3). The ball mean wear scar diameter for 0W20 with CB was approximately an order of magnitude higher compared to 0W20 (i.e., 873 µm and 312 µm, respectively).

Figure 3_soot-min

Figure 3. Wear of 0W20 and 0W20 with CB (an example of ball scar image). The wear values are average of two tests conducted for each case.

When the steel disk and ball were heated in 0W20 before a wear test in 0W20 with CB, severe grooves on the ball were observed for CB (0W20). This was in contrast with the smoother ball surface from 0W20 with CB (see Fig. 4). Ball mean wear scar diameter for CB (0W20) was about 8 % higher than 0W20 with CB (i.e., 939 µm and 873 µm, respectively).

Figure 4_soot-min

Figure 4. Wear for nonpreconditioned case 0W20 with CB and preconditioned CB(0W20)(an example of ball scar image). The wear values are average of two tests conducted for each case.

The magnitude and mechanism of wear were certainly influenced by the preconditioned state of steel surfaces. Higher wear and a rougher surface were exhibited on balls tested in CB (0W20) compared with nonpreconditioned surface 0W20 with CB.

A possible explanation is the interaction of zinc dialkyldithiophosphates (ZDDP) with the steel surfaces. ZDDP, the most popular polar additive used in engine oils, interacts with the steel surface to form antiwear phosphate films like zinc phosphate and iron phosphate and thus protects the steel during rubbing.

In general, chemisorption of ZDDP on steel surface prior to the wear test increased wear from CB, which was related to the removal of antiwear phosphate film because of the antagonistic reaction between ZDDP on steel surface and CB from the 0W20. Finally, HFRR preconditioning of steel surfaces can have a profound effect on the soot wear mechanism. Read the full article on ASTM International.

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