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The development of Ferrography as a laboratory wear measurement method for the study of engine operating conditions on diesel engine wear
Wear, 44 (1977) 183 - 199 0 Elsevier Sequoia S.A., Lausanne
183 - Printed
in the Netherlands
THE DEVELOPMENT OF FERROGRAPHY AS A LABORATORY WEAR MEASUREMENT METHOD FOR THE STUDY OF ENGINE OPERATING CONDITIONS ON DIESEL ENGINE WEAR
MICHAEL
V. HOFMAN
and JOHN H. JOHNSON
Mechanical Engineering-Engineering Mechanics Department, University, Houghton, Mich. (U.S.A.) (Received
Michigan Technological
April 20, 1977)
Summary Ferrographic oil analysis techniques were used in a laboratory study of diesel engine wear. Data were developed supporting the concept of using the Severity Index I, to rank the effect of engine operating conditions on wear. Results analyzing the Severity Index as a function of time and as a function of engine operating variables are presented. The Severity Index is also linearly correlated to spectrometric data (iron and lead concentrations in the used oil samples). Engine wear tends to increase with increase of either oil or coolant temperature. However, brake specific fuel consumption tends to decrease as oil and coolant temperatures increase, indicating a need for accurate temperature control for both mediums to minimize fuel consumption and wear. The heated Ferrogram analysis (HFA) technique was used to determine changes in the wear rates of specific engine parts with variation of the oil and coolant temperatures.
Introduction A diesel engine dynamometer test program was conducted to study the effects of engine operating conditions on fuel consumption and wear in order to separate the basic engine characteristics from the dynamic effect of the vehicle on the road. Provisions were made to control accurately the test cell environment and all important variables in the operation of a Cummins VT-903 Vee-type eight-cylinder direct-injection four-cycle diesel engine, turbocharged and non-aftercooled, with 20% torque rise. It was a well broken in engine with several hundred hours of running time prior to the wear tests reported. The full load performance curves, and descriptions of the instrumentation, controls and the test procedure are given elsewhere [ 11. The oil used for the wear tests was a typical diesel engine lubricant, Mobil Delvac 1330 (SAE no. 30), and the fuel used was an ASTM no. 2 diesel fuel oil of 39” API and 0.23% sulfur by mass.
184
The engine condition test matrix consisted of various combinations of the four independent operating variables: speed, load, inlet oil temperature, and outlet coolant temperature. Each test was run separately on new oil for 6 h at the specified engine operating condition. At the end of the 6 h period, an oil sample was taken for oil analysis purposes [ 1 ] . The engine oil was then changed according to previously reported procedure [ 11 and another test was started. The study was conducted to investigate the effects of the range of coolant temperatures and oil temperatures encountered in various climates and the effect of thermal environment on the wear of different surfaces inside a diesel engine. A schematic arrangement of the test set-up is shown in Fig. 1. Control of the coolant was achieved by using a temperature-controlled metering valve. The coolant temperature used for the tests was measured at the engine outlet, or position 1, as shown in Fig. 1. The oil and fuel were water cooled in tube-type heat exchangers. The inlet oil temperature used for the tests was measured after the oil was cooled (shown as position 2 in Fig. 1). The oil filter normally used during engine operation had been removed from the system because of its role as a particle removal mechanism. A sensitive method of wear analysis was desired to reveal slight changes in the wear rate and to predict from which surface the various wear particles originated. A number of analysis methods were used simultaneously in order to evaluate wear. The three primary wear measurement methods used were spectroscopy, radioactive tracer of the top compression ring and Ferrography. The portion of the results concerned with the development of Ferrography as a diesel engine wear measurement method is the primary subject of this paper. Ferrography is a technique which enables magnetic separation of wear debris from a lubricant sample. These wear particles are systematically arranged according to size on a glass substrate (Ferrogram) for microscopic
FROM WATER
CONTROL ~
VALVE.
- -.-
FRbM WATER SUPPLY TO i)RAIN
----~ CONTROL
FROM WATER ----SUPPLY
Fig. 1. Schematic
-
VALVE
-.~-J
arrangement
of the test set-up.
185
examination. A complete explanation of the instrumentation and techniques of Ferrographic oil analysis are given elsewhere [ 2 - 51. Ferrography has the significant advantage of being an extremely sensitive wear measurement method. Past applications of Ferrography have largely applied the technique to field failure detection analysis. This paper is directed at applying it to relatively short term (less than 10 h) performance tests of a diesel engine operated in the laboratory under relatively contaminate free conditions. The paper is divided into three main parts: (1) development of Ferrography for laboratory diesel engine wear analysis; (2) engine wear results as revealed by Ferrography; and (3) comparison of Ferrography, spectroscopy and radioactive tracer wear measurement results.
Ferrography
development
for diesel engine wear measurement
One of the techniques used to monitor the effect of a change in engine operating variables on wear employs the Ferrogram reader. The optical density of the deposit at different locations along the Ferrogram is measured to obtain an indication of the size distribution of particles found in the oil system. Previous investigations have shown that the operating condition of a machine can be determined by the study of the sizes, shapes, and relative quantities of these wear particles [3,4, 6 - 81. It has been shown that an indication of the severity of the system wear mode can be indicated by an empirical function, the Severity Index I,. This function is based on the relationship between the large particles (>5 pm) and the smaller particles (1 - 2 pm) using percentage area covered readings (optical density) obtained by the Ferrogram reader. It is a widely accepted concept that, as a machine tends to failure, both the total particle production rate and the difference in the production of the large particles to the small particles increase dramatically [4, 5,8, 91. The manner in which the particles are deposited on the Ferrogram lends itself favorably to the measurement of two quantities. The large particles appear primarily in the entry deposit near the oil entry location (approximately 56 mm) and the smaller particles (1 - 2 pm) appear further down the substrate at approximately the 50 mm position. A formula for the calculation of the Severity Index [12] is 1, = (AL + A,)(A,
-A,)
= A”L ---A;
where AL is the maximum percentage area covered reading of the entry deposit (~56 mm), and As is the maximum percentage area covered reading at 50 mm. The Severity Index can be seen to include both the number of total particles produced and the difference in the quantity of the large and small particles, each giving greater sensitivity to the formula and increasing its ability to be used as a tool for failure detection. The percentage area covered readings uersus Ferrogram distance for a number of engine operating
186
&RV
!lSTANCE
Fig. 2. Percentage conditions.
