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Abrasive concentration effects on wear under reciprocating conditions
Wear, 140 (1990)
359-367
Abrasive concentration conditions*
359
effects on wear under reciprocating
Ronald A. Mayville Arthur D. Little Inc., Cambridge, MA 02140 (U.S.A.)
Abstract Experiments were conducted to investigate wear under abrasive conditions present at the piston ring-cylinder liner interface in coal-fueled diesel engines. A reciprocating apparatus was designed and fabricated to study the effects on wear of ash content in coal and of abrasive concentration in oil. The wear rate was found to be a linear function of ash concentration regardless of whether the coal had been combusted or not. A linear dependence of wear rate on abrasive concentration in oil was also observed for both coal and pure silica. These effects have important implications on wear mitigation and research methodologies for coal-fueled engines.
1. Introduction
Development of coal-fueled diesel engine technology is under way in several laboratories throughout the U.S.A. The previous large effort in this area that took place in the 1930s and 1940s in Germany was based on the use of dry coal powder as the fuel [ 11. Today, coal-water shnry is the fuel of greatest interest because of easier injection and lower risk of unplanned combustion; the latter problem plagued Rudolph Diesel in his attempts to use coal. Regardless of the fuel form, successful operation of these engines is seriously hampered by the wear caused in such components as the piston rings. Much effort is being expended to mitigate the wear problems. Hard materials are the primary defense being pursued but other solutions, such as geometric changes to reduce pressure and special lubrication, may also be needed if practical implementation of this technology is to be achieved. The key to the selection of materials and alternative design solutions is an understanding of the wear mechanisms and the parameters on which material loss depends. Three-body abrasion is the most likely source of piston ring and cylinder liner wear for the coal-fueled engine. The abrasive medium in this case is probably combusted and perhaps uncombusted coal that lands on the cylinder liner, which is also coated with a thin layer of lubricant. In this scenario, a f&n of coal-oil slurry forms the lubricating film between *Paper presented at the International Conference on Wear of Materials, Denver, CO, U.S.A., April 8-14, 1989.
0043-1648/90/$3.50
Q Elsevier Sequoia/Printed in The Netherlands
360
ring and liner over most of if not ail the piston stroke. The dependence of wear on the characteristics of the coal-oil slurry is useful ~fo~a~on for the design and testing of wear-reducing solutions. The subject of this paper is the experimental investigation of the effects of abrasive concentration in oil and of ash concentration in coal on threebody abrasive wear under reciprocating conditions.
2. Experiments 2.1. Apparatus The apparatus developed for this investigation utilizes a configuration in which the ring specimen is stationary and the liner specimen moves. A sketch and a photograph of the mixture are shown in Fig. 1. Reciprocating motion is achieved by a horizontal crank mechanism, which is driven by a 1.5 hp d.c. motor. The motor is tied to a large block of concrete, bolted to the floor to prevent displacements from the unbalanced shaft loading. A straight ahuninum plate, to which the liner specimen is attached, rides on a brass block forming a simple slider bearing that is generously lubricated
TOP
counterweight
Load _
Tonearm
0
Ring Specimen
1 a/o-
‘16
Abrasive
I
Slurry Liner Specimen
6
r----------i Slider
I I, I I I 1
Block
SIDE
I
4I, II
I
I
FRONT
Fig.
1. Sketch (not to scale) and photograph of the reciprocating-wear apparatus.
with a mixture of SAE 1OW oil and MO&. The mechanism operates with a stroke of 125 mm (5 in) up to speeds of 1000 rev min-‘. The ring specimen is held in a “tone-arm” device whose axis is parallel to the reciprocating motion. The load is applied to the ring specimen through a ball-bearing situated on the head of the tone-arm; this minimizes the transfer of tangential load to the ring specimen owing to small longitudinal arm displacements. A lever is used to apply the normal load at a ratio of 5:l with lead shot used as the dead weight. Abrasive is dripped onto one side of the moving liner specimen as a liquid-solid particle shnry. The mixture is fed continuously by a peristaltic pump at a relatively constant rate of 0.6 ml min- ‘. The slurry is kept in a reservoir that is constantly stirred to prevent settling and a change in particle concentration.
