SOOT

Introduction

Soot is an extremely carbonaceous by-product formed because of partial burning in fuel-rich environments (Turns et al., 2000) operated under high temperature and by buildup of vapors from fuel which are depleted of oxygen due to inappropriate amalgamation or mixing (Crua et al., 2003). This definition itself makes it clear that soot is a prominent problem in diesel engine than gasoline engine. In diesel engine, there is a spontaneous ignition of fuel and air due to high pressure in combustion chamber resulting in fuel dense pockets producing soot. Most of the soot moves out to atmosphere through the exhaust while rest remains in the oil as engine soot. With growing attention towards environmental concerns for air pollution along with advances in heavy duty transportation technology requiring extended intervals for oil draining and better lubrication, study for better characteristics of engine and exhaust soot is highly desirable.

Structure of Soot

Information about size and structure of soot is beneficial and very important for environment as well as engine maintenance. Physical structure and surface chemistry of soot is largely dependent upon fuel composition. Agglomerates (range up to 0.5mm) of small, spherical particles with diameter about 0.02 mm make structure of soot. Besides carbon, hydrogen, oxygen, sulfur, phosphorous, zinc and calcium are normally present in soot (Rounds, 1981). Density of soot is researched to be in the range of 1.8 – 2.0 g/cm³ (Tree et al., 2007). Research has indicated that soot agglomerates more than 0.03 mm contaminating lubricant oil is one of the prominent reason for engine wear. Size and circulation of soot particulate influences the growth stage from nucleation to coagulation, agglomeration, oxidation and transportation to engine liner and lastly mixing in engine oil.

Soot in Engine Oil

A number of factors are responsible for presence of excessive soot and its emission from diesel engine. Soot moves into the oil on cylinder wall layers by the scraping action of the piston rings under complex fluid motion (Dahlen, 2002). Extreme idle periods, use of worn out piston rings, injectors with poor fuel spray configuration for atomization and clogged air filters providing improper mixture of air and fuel and turbulent conditions are some the important factors. Use of high top rings and delay in fuel injection timing and boosting EGR (Exhaust gas recirculation) ratio also increases soot in engine oil (McGeehan, 1991). Greeves et al., has given a conceptual model detailing soot formation process in diesel engine. As soot concentration builds up with, their interaction with engine and additive also raises up. A number of researchers have studied the presence of soot and their consequent effects in engine oil.  Excessive soot ultimately forming sludge, attaches to engine surface, retards oil movement through engine and filter resulting in reduced lubrication. Lubricant oils generally contain base oil and various additives such as viscosity improvers, detergents and anti wear agents. Soot changes the chemical properties of engine lubricating oil resulting in increased viscosity of engine oils effecting fuel injection timing and composition (Covitch et al., 1985) causing pumpability problems, thus leading to engine wear. Presence of soot changes oil quality thus limiting oil change service interval.

Soot-Induced Engine Wear Mechanism

Considering mechanical and chemical surface damage, engine wear is classified into 5 categories: Wear due to abrasion, Adhesive Wear, Fatigue, Corrosion and Lubricant breakdown. A number of friction test have been conducted to propose wear mechanism of soot particle in engine (Narita et al., 1997). It is indicated by research that soot with its abrasive action is the key wear mechanism in diesel engines (Gautam et al., 1999) (Jao, 2006) (Kim et al., 1992). Round was first to propose that wear mechanism was a possible combination of the antiwear film removal and adsorption of ZDP by soot (Rounds, 1987). Average wear is higher with soot contamination than without soot contamination. Diesel soot reduces the oil’s anti-wear properties; presumably by three body abrasive wear mechanism involving piston, cylinder and soot in between them (Gautam et al., 1999). Under limited oxygen and high temperature, high concentration of soot produces a transition from anti-wear Fe3O4 to prowear FeO (Corso et al., 1984). In general, the soot particle size and soot concentration has direct influence on engine wear (Mainwaring, 1997). Oil supply is limited by accumulation of soot at entrance of wear surface causing metal-metal contact (Yoshida et al., 1990) (Colacicco et al., 1995). Though researches are still being done regarding soot induced wear mechanism; considering surface interaction of soot with engine components and reduction of additives due to chemical reactions, five wear effects of soot on engine has been established since the early 1970s.

·         Soot acting as abrasive adsorb ZDP (Zinc dithiophosphate) decomposition products thereby reducing the antiwear protection on metal surfaces, leading to increased metal to metal contact (Rounds, 1978).

·         Soot accumulates on metal surface reducing coverage area for ZDP (Berbezier et al., 1986).