ON
FERROGRAM
area covered
Cmn:
as a function
of the Ferrogram
distance
for several engine
conditions are shown in Fig. 2. Ferrogram preparation procedure*, engine operating period on the lubricant (6 h) and all engine operational variables except the four independent variables listed were held constant. The Severity Index for the four operating conditions was also calculated, and the sensitivity of the formula in predicting wear rate changes for changes in operating conditions can be seen. Indications have been reported [3 - 5, 7, lo] that a machine operating in a normal manner reaches a state of equilibrium in which the particle loss rate equals the particle production rate. Any change in the wear rate causes the density of particles to increase or decrease. The particle population in an oil sample has been reported to be influenced by many factors such as oil consumption, inputs of fresh oil, period of time on the oil, oil filtration (if a filter is used) and the continuous grinding down mechanism of the large particles to smaller ones. Particles have also been seen to adhere to the internal walls of a machine. Other loss mechanisms may operate in specific cases. Owing to these dynamic interactions, the commonly used methods of spectroscopic oil analysis are not always an accurate indication of the true operating conditions of the machine [ 4,10 - 121. It is believed that, owing to the size selection possibilities found using Ferrography and the fact that the only particle sizes used in the calculation of the Severity Index are *In both these results and the results obtained for the oil and coolant temperature variation study (Figs. 2, 5 - 11) the standard Ferrogram preparation procedure of using 3 cm3 of oil and 1 cm3 of fixer is employed. These mixtures all had run periods of 3 min followed by a 10 min wash (fixer) period, The analysis with time results (Figs. 3, 4) had a slightly different Ferrogram preparation procedure using 1 cm3 of oil and 3 cm3 of fixer with a run period of 5 min and a wash (fixer) period of 10 min. Indications are that Ferrogram preparation procedure is a variable which should be controlled and kept constant in order to produce results which are comparable.
187
above 1 pm in size, the particle production rate/particle grinding mechanism/ oil filtration interactions approach some finite value with time for a fixed engine operating condition. Ideally, the Severity Index equation is then able to predict the wear severity by a finite rating of engine condition wear. Figure 3 shows the percentage area covered readings with time for data taken using the diesel engine test set-up; an equilibrium wear rate of the large particles of interest is established within the first 2 h of operation. The locus of maximum readings with time for each Ferrogram location indicates that a grinding mechanism is operative. A typical curve of the Severity Index uersus time relationship for the diesel engine test set-up is shown in Fig. 4. The hook at the 5 h point is hypothesized to be due to the interaction of wear particles in the wear interface. Because of the radioactive tracer wear test methods used during our investigation, the oil filtration portion of the lubrication system has been removed [ 11. The lack of proper filtration seemed to have an effect on the equilibrium particle production rate, increasing the severity of wear after a period of time. This seems to give an indication of the importance of proper oil filtration. A study is currently in progress on the interaction of filtration and filtration variables on the wear rates in the diesel engine test system. The overall shape of the Severity Index uersus time curve is a function of several processes. First, there is the rebreaking-in of the surface after starting with a fresh oil change and bringing the engine surfaces, oil system
. 0
I
10
I
I
20 TIME
30 (hours)
I
I
I
40
50
60
Fig. 3. Percentage area covered as a function of the test time. Fig. 4. The Severity Index as a function of the test time.
TIME
(hours)
188
and coolant system up to operating temperatures. The Severity Index increases rapidly between 0 and 1.5 h because of the higher rate of wear during rebreak-in and the fact that the oil has an insignificant number of wear particles at zero time. It was also desirable to determine which part or parts were contributing the wear particles as well as to determine the effects of operating variable changes on wear rates of the various components. For this analysis, a technique designated heated Ferrogram analysis (HFA) was applied. Basic knowledge of the various components in the engine and the tendency of the different materials to take on different thicknesses of oxide layers upon heating at various temperatures was applied. Table 1 gives a list of the various wear surfaces in the test engine, listed in order of importance of engine wear for the engine. It has been established that heating the Ferrogram* for 90 s at 625 “F (~330 “C) turns low carbon and/or alloy steels blue, cast iron brown but does not affect lead. Heating of the Ferrogram to 700 “F (=370 “C) for a further 90s changes low carbon and/or alloy steels to a lighter or silver blue, cast iron to dark brown with slight dark blue areas. Lead remains unaffected by heating. These effects can be seen in Fig. 5. TABLE
1
VT-903 wear surfaces considerations)
(listed
in order
Surfaces
(1) (2) (3) (4) (3) (6) (7) (3) (9) (10) (11)
due to engine
development
wear
Metals
Main bearings* Connecting rod bearings Camshaft bushings Camshaft Tappet roller pins Tappet rollers Piston rings Cylinder liner* Valve guides Piston ring bushings Crankshaft*
*Dominates
of importance
ferrogram
based on HFA
Lead-tin/lead/copper--lead tin Lead-tin/lead/copper -lead tin Lead--tin/copper-lead tin Low carbon low alloy steel - AISI 6060 steel Copper-lead-tin (leaded phosphor bronze) Low carbon low alloy steel Cast iron Cast iron Cast iron Lead-tin/copper--lead-tin Low carbon low alloy steel and examination
AISI 4340-H
steel
of parts.
A study of the relative effects of heating the Ferrogram, correlated with the material specifications enabled the microscope observation of the effects of primarily three materials: (a) low carbon alloy steel, (b) cast iron and (c) lead. The percentages of each type of material found upon heating correlated with the percentage area covered reading uersus Ferrogram *The Ferrogram is heated on a “porcelain top” type laboratory ature is measured with a hotplate surface thermometer.
hotplate.