2.2. Matevials AU specimens, ring and Liner, were fabricated from gray cast iron with a hardness of 225 kg mn-‘, type A graphite and a free ferrite content of less than 3%. The density of the cast iron was 7.2 g cmm3. The Linerspecimen has dimensions 150 mmX 12.5 mmx3.2 mm and the ring specimen 12.5 mm X 12.5 mm X 3.2 mm. The contact area between ring and liner specimens is 12.5 mmx 3.2 mm. Surfaces were finished by grinding and then running in the apparatus with a diIute siIica_oiI abrasive shrrry. Surface roughness was not measured. Two types of abrasive powder were used in this investigation: coal and pure silica. CoaI was tested in both its combusted and its tmcombusted form. It was eastern Kentucky bituminous coaI in alI cases with a mean particle size of 10-20 pm. The ash content ranged from 0.6% to 25% depending on the amount by which it was cleaned and whether it had been combated or not. Chemical and microscopy analysis showed sihca (quartz) to be the major hard constituent of the ash [2]. The pure silica powder also had a mean particle size of 20 pm. The hardnesses of the abrasives were measured using a procedure developed by Funs and Rabinowicz [ 3 J. The method utilizes the change in relationship between wear rate and hardness that occurs once the hardness of the workpiece exceeds 0.8 times the hardness of the abrasive. Several metals, each with a different hardness, are loaded equally in a wheel polishing device in which the abrasive of interest mixed with a lubricant acts as the polishing medium. If I&, is the metal hardness at which the mass loss-metal hardness curve changes slope, then the abrasive hardness is taken as Ha =HJ 0.8. The results of these tests (Table 1) show that the hardnesses of the coal and silica are substantially higher than the cast iron ring and liner hardness, which is 225 kgf mmm2. The SAE 1OW oil with which aII abrasive powders were mixed had a viscosity of 100 cSt at 21 “C. 2.3. Test procedure All experiments were performed in laboratory air at a speed of 500 rev min-’ and a normaI load of 44.4 N (10 lbf). This corresponds to a ring specimen face pressure of 1100 kPa (160 Ibf ine2) and an average surface speed of 2.1 m s-i (6.9 ft s-l). A typical stationary power, i.e. the diesel TABLE 1 Hardnesses of abrasives
Abrasive
Hardness t&f mm-*>
4% asbqnbumed coal 11% ash, burned coal Pure silica
500 400 660
363
engine brake mean engine pressure and the average ring surface speed are 1000 kPa (150 lbf in-“) and 6 m s-’ (20 ft s-‘) respectively. Test durations ranged from 6 to 8 h and the mass loss measurements were made every 1 or 2 h. Before their masses were determined, both ring and liner specimens were thoroughly cleaned with a dry white paper towel until there was absolutely no evidence of slurry or wear debris. The mass loss was found to be nearly linear with time and there was no perceptible incubation period for the conditions used in these experiments. The ring and liner specimens showed approximately the same mass losses and only the ring mass loss is reported here. Two series of experiments were performed. (1) The effect of abrasive powder concentration in oil (concentrations ranging from 0 to 20 wt.%) were investigated for both 4% ash, uncombusted coal and pure silica. (2) The effect of ash content in coal (ash concentrations ranging from 0 to 25%) were investigated. 3. Results The results show a linear relationship between wear rate and both abrasive concentration in oil and ash concentration in coal. Table 2 lists the data for wear rate at four abrasive concentrations in the SAE 1OW oil. Silica causes nearly 100 times more wear than does the 4% ash coal for all concentrations. Figure 2 shows the normalized ring specimen wear rate as a function of abrasive concentration in the oil for 4% ash, uncombusted coal and pure silica. It should be noted that the data coincide at the 5 wt.% value at which they were normalized. Further experimental work is under way to characterize wear at higher abrasive concentrations. Table 3 shows the wear rates obtained for different coals and pure silica when tested at a 5 wt.% concentration in oil. The pure silica is observed to be more abrasive by a factor of 10 than the coal with the highest ash content. Included for reference in the table are the wear coefficients k defined by the Archard equation in the form TABLE 2 Wear rates at various concentrations of abrasive in oil
cmentratti
Wear rate (mg h-l)
(wt.%) 5 10 15 20
4% ash coal
Pure silica
0.8 1.4 3.1
75.1 105.0 207.5 301.5
364
A 6-
5w 2 n
P/’
4-
2 w
/o /
I &
3-
,Eii’
i
/ /
2
2-
E z
/; l-
/o//
/-
/ I
0 0
ASRASI,“,
CONCE&iATION
I
I
I
I,’IOWO!L%+ %)
.