·         Soot weakens the antiwear film’s mechanical strength and adherence property to the metal surface.

·         Soot agglomeration brings pumpability problems.

·         Soot agglomerates leads to abrasive wear.

Maximum abrasive wear occurs at the Top Dead Center (TDC) and the Bottom Dead Center (BDC) positions of the engine duty cycle. When engine works under low speed and high load during startup, shutdown and high torque conditions, boundary lubrication permits contact of soot with engine surface resulting in wear which results in accumulation of metal particle debris due to chain reaction of wear. Valve-bridges and fuel injector adjusting screws are most prone to abrasive wear as they work under boundary lubrication conditions (McGeehan et al., 1991). Under high soot presence, grooves of piston rings can accumulate large amount of carbon resulting in oil seal deterioration between the piston rings and cylinder liner leading to abrasion. The space amid the rings and liner increases due to successive abrasion inviting large amount of combustion byproducts into the crankcase. Eventually, it leads to loss of engine horsepower and fuel efficiency with reduction in the cylinder compression and deteriorating ability of expanding to push the piston down.

Conclusion

It is now established by research that high concentration of engine soot can lead to excessive levels of wear in engine and its parts, bears, chains and piston liners. Since soot concentration in engine oil is unavoidable, all engine lubricants must be must be formulated to mitigate this harmful effect as far as possible.

References

1.      Berbezier, I., Martin, J.M., Kapsa, Ph.: The role of carbon in lubricated mild wear. Tribol. Int. 19, 115–122 (1986)

2.      Colacicco, P., Mazuyer, D.: The role of soot aggregation on the lubrication of diesel-engines. Tribol. Trans. 38, 959–965 (1995)

3.      Corso, S., Adamo, R.: The effect of diesel soot on reactivity of oil additives and valve train materials. SAE Paper 841369 (1984).

4.      Covitch M.J., Humphrey B.K. and Ripple U.E.: SAE Paper 852126 (1985).

5.      Crua, C., Kennaird, D.A. and Heikal, M.R., Laser-induced incandescence study of diesel soot formation in a rapid compression machine at elevated pressures. Combustion and Flame, 2003. 135: p. 475-488.

6.      Dahlen, L., On Applied CFD and Model Development in Combustion Systems Development for DI Diesel Engines: Prediction of Soot Mediated Oil Thickening. 2002, Royal Institute of Technology: Stockholm.

7.      Gautam, M., Chitoor, K., Durbha, M., Summers, J.C.: Effect of soot contaminated oil on engine wear–investigation of novel oil formulations. Tribol. Int. 32, 687–699 (1999).

8.      Jao, T.-C., Li, S., Yatasumi, K., Chen, S.J., Csontos, A.A., Howe, J.M.: Soot characterisation and diesel engine wear. Lubr. Sci. 16, 111–126 (2004).

9.      Kim, C., Passut, C.A. and Zang, D.M., Relationships among Oil Composition Combustion-Generated Soot, and Diesel Engine Valve Train Wear. SAE Papers, 1992(922199).

10.  Mainwaring, R.: Soot and wear in heavy duty diesel engines. SAE Paper 971631 (1997)

11.  McGeehan J.A., Alexander W., Ziemer J.N., Roby S.H., Graham J.P. (1999) ‘The pivotal role of crankcase oil in preventing soot wear and extending filter life in low emission diesel engines. 12th Int. Colloquium: “Tribology 2000-plus”, 11.-13. January 2000, Volume I,, p. 425-449

12.  McGeehan J.A., Runterford J.A. and Couch M.C.: SAE Paper 912342 (1991).

13.  Narita, K.: The effects of diesel soot on engine oil performance. Jpn J. Tribol. 42, 677–683 (1997)

14.  Rounds FG. Soots from used diesel engine oils — their effects on wear as measured in 4-ball wear tests. SAE 810499, 1981.

15.  Rounds, F.G.: Effect of lubricant additives on the prowear characteristics of synthetic diesel soot. Lubr. Eng. 43, 273–282 (1987).

16.  Tree, D.R. and Svensson, K.I., Soot processes in compression ignition engines. Progress in Energy and Combustion Science, 2007. 33: p. 272-309.

17.  Turns, S.R., An Introduction to Combustion: Concepts and Applications. 2nd ed. 2000, Singapore: McGraw-Hill. p. 550.

18.  Yoshida, K.: Effects of sliding speed and temperature on tribological behavior with oils containing a polymer additive or soot. Tribol. Trans. 33, 221–228 (1990).