Temper-
Location:
LOW ALLOY
55.5 mm T-----
STEEL
‘-
CAST IRON
54.5 mm
51.5 mm
I
(e)
CARBON STEEL Fig. 5. Heated Ferrogram analysis results: (a), (b), (c) room temperature; (d), (e), (f) 625 “F; (g), (h), (i) 700 OF (magnifications 600 X ).
I
193
25. e : F
A ior*r=735
-60
50 DISTANCE
40
30
ON FERROGRAM
20
10
(mm)
Fig. 6. Percentage area covered and area under the curve for lead, cast iron and carbon steel.
distance and provided data on the approximate relative amounts of the different materials as illustrated in Fig. 6. The area under the curves for each of the three materials is then calculated and can be compared with other engine operating conditions to indicate shifts in wear rates of certain engine components. By observation of the various particles and a correlation of the data in Table 1, the majority of the particles seen on the Ferrogram prepared from oil from the test engine were from either the cylinder liner/piston ring or the crankshaft/main bearing wear interfaces. The wear material of the crankshaft, being a low carbon low alloy steel, was indicated by the portion of the particles that turned blue at 625 “F. The wear of the piston rings and cylinder liners, both being cast iron, was monitored by the percentage of particles taking on the characteristic brownish color upon heating. Lead, being non-magnetic, did not appear to show up at any specific area of the Ferrogram, but did seem to deposit a relative portion of the particles present, usually towards the exit end of the slide (<20 mm). Owing to the interaction of lead wear particles with ferrous particles from the crankshaft, sufficient embedded iron is present to cause deposition of a representative portion of lead particles (of the order of 10% compared with 100% in the case of iron).
194
Engine wear results using Ferrography Using the Severity Index as the wear indicator, the effect of outlet coolant temperature on the overall wear of the test engine is shown in Fig. 7 The same data, rearranged in Fig. 8, shows the effect of the inlet oil temperature on the total engine wear. The standard deviation of the mean value of the Severity Index for three separate sets of Severity Index calculations for the same Ferrogram is also presented in the form of error bars. Generally, the variations in Severity Index increase with increase in Severity Index values, mainly as a result of the general construction of the Severity Index equation.
.L.-.-.. .__i_.~._ _i_.._ 170
135
OWiET
CCXXANT
205
IEMP
/ {J
ii__
__-
155
s*c- !
.-I_ _. IliiF’
1~. _. ._._.. 180 GIL TFL*,‘t’F j
-.
,_._ :’ e 5
Fig. 7. The Severity Index as a function of the coolant temperature. Fig. 8. The Severity Index as a function of the oil temperature.
Total wear increases for both increasing oil and coolant temperatures. This was also indicated by the results obtained by other methods of wear measurement reported earlier [ 1,131. Figures 9 and 10 were plotted from published data [l, 131 and give an indication of the wear of various components by analysis of the area under the curves using the heated Ferrogram analysis techniques. The carbon steel area values indicate primarily the wear of the crankshaft, the cast iron area under the curve indicating primarily the wear at the cylinder liner/piston ring interface, and the lead area indicating the main bearing wear condition.
195
TOP
RING CHROMIUW ‘&EAR RATE Cug /RING/HH.;
’ eu 60 JO
-
RINGWEAR
u
0
IRON CO~iCENTRATION AT 6 HOURS-AE (PPm)
i
LEADCONCENTRATION
10
AT 6 HOURS-AE iwpm)
I
I AREAUNDER CURVE COVERED BY CAST IRON
400 200
/
AREA 100
--O_-
I
600
500
FERROGRAPHY
_
UNDER CURVE COVERED BY CARBON STEEL
zoo
CARBON STEEL
__--
50
AREA UNDER CURVE COVER ED BY LEAD
?O
SEVERITY (Isi
IROO
-
!SGO
-
1200
-
900
-
600
-
INDEX
300
0
I 100
135 OUTLET
COOL ANT
170 TEMPERATURE
(OF
)
205
Fig. 9. The effect of engine coolant temperature on wear using radioactive tracer, spectroscopy and Ferrography wear measurement methods.
Comparison of Ferrography, spectroscopy measurement results
and radioactive tracer wear
A comparison of the different methods of wear measurement used in engine wear investigation is now possible. As indicated in Figs. 9 and 10, piston ring wear as analyzed by the radioactive tracer method [I] seems greatly affected by outlet coolant temperature variation and only slightfy affected by variations in inlet oil temperature. Indications of iron wear given the
196 2c IO-
-
1-Z to-’
OUTLET COOLANT
li ‘O-
205’F --D_._
TEMP TOPRINGCHROMIUM WEAR
RADIOACTIVE
RATE
@g/RING/HR
TRACER
c 1
RiNG
---_a
_-
WEAR
E10IO-
o-
SPECTROSCOPY
I5
lo
IRON CONCENTRATION AT 6 HOURS-AE
3
(pm)
5LEAD
1 o-
CONCENTRATION
AT 6 HOURS-AE (wm)
5o500 loo _.
AREA
UNDER
CURVE
COVERED BY CARBON STEEL !O”
6C )O AREA UNDER CURVE COVERED BY CAST IRON
4 )O 2( )O -
i0
o-
10 AREA UNDER CURVE COVERED BY LEAD
-2 !O 16C
-c
)O -
15C)O 12c)O SEVERITY (I,)
INDEX 9c )O 6C 10 3( )O J
oINLET
OIL TEMPERATURE
(‘F)
Fig. 10. The effect of engine oil temperature on wear using radioactive tracer, spectroscopy and Ferrography wear measurement methods.
by the atomic emission (AE) spectroscopy data seem to correspond quite accurately to the indications of both the carbon steel and the cast iron areas under the curves and in general to the Severity Index (which essentially indicates the total iron wear trends). Only general comparison between Ferrography and AE spectroscopy of the wear trends of lead can be made. However, the same general trends are present. An attempt is now presented to relate generally the empirical Severity Index equation to the other wear metal analysis methods used in this
197
investigation. This investigation has studied the relative changes in the wear trends of only slight changes of wear in a system operating normally. There were therefore no major changes in the modes of wear observed during the investigation. The data are presented in the form of crossplots in Fig. 11. Care has been taken in the preparation of this figure to pick scales that do not misrepresent the precision and accuracy of the data. 1 ppm, being the unit of distin~isable change for the spectrometric data, is compared to a 1600 1400 1200 AREA UNDER CURVE COVERED BY CAST IRON AND CARBON STEEL
1000 800 600
6 IRON CONCENTRATION (pm) AT 6 HOURS-PIE
4/ 6
40 AREA UNDER CURVE COVERED BY LEAD
3. 20 10
L~;~O’~[-‘~~;ION
‘:>; 0
200
400
,, 600
800 1000 1200 SEVERITY INDEX (I,)
1400
1600
1800
2000
INLETOIL TEMPERATURE 0 205.F
Fig. 11. Correlation of severity index with the area under the curve and spectroscopy wear analysis methods.