25
2. Normalized ring specimen wear rate (normalized a function of abrasive concentration in oil.
Fig.
TABLE
by rate at a concentration
of 5%) as
3
Wear rates for various abrasives at a 5 wt.% abrasive concentration
in oil
Abrasive
Wear rate 0% h-9
k
None 0.6% ash, uncombusted coal 4% ash, uncombusted coal 11% ash, combusted coal 22% ash, uncombusted coal 25% ash, combusted coal Pure silica
0.1 0.2 0.8 2.5 5.2 7.7 75.1
8.0X 1.6x 6.4x 2.0x 4.2 x 6.2 x 6.0x
Y’his value was obtained at a concentration
10-s 1O-7” lo-’ 1o-6 lo-’ lo-’ lo-’
of 20 wt.% in oil and divided by 4.
where W is the wear volume, N the normal load, x the sliding distance and H the specimen hardness. Figure 3 is a plot of the ring specimen wear rate as a function of ash content in coal and shows that wear rate is also a linear function of ash content regardless of whether the coal has been combusted or not. 4. Discussion The results of this investigation show the existence of a linear relationship between three-body abrasive wear rate and both coal ash content and abrasive
365
0
Uncambusted
I3 Cornbusted
COAL
ASH
Cool Coal
CONTENT
(%I
Ring specimen wear rate (at a cod concentration in oil of 5%) as a function of coal ash content.
Fig.
3.
concentration in oil under reciprocating displacement. The data also cover wider ranges of these parameters than studied in previous research. Levy et al. [4] found a linear relationship between mass loss and the weight percentage of silica and alumina in the ash for coals with approximately 25% ash content. Their experiments were performed in a $1~ pot wear test and the coal was mixed at a 30 wt.% concentration with kerosene. Severd studies have also revealed a linear variation in wear with abrasive concentration in oil [5-S]. However, the highest concentration in these investigations was only 1 wt.%. Data gathered in this study on the rela~ons~p between wear and coal ash content are consistent with the physical characteristics of the coals. Although the exact species of the ash that cause wear have not yet been unambiguously identified, preliminary research shows that the sixes and phases of the hard ash particles are unaltered by the coal-cleaning processes used to reduce ash content or by the stationary power diesel engine combustion process [9]. An exception is the transformation of a small percentage of the ash into glassy spherical particles. Thus the effect of fewer cleaning steps on coal combustion is to cause a higher concentration of the same size and form of abrasive ash. Since the wear rate is observed to increase linearly with abrasive concentration, the wear rate is also expected to increase linearly with ash content. The pure sihca results are also consistent with the coal ash data. Ash in the Kentucky bituminous coal consists of aproximately 30% silica particles. Thus a 25% ash coal has actually only 8% silica. On the basis of the linear relationship between wear rate and abrasive concen~a~on, one would expect a factor of 12 (= l/0.08) difference in wear rate between 25% ash coal and
pure silica. The difference measured in the experiments in this study (Table 3) is a factor of 10. This is reasonably close considering the probable diBerence in silica particle sizes between the two samples and the presence of other abrasive species in the ash. The linear dependence of wear rate on ash content implies that more efficient combustion, which results in a higher coal burn-out rate and a correspondingly higher ash content in the exhaust particulate, will result in higher wear of rings and liner. Higher ash fuels should have the same effect, since the exhaust particulate from such fuels is also observed to be higher in ash content [Z]. This is conkned by our own coal-fueled diesel engine tests [lo] and those of Clingenpeel et al, [ 111. An additional implication of the results is that experiments designed to simulate the coal-fueled diesel engine environment can be performed with more readily available high ash coal that is not combusted or a very dilute slung of pure silica in oil. The former is being used in small engine wear tests in which a 20% ash content coal is added to lubricating oil of a natural gas diesel engine; exhaust particulate from our coal-fueled diesel engine has an ash content that ranges from 10% to 20%. The linear re~tio~~p between wear rate and abrasive concen~at~on in oil can also be taken advantage of in reducing coal-fueled engine wear and in analyzing the abrasiveness of exhaust particulate. Although the concentration of abrasive on the cylinder walls has not yet been characterized, this study shows how its dilution will reduce wear. The possibility of practically increasing lubricant flow rates to dilute the abrasive is being investigated. l?inally, the linear relationship allows one to rank correctly the relative abrasiveness values of various exhaust particuIates at low concentrations in oil, which is very important when only small samples can be obtained from experimental engine runs. 6. Conclusions (1) Pure silica is approximately ten times more abrasive than a 25% ash content coal powder with comparable particle size, This is because less than only one half of the ash is as abrasive as pure silica. (2) The wear rate is a linear function of ash content in the coal, at least for conditions in which the coal is from the same source and has comparable particle sizes. (3) The linear rela~o~~p between wear rate and coal ash content does not depend on whether the coal has been combusted or not. (4) There is also a linear dependence of wear rate on abrasive concentration in oil for both coal and pure silica, at least up to 20% concentration. Acknowledgments This research was supported by the U.S. Department of Energy, Morgantown Energy Technology Center under Contract 21-85MC22182. The
367
author gratefully acknowledges the assistance of Messrs. Thomas Warren, William W&on and Arturo SanRoman in performing several of the experiments. The assistance of Professor E. Rabinowicz, Massachusetts Institute of Technology, in determining abrasive hardness is also acknowledged.
References 1 E. E. Soehngen, Development of coal-burning diesel engines in Germany, NTIS Rep. F!E/ WApo/3387-1, August 1976 (National Technical Information Service). 2 K. Benedek, Arthur D. Little Inc., Cambridge, MA, personal communication, December 1988. 3 E. Func and E. Rabinowics, Abrasiveness of fine grain particles in slurry machinery, Proc. 7th Tech. Co@ cm Slumy Transportation, Lake Tahoe, NV, 1982. 4 A. V. Levy, N. Jee and G. Sorell, Erosivity of coal particles in coal-solvent slurries, in J. E. Miller and F. E. Schmidt, Jr. (eds.), Slurry Em Uses, Applications and Test Methods, AS!f!iU Spec. Tech. PubL 946, 1987, pp. 62-77.