198
variation of the Severity Index of about 50 units ls. This is considered to be approximately the range of operating precision of the Ferrography equipment for the data presented. For the area under the curve data for iron (cast iron and carbon steel) the possible error variation for the data was considered to be 100 units and is seen to compare generally to 50 units I, variation. The area under the curve data for lead is also considered with the error variation of approximately 10 units in direct comparison with 100 units variation for I, values. The relative slope of bhe band of readings gives a relative indication of the sensitivity of one method with respect to another, with the width of the band being the precision of the correlation of the two methods. Iron wear rates, as indicated by the combined area under the curve of carbon steel and cast iron, correlated well with the Severity Index. The relative slope of the band indicates that the Severity Index is much more sensitive to actual change in the wear trends than is the heated area under the curve methods. Similarly, the AE spectroscopy data for both iron and lead give a good linear correlation to the Severity Index with the Severity Index being the morrx sensitive. It is interesting to note that very little correlation of the area of lead (from the heated Ferrogram analysis) to the Severity Index was obtained and the wide scatter of values indicates that a method needed.
for improved
lead particle
retention
during
Ferrogram
preparation
is
Conclusions The Ferrographic oil analysis Severity Index I,, previously reported to be a useful failure detection technique, appears to be also applicable to the detection of slight changes in wear rate due to changes in the operating conditions of diesel engines. Use of the heated Ferrogram analysis (HFA) techniques enables slight changes in the wear rates of various ferrous wear surfaces to be monitored. The HFA technique represents a breakthrough in the wear analysis of specific component wear; this was previously only possible via radioactive tracer wear analysis methods. The Ferrographic oil analysis Severity Index appears to correlate well with the wear results obtained by AE spectroscopy for both iron and lead. The Severity Index has greater sensitivity than the AE spectroscopy methods. The results indicate that both oil and coolant operating temperatures are critical to the operation of the diesel engine under the test conditions from a wear standpoint. Wear trends generally indicate that increasing either the inlet oil temperature or the outlet coolant temperature tends to increase the amount of wear. Published data (141 indicate that fuel consumption decreases as coolant and oil temperatures increase. The critical nature of both temperatures indicates that accurate temperature control of both mediums could produce substantial reductions in fuel consumption and total engine wear.
199
Acknowledgments The authors thank Kysor of Cadillac for funding the project, the Cummins Engine Company for the engine donation and Mr. Henry Becker of Cummins for providing information on the engine components. The assistance of Mr. Peter Senholzi of the Naval Air Engineering Center in carrying out the spectroscopy and in making many of the Ferrograms is greatly appreciated. Special thanks are extended to Mr. Dave Bolis for his part in the engine test set-up, operation and oil sample collection which made the Ferrography analysis possible. References 1 D. A. Bolis, J. H. Johnson and D. A. Daavettila, The effect of oil and coolant temperature on diesel engine wear, SAE Paper No. 770086, presented at the SAE International Congress, February 28 - March 4, 1977. 2 D. Scott, W. W. Siefert and V. C. Westcott, The particles of wear, Sci. Am., 230 (5) (1974) 88 - 97. 3 D. Scott, W. W. Siefert and V. C. Westcott, Ferrography - an advanced design aid for the 80’s, Wear, 34 (1975) 251 - 260. 4 W. W. Siefert and V. C. Westcott, A method for the study of wear particles in lubricating oil, Wear, 21 (1972) 27 - 42. 5 E. R. Bowen, D. Scott, W. W. Siefert and V. C. Westcott, Ferrography, Tribol. Int., 9 (1976) 109 - 115. 6 E. R. Bowen and V. C. Westcott, Wear particle atlas, July 1976, prepared for the Naval Air Engineering Center, Lakehurst, N. J., under Contract no. N00156-74-C1682. 7 A. A. Reda, E. R. Bowen and V. C. Westcott, Characteristics of particles generated at the interface between sliding steel surfaces, Wear, 34 (1975) 261 - 273. 8 J. L. Middleton, V. C. Westcott and R. W. Wright, The number of spherical particles emitted by propagating fatigue cracks in rolling bearings, Wear, 30 (1974) 275 - 277. 9 V. C. Westcott, Ferrographic oil and grease analysis as applied to earthmoving machinery, SAE Paper no. 750555, presented at the SAE Earthmoving Industry Conf., April 15 - 16, 1975. 10 E. R. Bowen and W. W. Siefert, Ferrography - a new tool for analyzing wear conditions, paper presented at the 1976 Fluid Power Testing Symp., August 16 - 18, 1976. 11 V. C. Westcott and W. W. Siefert, Investigation of iron content of lubricating oil using a Ferrograph and an emission spectrometer, Wear, 23 (1973) 239 - 249. 12 D. R. Jackson, Comparison of atomic absorption and emission spectroscopy in the evaluation of lubrication in normally operating diesel engines, Lubr. Eng., 28 (5) (1972) 76 - 81. 13 D. A. Bolis, The effect of oil and coolant temperatures on diesel engine specific fuel consumption and wear, Masters Thesis, Michigan Technological University, 1976. 14 D. A. Bolis, J. H. Johnson and R. W. Callen, A study of the effect of oil and coolant temperature on diesel engine brake specific fuel consumption, SAE Paper No. 770313, presented at the SAE International Congress, February 28 - March 4, 1977.