5 R. L. Mehan, The wear of selected materials in mineral oil containing a solid contaminant, Wear, 124 (1988) 65-85. 6 S. Odi-Cwei and B. J. Roylance, The effect of solid contaminants on the wear and critical failure load in a sliding lubricated contact, Wear, 212 (1986) 239-255. 7 J. A. Perrotto, R. R. Rio and S. F. Murray, Effect of abrasive contamination on ball bearing performance, Lt.&r. Eng., M(12) (1979) 698-705. 8 B. Fitzsimmons and H. D. Clevenger, Contaminated lubricants and tapered roller bearing wear, ASLE Trans., 20(2) (1977) 97-107. 9 Examination of particulate material from engine exhaust, Report to K. Bemdek, December 1988 (Battelle Columbus Laboratories). 10 A. K. Rao et aL, Cooper-Bessemer coal-fueled engine system-progress report, ASi%!? Energy Sources Technology Coqf , Houston, lX, Januam 1989. 11 J. M. Clingenpeel, M. D. Gurney and D. B. Eccleston, A combustion and wear analysis of a compression-ignition engine using coal slurry fuels, ASMZ Paper 84-DGP-8, 1984, pp. l-6 (American Society for Mechanical Engineers).
359-367
Abrasive concentration conditions*
359
effects on wear under reciprocating
Ronald A. Mayville Arthur D. Little Inc., Cambridge, MA 02140 (U.S.A.)
Abstract Experiments were conducted to investigate wear under abrasive conditions present at the piston ring-cylinder liner interface in coal-fueled diesel engines. A reciprocating apparatus was designed and fabricated to study the effects on wear of ash content in coal and of abrasive concentration in oil. The wear rate was found to be a linear function of ash concentration regardless of whether the coal had been combusted or not. A linear dependence of wear rate on abrasive concentration in oil was also observed for both coal and pure silica. These effects have important implications on wear mitigation and research methodologies for coal-fueled engines.
1. Introduction
Development of coal-fueled diesel engine technology is under way in several laboratories throughout the U.S.A. The previous large effort in this area that took place in the 1930s and 1940s in Germany was based on the use of dry coal powder as the fuel [ 11. Today, coal-water shnry is the fuel of greatest interest because of easier injection and lower risk of unplanned combustion; the latter problem plagued Rudolph Diesel in his attempts to use coal. Regardless of the fuel form, successful operation of these engines is seriously hampered by the wear caused in such components as the piston rings. Much effort is being expended to mitigate the wear problems. Hard materials are the primary defense being pursued but other solutions, such as geometric changes to reduce pressure and special lubrication, may also be needed if practical implementation of this technology is to be achieved. The key to the selection of materials and alternative design solutions is an understanding of the wear mechanisms and the parameters on which material loss depends. Three-body abrasion is the most likely source of piston ring and cylinder liner wear for the coal-fueled engine. The abrasive medium in this case is probably combusted and perhaps uncombusted coal that lands on the cylinder liner, which is also coated with a thin layer of lubricant. In this scenario, a f&n of coal-oil slurry forms the lubricating film between *Paper presented at the International Conference on Wear of Materials, Denver, CO, U.S.A., April 8-14, 1989.
0043-1648/90/$3.50
Q Elsevier Sequoia/Printed in The Netherlands
360
ring and liner over most of if not ail the piston stroke. The dependence of wear on the characteristics of the coal-oil slurry is useful ~fo~a~on for the design and testing of wear-reducing solutions. The subject of this paper is the experimental investigation of the effects of abrasive concentration in oil and of ash concentration in coal on threebody abrasive wear under reciprocating conditions.
2. Experiments 2.1. Apparatus The apparatus developed for this investigation utilizes a configuration in which the ring specimen is stationary and the liner specimen moves. A sketch and a photograph of the mixture are shown in Fig. 1. Reciprocating motion is achieved by a horizontal crank mechanism, which is driven by a 1.5 hp d.c. motor. The motor is tied to a large block of concrete, bolted to the floor to prevent displacements from the unbalanced shaft loading. A straight ahuninum plate, to which the liner specimen is attached, rides on a brass block forming a simple slider bearing that is generously lubricated
TOP
counterweight
Load _
Tonearm
0
Ring Specimen
1 a/o-
‘16
Abrasive
I
Slurry Liner Specimen
6
r----------i Slider
I I, I I I 1
Block
SIDE
I
4I, II
I
I
FRONT
Fig.