183 - Printed
in the Netherlands
THE DEVELOPMENT OF FERROGRAPHY AS A LABORATORY WEAR MEASUREMENT METHOD FOR THE STUDY OF ENGINE OPERATING CONDITIONS ON DIESEL ENGINE WEAR
MICHAEL
V. HOFMAN
and JOHN H. JOHNSON
Mechanical Engineering-Engineering Mechanics Department, University, Houghton, Mich. (U.S.A.) (Received
Michigan Technological
April 20, 1977)
Summary Ferrographic oil analysis techniques were used in a laboratory study of diesel engine wear. Data were developed supporting the concept of using the Severity Index I, to rank the effect of engine operating conditions on wear. Results analyzing the Severity Index as a function of time and as a function of engine operating variables are presented. The Severity Index is also linearly correlated to spectrometric data (iron and lead concentrations in the used oil samples). Engine wear tends to increase with increase of either oil or coolant temperature. However, brake specific fuel consumption tends to decrease as oil and coolant temperatures increase, indicating a need for accurate temperature control for both mediums to minimize fuel consumption and wear. The heated Ferrogram analysis (HFA) technique was used to determine changes in the wear rates of specific engine parts with variation of the oil and coolant temperatures.
Introduction A diesel engine dynamometer test program was conducted to study the effects of engine operating conditions on fuel consumption and wear in order to separate the basic engine characteristics from the dynamic effect of the vehicle on the road. Provisions were made to control accurately the test cell environment and all important variables in the operation of a Cummins VT-903 Vee-type eight-cylinder direct-injection four-cycle diesel engine, turbocharged and non-aftercooled, with 20% torque rise. It was a well broken in engine with several hundred hours of running time prior to the wear tests reported. The full load performance curves, and descriptions of the instrumentation, controls and the test procedure are given elsewhere [ 11. The oil used for the wear tests was a typical diesel engine lubricant, Mobil Delvac 1330 (SAE no. 30), and the fuel used was an ASTM no. 2 diesel fuel oil of 39” API and 0.23% sulfur by mass.
184
The engine condition test matrix consisted of various combinations of the four independent operating variables: speed, load, inlet oil temperature, and outlet coolant temperature. Each test was run separately on new oil for 6 h at the specified engine operating condition. At the end of the 6 h period, an oil sample was taken for oil analysis purposes [ 1 ] . The engine oil was then changed according to previously reported procedure [ 11 and another test was started. The study was conducted to investigate the effects of the range of coolant temperatures and oil temperatures encountered in various climates and the effect of thermal environment on the wear of different surfaces inside a diesel engine. A schematic arrangement of the test set-up is shown in Fig. 1. Control of the coolant was achieved by using a temperature-controlled metering valve. The coolant temperature used for the tests was measured at the engine outlet, or position 1, as shown in Fig. 1. The oil and fuel were water cooled in tube-type heat exchangers. The inlet oil temperature used for the tests was measured after the oil was cooled (shown as position 2 in Fig. 1). The oil filter normally used during engine operation had been removed from the system because of its role as a particle removal mechanism. A sensitive method of wear analysis was desired to reveal slight changes in the wear rate and to predict from which surface the various wear particles originated. A number of analysis methods were used simultaneously in order to evaluate wear. The three primary wear measurement methods used were spectroscopy, radioactive tracer of the top compression ring and Ferrography. The portion of the results concerned with the development of Ferrography as a diesel engine wear measurement method is the primary subject of this paper. Ferrography is a technique which enables magnetic separation of wear debris from a lubricant sample. These wear particles are systematically arranged according to size on a glass substrate (Ferrogram) for microscopic
FROM WATER
CONTROL ~
VALVE.
- -.-
FRbM WATER SUPPLY TO i)RAIN
----~ CONTROL
FROM WATER ----SUPPLY
Fig. 1. Schematic
-
VALVE
-.~-J
arrangement
of the test set-up.
185
examination. A complete explanation of the instrumentation and techniques of Ferrographic oil analysis are given elsewhere [ 2 - 51. Ferrography has the significant advantage of being an extremely sensitive wear measurement method. Past applications of Ferrography have largely applied the technique to field failure detection analysis. This paper is directed at applying it to relatively short term (less than 10 h) performance tests of a diesel engine operated in the laboratory under relatively contaminate free conditions. The paper is divided into three main parts: (1) development of Ferrography for laboratory diesel engine wear analysis; (2) engine wear results as revealed by Ferrography; and (3) comparison of Ferrography, spectroscopy and radioactive tracer wear measurement results.
Ferrography
development
for diesel engine wear measurement
One of the techniques used to monitor the effect of a change in engine operating variables on wear employs the Ferrogram reader. The optical density of the deposit at different locations along the Ferrogram is measured to obtain an indication of the size distribution of particles found in the oil system. Previous investigations have shown that the operating condition of a machine can be determined by the study of the sizes, shapes, and relative quantities of these wear particles [3,4, 6 - 81. It has been shown that an indication of the severity of the system wear mode can be indicated by an empirical function, the Severity Index I,. This function is based on the relationship between the large particles (>5 pm) and the smaller particles (1 - 2 pm) using percentage area covered readings (optical density) obtained by the Ferrogram reader. It is a widely accepted concept that, as a machine tends to failure, both the total particle production rate and the difference in the production of the large particles to the small particles increase dramatically [4, 5,8, 91. The manner in which the particles are deposited on the Ferrogram lends itself favorably to the measurement of two quantities. The large particles appear primarily in the entry deposit near the oil entry location (approximately 56 mm) and the smaller particles (1 - 2 pm) appear further down the substrate at approximately the 50 mm position. A formula for the calculation of the Severity Index [12] is 1, = (AL + A,)(A,
-A,)
= A”L ---A;
where AL is the maximum percentage area covered reading of the entry deposit (~56 mm), and As is the maximum percentage area covered reading at 50 mm. The Severity Index can be seen to include both the number of total particles produced and the difference in the quantity of the large and small particles, each giving greater sensitivity to the formula and increasing its ability to be used as a tool for failure detection. The percentage area covered readings uersus Ferrogram distance for a number of engine operating
186
&RV
!lSTANCE
Fig. 2. Percentage conditions.
ON
FERROGRAM
area covered
Cmn:
as a function
of the Ferrogram
distance
for several engine
conditions are shown in Fig. 2. Ferrogram preparation procedure*, engine operating period on the lubricant (6 h) and all engine operational variables except the four independent variables listed were held constant. The Severity Index for the four operating conditions was also calculated, and the sensitivity of the formula in predicting wear rate changes for changes in operating conditions can be seen. Indications have been reported [3 - 5, 7, lo] that a machine operating in a normal manner reaches a state of equilibrium in which the particle loss rate equals the particle production rate. Any change in the wear rate causes the density of particles to increase or decrease. The particle population in an oil sample has been reported to be influenced by many factors such as oil consumption, inputs of fresh oil, period of time on the oil, oil filtration (if a filter is used) and the continuous grinding down mechanism of the large particles to smaller ones. Particles have also been seen to adhere to the internal walls of a machine. Other loss mechanisms may operate in specific cases. Owing to these dynamic interactions, the commonly used methods of spectroscopic oil analysis are not always an accurate indication of the true operating conditions of the machine [ 4,10 - 121. It is believed that, owing to the size selection possibilities found using Ferrography and the fact that the only particle sizes used in the calculation of the Severity Index are *In both these results and the results obtained for the oil and coolant temperature variation study (Figs. 2, 5 - 11) the standard Ferrogram preparation procedure of using 3 cm3 of oil and 1 cm3 of fixer is employed. These mixtures all had run periods of 3 min followed by a 10 min wash (fixer) period, The analysis with time results (Figs. 3, 4) had a slightly different Ferrogram preparation procedure using 1 cm3 of oil and 3 cm3 of fixer with a run period of 5 min and a wash (fixer) period of 10 min. Indications are that Ferrogram preparation procedure is a variable which should be controlled and kept constant in order to produce results which are comparable.
187
above 1 pm in size, the particle production rate/particle grinding mechanism/ oil filtration interactions approach some finite value with time for a fixed engine operating condition. Ideally, the Severity Index equation is then able to predict the wear severity by a finite rating of engine condition wear. Figure 3 shows the percentage area covered readings with time for data taken using the diesel engine test set-up; an equilibrium wear rate of the large particles of interest is established within the first 2 h of operation. The locus of maximum readings with time for each Ferrogram location indicates that a grinding mechanism is operative. A typical curve of the Severity Index uersus time relationship for the diesel engine test set-up is shown in Fig. 4. The hook at the 5 h point is hypothesized to be due to the interaction of wear particles in the wear interface. Because of the radioactive tracer wear test methods used during our investigation, the oil filtration portion of the lubrication system has been removed [ 11. The lack of proper filtration seemed to have an effect on the equilibrium particle production rate, increasing the severity of wear after a period of time. This seems to give an indication of the importance of proper oil filtration. A study is currently in progress on the interaction of filtration and filtration variables on the wear rates in the diesel engine test system. The overall shape of the Severity Index uersus time curve is a function of several processes. First, there is the rebreaking-in of the surface after starting with a fresh oil change and bringing the engine surfaces, oil system
. 0
I
10
I
I
20 TIME
30 (hours)
I
I
I
40
50
60
Fig. 3. Percentage area covered as a function of the test time. Fig. 4. The Severity Index as a function of the test time.
TIME
(hours)
188
and coolant system up to operating temperatures. The Severity Index increases rapidly between 0 and 1.5 h because of the higher rate of wear during rebreak-in and the fact that the oil has an insignificant number of wear particles at zero time. It was also desirable to determine which part or parts were contributing the wear particles as well as to determine the effects of operating variable changes on wear rates of the various components. For this analysis, a technique designated heated Ferrogram analysis (HFA) was applied. Basic knowledge of the various components in the engine and the tendency of the different materials to take on different thicknesses of oxide layers upon heating at various temperatures was applied. Table 1 gives a list of the various wear surfaces in the test engine, listed in order of importance of engine wear for the engine. It has been established that heating the Ferrogram* for 90 s at 625 “F (~330 “C) turns low carbon and/or alloy steels blue, cast iron brown but does not affect lead. Heating of the Ferrogram to 700 “F (=370 “C) for a further 90s changes low carbon and/or alloy steels to a lighter or silver blue, cast iron to dark brown with slight dark blue areas. Lead remains unaffected by heating. These effects can be seen in Fig. 5. TABLE
1
VT-903 wear surfaces considerations)
(listed
in order
Surfaces
(1) (2) (3) (4) (3) (6) (7) (3) (9) (10) (11)
due to engine
development
wear
Metals
Main bearings* Connecting rod bearings Camshaft bushings Camshaft Tappet roller pins Tappet rollers Piston rings Cylinder liner* Valve guides Piston ring bushings Crankshaft*
*Dominates
of importance
ferrogram
based on HFA
Lead-tin/lead/copper--lead tin Lead-tin/lead/copper -lead tin Lead--tin/copper-lead tin Low carbon low alloy steel - AISI 6060 steel Copper-lead-tin (leaded phosphor bronze) Low carbon low alloy steel Cast iron Cast iron Cast iron Lead-tin/copper--lead-tin Low carbon low alloy steel and examination
AISI 4340-H
steel
of parts.
A study of the relative effects of heating the Ferrogram, correlated with the material specifications enabled the microscope observation of the effects of primarily three materials: (a) low carbon alloy steel, (b) cast iron and (c) lead. The percentages of each type of material found upon heating correlated with the percentage area covered reading uersus Ferrogram *The Ferrogram is heated on a “porcelain top” type laboratory ature is measured with a hotplate surface thermometer.
hotplate.