1. Sketch (not to scale) and photograph of the reciprocating-wear apparatus.
with a mixture of SAE 1OW oil and MO&. The mechanism operates with a stroke of 125 mm (5 in) up to speeds of 1000 rev min-‘. The ring specimen is held in a “tone-arm” device whose axis is parallel to the reciprocating motion. The load is applied to the ring specimen through a ball-bearing situated on the head of the tone-arm; this minimizes the transfer of tangential load to the ring specimen owing to small longitudinal arm displacements. A lever is used to apply the normal load at a ratio of 5:l with lead shot used as the dead weight. Abrasive is dripped onto one side of the moving liner specimen as a liquid-solid particle shnry. The mixture is fed continuously by a peristaltic pump at a relatively constant rate of 0.6 ml min- ‘. The slurry is kept in a reservoir that is constantly stirred to prevent settling and a change in particle concentration.
2.2. Matevials AU specimens, ring and Liner, were fabricated from gray cast iron with a hardness of 225 kg mn-‘, type A graphite and a free ferrite content of less than 3%. The density of the cast iron was 7.2 g cmm3. The Linerspecimen has dimensions 150 mmX 12.5 mmx3.2 mm and the ring specimen 12.5 mm X 12.5 mm X 3.2 mm. The contact area between ring and liner specimens is 12.5 mmx 3.2 mm. Surfaces were finished by grinding and then running in the apparatus with a diIute siIica_oiI abrasive shrrry. Surface roughness was not measured. Two types of abrasive powder were used in this investigation: coal and pure silica. CoaI was tested in both its combusted and its tmcombusted form. It was eastern Kentucky bituminous coaI in alI cases with a mean particle size of 10-20 pm. The ash content ranged from 0.6% to 25% depending on the amount by which it was cleaned and whether it had been combated or not. Chemical and microscopy analysis showed sihca (quartz) to be the major hard constituent of the ash [2]. The pure silica powder also had a mean particle size of 20 pm. The hardnesses of the abrasives were measured using a procedure developed by Funs and Rabinowicz [ 3 J. The method utilizes the change in relationship between wear rate and hardness that occurs once the hardness of the workpiece exceeds 0.8 times the hardness of the abrasive. Several metals, each with a different hardness, are loaded equally in a wheel polishing device in which the abrasive of interest mixed with a lubricant acts as the polishing medium. If I&, is the metal hardness at which the mass loss-metal hardness curve changes slope, then the abrasive hardness is taken as Ha =HJ 0.8. The results of these tests (Table 1) show that the hardnesses of the coal and silica are substantially higher than the cast iron ring and liner hardness, which is 225 kgf mmm2. The SAE 1OW oil with which aII abrasive powders were mixed had a viscosity of 100 cSt at 21 “C. 2.3. Test procedure All experiments were performed in laboratory air at a speed of 500 rev min-’ and a normaI load of 44.4 N (10 lbf). This corresponds to a ring specimen face pressure of 1100 kPa (160 Ibf ine2) and an average surface speed of 2.1 m s-i (6.9 ft s-l). A typical stationary power, i.e. the diesel TABLE 1 Hardnesses of abrasives
Abrasive
Hardness t&f mm-*>
4% asbqnbumed coal 11% ash, burned coal Pure silica
500 400 660
363