Temper-
Location:
LOW ALLOY
55.5 mm T-----
STEEL
‘-
CAST IRON
54.5 mm
51.5 mm
I
(e)
CARBON STEEL Fig. 5. Heated Ferrogram analysis results: (a), (b), (c) room temperature; (d), (e), (f) 625 “F; (g), (h), (i) 700 OF (magnifications 600 X ).
I
193
25. e : F
A ior*r=735
-60
50 DISTANCE
40
30
ON FERROGRAM
20
10
(mm)
Fig. 6. Percentage area covered and area under the curve for lead, cast iron and carbon steel.
distance and provided data on the approximate relative amounts of the different materials as illustrated in Fig. 6. The area under the curves for each of the three materials is then calculated and can be compared with other engine operating conditions to indicate shifts in wear rates of certain engine components. By observation of the various particles and a correlation of the data in Table 1, the majority of the particles seen on the Ferrogram prepared from oil from the test engine were from either the cylinder liner/piston ring or the crankshaft/main bearing wear interfaces. The wear material of the crankshaft, being a low carbon low alloy steel, was indicated by the portion of the particles that turned blue at 625 “F. The wear of the piston rings and cylinder liners, both being cast iron, was monitored by the percentage of particles taking on the characteristic brownish color upon heating. Lead, being non-magnetic, did not appear to show up at any specific area of the Ferrogram, but did seem to deposit a relative portion of the particles present, usually towards the exit end of the slide (<20 mm). Owing to the interaction of lead wear particles with ferrous particles from the crankshaft, sufficient embedded iron is present to cause deposition of a representative portion of lead particles (of the order of 10% compared with 100% in the case of iron).
194
Engine wear results using Ferrography Using the Severity Index as the wear indicator, the effect of outlet coolant temperature on the overall wear of the test engine is shown in Fig. 7 The same data, rearranged in Fig. 8, shows the effect of the inlet oil temperature on the total engine wear. The standard deviation of the mean value of the Severity Index for three separate sets of Severity Index calculations for the same Ferrogram is also presented in the form of error bars. Generally, the variations in Severity Index increase with increase in Severity Index values, mainly as a result of the general construction of the Severity Index equation.
.L.-.-.. .__i_.~._ _i_.._ 170
135
OWiET
CCXXANT
205
IEMP
/ {J
ii__
__-
155
s*c- !
.-I_ _. IliiF’
1~. _. ._._.. 180 GIL TFL*,‘t’F j
-.
,_._ :’ e 5
Fig. 7. The Severity Index as a function of the coolant temperature. Fig. 8. The Severity Index as a function of the oil temperature.
Total wear increases for both increasing oil and coolant temperatures. This was also indicated by the results obtained by other methods of wear measurement reported earlier [ 1,131. Figures 9 and 10 were plotted from published data [l, 131 and give an indication of the wear of various components by analysis of the area under the curves using the heated Ferrogram analysis techniques. The carbon steel area values indicate primarily the wear of the crankshaft, the cast iron area under the curve indicating primarily the wear at the cylinder liner/piston ring interface, and the lead area indicating the main bearing wear condition.
195
TOP
RING CHROMIUW ‘&EAR RATE Cug /RING/HH.;
’ eu 60 JO
-
RINGWEAR
u
0
IRON CO~iCENTRATION AT 6 HOURS-AE (PPm)
i
LEADCONCENTRATION
10
AT 6 HOURS-AE iwpm)
I
I AREAUNDER CURVE COVERED BY CAST IRON
400 200
/
AREA 100
--O_-
I
600
500
FERROGRAPHY
_
UNDER CURVE COVERED BY CARBON STEEL
zoo
CARBON STEEL
__--
50
AREA UNDER CURVE COVER ED BY LEAD
?O
SEVERITY (Isi
IROO
-
!SGO
-
1200
-
900
-
600
-
INDEX
300
0
I 100
135 OUTLET
COOL ANT
170 TEMPERATURE
(OF
)
205
Fig. 9. The effect of engine coolant temperature on wear using radioactive tracer, spectroscopy and Ferrography wear measurement methods.
Comparison of Ferrography, spectroscopy measurement results
and radioactive tracer wear
A comparison of the different methods of wear measurement used in engine wear investigation is now possible. As indicated in Figs. 9 and 10, piston ring wear as analyzed by the radioactive tracer method [I] seems greatly affected by outlet coolant temperature variation and only slightfy affected by variations in inlet oil temperature. Indications of iron wear given the
196 2c IO-
-
1-Z to-’
OUTLET COOLANT
li ‘O-
205’F --D_._
TEMP TOPRINGCHROMIUM WEAR
RADIOACTIVE
RATE
@g/RING/HR
TRACER
c 1
RiNG
---_a
_-
WEAR
E10IO-
o-
SPECTROSCOPY
I5
lo
IRON CONCENTRATION AT 6 HOURS-AE
3
(pm)
5LEAD
1 o-
CONCENTRATION
AT 6 HOURS-AE (wm)
5o500 loo _.
AREA
UNDER
CURVE
COVERED BY CARBON STEEL !O”
6C )O AREA UNDER CURVE COVERED BY CAST IRON
4 )O 2( )O -
i0
o-
10 AREA UNDER CURVE COVERED BY LEAD
-2 !O 16C
-c
)O -
15C)O 12c)O SEVERITY (I,)
INDEX 9c )O 6C 10 3( )O J
oINLET
OIL TEMPERATURE
(‘F)
Fig. 10. The effect of engine oil temperature on wear using radioactive tracer, spectroscopy and Ferrography wear measurement methods.
by the atomic emission (AE) spectroscopy data seem to correspond quite accurately to the indications of both the carbon steel and the cast iron areas under the curves and in general to the Severity Index (which essentially indicates the total iron wear trends). Only general comparison between Ferrography and AE spectroscopy of the wear trends of lead can be made. However, the same general trends are present. An attempt is now presented to relate generally the empirical Severity Index equation to the other wear metal analysis methods used in this
197
investigation. This investigation has studied the relative changes in the wear trends of only slight changes of wear in a system operating normally. There were therefore no major changes in the modes of wear observed during the investigation. The data are presented in the form of crossplots in Fig. 11. Care has been taken in the preparation of this figure to pick scales that do not misrepresent the precision and accuracy of the data. 1 ppm, being the unit of distin~isable change for the spectrometric data, is compared to a 1600 1400 1200 AREA UNDER CURVE COVERED BY CAST IRON AND CARBON STEEL
1000 800 600
6 IRON CONCENTRATION (pm) AT 6 HOURS-PIE
4/ 6
40 AREA UNDER CURVE COVERED BY LEAD
3. 20 10
L~;~O’~[-‘~~;ION
‘:>; 0
200
400
,, 600
800 1000 1200 SEVERITY INDEX (I,)
1400
1600
1800
2000
INLETOIL TEMPERATURE 0 205.F
Fig. 11. Correlation of severity index with the area under the curve and spectroscopy wear analysis methods.
198
variation of the Severity Index of about 50 units ls. This is considered to be approximately the range of operating precision of the Ferrography equipment for the data presented. For the area under the curve data for iron (cast iron and carbon steel) the possible error variation for the data was considered to be 100 units and is seen to compare generally to 50 units I, variation. The area under the curve data for lead is also considered with the error variation of approximately 10 units in direct comparison with 100 units variation for I, values. The relative slope of bhe band of readings gives a relative indication of the sensitivity of one method with respect to another, with the width of the band being the precision of the correlation of the two methods. Iron wear rates, as indicated by the combined area under the curve of carbon steel and cast iron, correlated well with the Severity Index. The relative slope of the band indicates that the Severity Index is much more sensitive to actual change in the wear trends than is the heated area under the curve methods. Similarly, the AE spectroscopy data for both iron and lead give a good linear correlation to the Severity Index with the Severity Index being the morrx sensitive. It is interesting to note that very little correlation of the area of lead (from the heated Ferrogram analysis) to the Severity Index was obtained and the wide scatter of values indicates that a method needed.
for improved
lead particle
retention
during
Ferrogram
preparation
is
Conclusions The Ferrographic oil analysis Severity Index I,, previously reported to be a useful failure detection technique, appears to be also applicable to the detection of slight changes in wear rate due to changes in the operating conditions of diesel engines. Use of the heated Ferrogram analysis (HFA) techniques enables slight changes in the wear rates of various ferrous wear surfaces to be monitored. The HFA technique represents a breakthrough in the wear analysis of specific component wear; this was previously only possible via radioactive tracer wear analysis methods. The Ferrographic oil analysis Severity Index appears to correlate well with the wear results obtained by AE spectroscopy for both iron and lead. The Severity Index has greater sensitivity than the AE spectroscopy methods. The results indicate that both oil and coolant operating temperatures are critical to the operation of the diesel engine under the test conditions from a wear standpoint. Wear trends generally indicate that increasing either the inlet oil temperature or the outlet coolant temperature tends to increase the amount of wear. Published data (141 indicate that fuel consumption decreases as coolant and oil temperatures increase. The critical nature of both temperatures indicates that accurate temperature control of both mediums could produce substantial reductions in fuel consumption and total engine wear.
199
Acknowledgments The authors thank Kysor of Cadillac for funding the project, the Cummins Engine Company for the engine donation and Mr. Henry Becker of Cummins for providing information on the engine components. The assistance of Mr. Peter Senholzi of the Naval Air Engineering Center in carrying out the spectroscopy and in making many of the Ferrograms is greatly appreciated. Special thanks are extended to Mr. Dave Bolis for his part in the engine test set-up, operation and oil sample collection which made the Ferrography analysis possible. References 1 D. A. Bolis, J. H. Johnson and D. A. Daavettila, The effect of oil and coolant temperature on diesel engine wear, SAE Paper No. 770086, presented at the SAE International Congress, February 28 - March 4, 1977. 2 D. Scott, W. W. Siefert and V. C. Westcott, The particles of wear, Sci. Am., 230 (5) (1974) 88 - 97. 3 D. Scott, W. W. Siefert and V. C. Westcott, Ferrography - an advanced design aid for the 80’s, Wear, 34 (1975) 251 - 260. 4 W. W. Siefert and V. C. Westcott, A method for the study of wear particles in lubricating oil, Wear, 21 (1972) 27 - 42. 5 E. R. Bowen, D. Scott, W. W. Siefert and V. C. Westcott, Ferrography, Tribol. Int., 9 (1976) 109 - 115. 6 E. R. Bowen and V. C. Westcott, Wear particle atlas, July 1976, prepared for the Naval Air Engineering Center, Lakehurst, N. J., under Contract no. N00156-74-C1682. 7 A. A. Reda, E. R. Bowen and V. C. Westcott, Characteristics of particles generated at the interface between sliding steel surfaces, Wear, 34 (1975) 261 - 273. 8 J. L. Middleton, V. C. Westcott and R. W. Wright, The number of spherical particles emitted by propagating fatigue cracks in rolling bearings, Wear, 30 (1974) 275 - 277. 9 V. C. Westcott, Ferrographic oil and grease analysis as applied to earthmoving machinery, SAE Paper no. 750555, presented at the SAE Earthmoving Industry Conf., April 15 - 16, 1975. 10 E. R. Bowen and W. W. Siefert, Ferrography - a new tool for analyzing wear conditions, paper presented at the 1976 Fluid Power Testing Symp., August 16 - 18, 1976. 11 V. C. Westcott and W. W. Siefert, Investigation of iron content of lubricating oil using a Ferrograph and an emission spectrometer, Wear, 23 (1973) 239 - 249. 12 D. R. Jackson, Comparison of atomic absorption and emission spectroscopy in the evaluation of lubrication in normally operating diesel engines, Lubr. Eng., 28 (5) (1972) 76 - 81. 13 D. A. Bolis, The effect of oil and coolant temperatures on diesel engine specific fuel consumption and wear, Masters Thesis, Michigan Technological University, 1976. 14 D. A. Bolis, J. H. Johnson and R. W. Callen, A study of the effect of oil and coolant temperature on diesel engine brake specific fuel consumption, SAE Paper No. 770313, presented at the SAE International Congress, February 28 - March 4, 1977.