engine brake mean engine pressure and the average ring surface speed are 1000 kPa (150 lbf in-“) and 6 m s-’ (20 ft s-‘) respectively. Test durations ranged from 6 to 8 h and the mass loss measurements were made every 1 or 2 h. Before their masses were determined, both ring and liner specimens were thoroughly cleaned with a dry white paper towel until there was absolutely no evidence of slurry or wear debris. The mass loss was found to be nearly linear with time and there was no perceptible incubation period for the conditions used in these experiments. The ring and liner specimens showed approximately the same mass losses and only the ring mass loss is reported here. Two series of experiments were performed. (1) The effect of abrasive powder concentration in oil (concentrations ranging from 0 to 20 wt.%) were investigated for both 4% ash, uncombusted coal and pure silica. (2) The effect of ash content in coal (ash concentrations ranging from 0 to 25%) were investigated. 3. Results The results show a linear relationship between wear rate and both abrasive concentration in oil and ash concentration in coal. Table 2 lists the data for wear rate at four abrasive concentrations in the SAE 1OW oil. Silica causes nearly 100 times more wear than does the 4% ash coal for all concentrations. Figure 2 shows the normalized ring specimen wear rate as a function of abrasive concentration in the oil for 4% ash, uncombusted coal and pure silica. It should be noted that the data coincide at the 5 wt.% value at which they were normalized. Further experimental work is under way to characterize wear at higher abrasive concentrations. Table 3 shows the wear rates obtained for different coals and pure silica when tested at a 5 wt.% concentration in oil. The pure silica is observed to be more abrasive by a factor of 10 than the coal with the highest ash content. Included for reference in the table are the wear coefficients k defined by the Archard equation in the form TABLE 2 Wear rates at various concentrations of abrasive in oil
cmentratti
Wear rate (mg h-l)
(wt.%) 5 10 15 20
4% ash coal
Pure silica
0.8 1.4 3.1
75.1 105.0 207.5 301.5
364
A 6-
5w 2 n
P/’
4-
2 w
/o /
I &
3-
,Eii’
i
/ /
2
2-
E z
/; l-
/o//
/-
/ I
0 0
ASRASI,“,
CONCE&iATION
I
I
I
I,’IOWO!L%+ %)
.
25
2. Normalized ring specimen wear rate (normalized a function of abrasive concentration in oil.
Fig.
TABLE
by rate at a concentration
of 5%) as
3
Wear rates for various abrasives at a 5 wt.% abrasive concentration
in oil
Abrasive
Wear rate 0% h-9
k
None 0.6% ash, uncombusted coal 4% ash, uncombusted coal 11% ash, combusted coal 22% ash, uncombusted coal 25% ash, combusted coal Pure silica
0.1 0.2 0.8 2.5 5.2 7.7 75.1
8.0X 1.6x 6.4x 2.0x 4.2 x 6.2 x 6.0x
Y’his value was obtained at a concentration
10-s 1O-7” lo-’ 1o-6 lo-’ lo-’ lo-’
of 20 wt.% in oil and divided by 4.
where W is the wear volume, N the normal load, x the sliding distance and H the specimen hardness. Figure 3 is a plot of the ring specimen wear rate as a function of ash content in coal and shows that wear rate is also a linear function of ash content regardless of whether the coal has been combusted or not. 4. Discussion The results of this investigation show the existence of a linear relationship between three-body abrasive wear rate and both coal ash content and abrasive
365
0
Uncambusted
I3 Cornbusted
COAL
ASH
Cool Coal
CONTENT
(%I
Ring specimen wear rate (at a cod concentration in oil of 5%) as a function of coal ash content.
Fig.
3.
concentration in oil under reciprocating displacement. The data also cover wider ranges of these parameters than studied in previous research. Levy et al. [4] found a linear relationship between mass loss and the weight percentage of silica and alumina in the ash for coals with approximately 25% ash content. Their experiments were performed in a $1~ pot wear test and the coal was mixed at a 30 wt.% concentration with kerosene. Severd studies have also revealed a linear variation in wear with abrasive concentration in oil [5-S]. However, the highest concentration in these investigations was only 1 wt.%. Data gathered in this study on the rela~ons~p between wear and coal ash content are consistent with the physical characteristics of the coals. Although the exact species of the ash that cause wear have not yet been unambiguously identified, preliminary research shows that the sixes and phases of the hard ash particles are unaltered by the coal-cleaning processes used to reduce ash content or by the stationary power diesel engine combustion process [9]. An exception is the transformation of a small percentage of the ash into glassy spherical particles. Thus the effect of fewer cleaning steps on coal combustion is to cause a higher concentration of the same size and form of abrasive ash. Since the wear rate is observed to increase linearly with abrasive concentration, the wear rate is also expected to increase linearly with ash content. The pure sihca results are also consistent with the coal ash data. Ash in the Kentucky bituminous coal consists of aproximately 30% silica particles. Thus a 25% ash coal has actually only 8% silica. On the basis of the linear relationship between wear rate and abrasive concen~a~on, one would expect a factor of 12 (= l/0.08) difference in wear rate between 25% ash coal and
pure silica. The difference measured in the experiments in this study (Table 3) is a factor of 10. This is reasonably close considering the probable diBerence in silica particle sizes between the two samples and the presence of other abrasive species in the ash. The linear dependence of wear rate on ash content implies that more efficient combustion, which results in a higher coal burn-out rate and a correspondingly higher ash content in the exhaust particulate, will result in higher wear of rings and liner. Higher ash fuels should have the same effect, since the exhaust particulate from such fuels is also observed to be higher in ash content [Z]. This is conkned by our own coal-fueled diesel engine tests [lo] and those of Clingenpeel et al, [ 111. An additional implication of the results is that experiments designed to simulate the coal-fueled diesel engine environment can be performed with more readily available high ash coal that is not combusted or a very dilute slung of pure silica in oil. The former is being used in small engine wear tests in which a 20% ash content coal is added to lubricating oil of a natural gas diesel engine; exhaust particulate from our coal-fueled diesel engine has an ash content that ranges from 10% to 20%. The linear re~tio~~p between wear rate and abrasive concen~at~on in oil can also be taken advantage of in reducing coal-fueled engine wear and in analyzing the abrasiveness of exhaust particulate. Although the concentration of abrasive on the cylinder walls has not yet been characterized, this study shows how its dilution will reduce wear. The possibility of practically increasing lubricant flow rates to dilute the abrasive is being investigated. l?inally, the linear relationship allows one to rank correctly the relative abrasiveness values of various exhaust particuIates at low concentrations in oil, which is very important when only small samples can be obtained from experimental engine runs. 6. Conclusions (1) Pure silica is approximately ten times more abrasive than a 25% ash content coal powder with comparable particle size, This is because less than only one half of the ash is as abrasive as pure silica. (2) The wear rate is a linear function of ash content in the coal, at least for conditions in which the coal is from the same source and has comparable particle sizes. (3) The linear rela~o~~p between wear rate and coal ash content does not depend on whether the coal has been combusted or not. (4) There is also a linear dependence of wear rate on abrasive concentration in oil for both coal and pure silica, at least up to 20% concentration. Acknowledgments This research was supported by the U.S. Department of Energy, Morgantown Energy Technology Center under Contract 21-85MC22182. The
367
author gratefully acknowledges the assistance of Messrs. Thomas Warren, William W&on and Arturo SanRoman in performing several of the experiments. The assistance of Professor E. Rabinowicz, Massachusetts Institute of Technology, in determining abrasive hardness is also acknowledged.
References 1 E. E. Soehngen, Development of coal-burning diesel engines in Germany, NTIS Rep. F!E/ WApo/3387-1, August 1976 (National Technical Information Service). 2 K. Benedek, Arthur D. Little Inc., Cambridge, MA, personal communication, December 1988. 3 E. Func and E. Rabinowics, Abrasiveness of fine grain particles in slurry machinery, Proc. 7th Tech. Co@ cm Slumy Transportation, Lake Tahoe, NV, 1982. 4 A. V. Levy, N. Jee and G. Sorell, Erosivity of coal particles in coal-solvent slurries, in J. E. Miller and F. E. Schmidt, Jr. (eds.), Slurry Em Uses, Applications and Test Methods, AS!f!iU Spec. Tech. PubL 946, 1987, pp. 62-77.
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