Wear, 130 (1989) 189 - 201 189

TRIBOLOGICAL FAILURE DETECTION AND CONDITION MONITORING FOR DIESEL ENGINES*

M. S. OZOGAN and A. I. KHALIL

School of Engineering and Science, Humberside College of Higher Education, George Street, Hull (U.K.)

P. S. KATSOULAKOS

Lloyd’s Register of Shipping, Lloyd’s Register House, 29 Wellesley Road, Croydon (U.K.)

Summary

As part of a condition-monitoring project, the wear characteristics associated with the running-in of a diesel engine have been determined by using on-line debris monitors and oil sample analysis. The suitability of these techniques for monitoring running-in and detecting likely failures is evaluated and some preliminary experimental results are presented.

1. Introduction

In the course of normal operation, machines produce microscopic particles formed by the removal or deformation of surface material from the primary wearing components, such as ball and roller bearings, gears and sliding contacts. These wear particles, which have characteristic com- position, shape and size, are picked up by the lubricating oil of the engine and reach most of the wearing parts. Analysis of the properties and constit- uents of the used oil can provide invaluable information on the condition of the engine and so can help in predicting the reliability and durability of the wearing components, and in detecting the onset of machine problems and incipient failures.

A broad survey of mechanical breakdowns [ 11, suggests that well over half the total involves tribological failures implicating the working faces of mechanical contacts. Although the advantages of testing machine condition by the techniques of performance monitoring and vibration monitoring are widely accepted, the growing need for quality assurance when using vital machinery has led to the increasing utilization of machine monitoring by lubricant and wear debris analysis. A list of lubricant moni- toring techniques for assessing machinery condition is given in Table 1,

*Paper presented at the Nordic Symposium on Tribology, Trondheim, Norway, June 26 - 29,1988.

0043-1648/89/$3.50 0 Elsevier Sequoia/Printed in The Netherlands

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TABLE 1

A list of lubricant monitoring techniques for plant condition

Component Monitor-able Monitoring techniques

failure mode parameter

Critical wear Wear debris

Seal failure Seal debris

Contaminant ingress

Oil cooler failures

Coolant ingress

Filter inspection Spectrographic oil analysis Magnetic plugs Ferrography Particle counting (HiAc) Optical microscopy Scanning electron microscopy X-ray fluorescence spectroscopy Mossbauer spectroscopy Conducting filters Capacitative oil debris monitor Light scattering Induction techniques

Ferrography Filter inspection

Spectrographic oil analysis Gas liquid chromatography Differential IR spectroscopy

(Many others, specific to composition of expected contaminants)

Spectrographic oil analysis Differential IR spectroscopy

(Many others, specific to composition of coolant)

whereas Table 2 lists common contaminant metallic particles that could be present in the lubricant as a result of component wear, and their possible sources.

1.1. Debris-monitoring methods The most important element in an effective wear debris monitoring

programme is the ability to detect the wear particles being generated by a wearing component. A very large number of techniques have been applied to wear debris monitoring over the past decade, with varying success rates. The main categories available for use in debris monitoring are as follows.

1.1.1. Lube oil sampling analysis This method relies on specimen extraction in the form of an oil sample

taken periodically to assess its debris content while at the same time gener- ally checking the oil condition (ferrography and spectrometric oil analysis) [2, 31.

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TABLE 2

Wear particles and their possible sources

Element showing significant change in concentration

Typical associated machinery problems

Al Ingress of aluminaceous dirt Wear of oil-wetted aluminium components e.g. light alloy casings - pistons

torque converter (impellor-stator) aluminium bearings

Cr

cu

Fe

Pb

Wear of chrome-plated rings-liners-shafts Leakage of cooling water (chromate inhibitor)

Leakage of cooling water (copper pipes) Wear of copper lead bearings-bushes

bronze bearings-bushes cages of rolling element bearings

Clutch slippage (lubricated clutches) Washers

General wear; gears; sliding contacts; ball and roller bearings; piston heads; camshaft; crankshaft

Contamination by petrol, oils etc. Wear or corrosion, of copper-lead plain bearings Wear or corrosion, of white metal plain bearings Bushings

Ingress of siliceous dirt (air intake systems) Residual casting sand Silicone lubricant

Wear of tin-plated pistons Wear of Sn-Al plain bearings Wear of tin-based white metal bearings Timing gear

Leakage of cooling water, residual fuel

Coolant leak

Residual fuel contamination

Seals, brass components

Platings

Si

Sn

Na

B

Va

Zn

Ni

1.1.2. Debris collection methods Wear debris is collected from the flow path by a device which is conve-

nient to remove for debris extraction and examination (magnetic plug or filter) [ 41.

1.1.3. Direct detection methods Wear debris in the lubricant is detected in the machine by on-line

devices through which the oil flows, and which are sensitive to the presence

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of debris (debris sensors) [5]. On-line systems seem to offer a more suitable solution to the practical problems of wear debris monitoring, by eliminating the need for regular human intervention and by having a reasonably constant sampling efficiency. Because of its advantages, on-line oil contaminant analysis is increasingly preferred [ 61.

1.2. Trend monitoring Absolute values are not important when looking for potential fail-

ure conditions with an on-line monitoring device. It is far more important to determine whether there has been a change in the number of par- ticles rather than to know the total number of particles produced. A change showing an increase means that the stable conditions have been upset and that a failure is likely to develop. The time available before fail- ure takes place is dependent on the rate at which the increase occurs. The use of electronic equipment in a continuous monitoring mode provides trend information which permits the statistical elimination of spurious results as well as providing real time information on the wear state.

The use of trend analysis has relevance to the design of equipment which is to be used continuously. When a sampling technique is used there is a delay between taking the sample and the analysis, which may have catastrophic consequences in the case of rapid failure. For this reason, it is important to ensure the collection and presentation of information in a way readily indicating the trend data and giving immediate response when an exceptional situation occurs.

1.3. Present work 1.3.1. Overall project objectives The work described in this paper forms an integral part of a project

on diesel engine condition-performance monitoring and predictive systems with the ultimate objective of developing an advanced engine monitoring system. This system will be baaed on expert systems and simulation tech- niques for processing data received from a specially selected sensor pit [ 71. The sensory system will be defined using the expert system development approach, outlined in Fig. 1 [ 81. Diagnostic requirements as well as redun- dancy design policies need to be satisfied. The aim is to utilize existing sensors and non-intrusive sensors as far as possible.

In the category of non-intrusive sensors, debris monitors which are permanently installed in the engine and its lubrication system are expected to play an important role.

The experimental part of the project is being undertaken at Humber- side College and at the University of Newcastle upon Tyne. The work at Humberside College is largely concerned with oil debris monitoring. In addition, aspects of crankshaft and engine bearing monitoring are being investigated in detail.

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t

I Fault reqxnse I

Fig. 1. The expert system development approach.

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1.3.2. Lubricating oil-debris monitoring A primary objective of the experimental programme at Humberside

College is to determine experimentally the wear particle characteristics associated firstly with the “normal” operation of a diesel engine and then with artificially imposed “failure modes”. The failure conditions being investigated are given in Table 3. The wear particle characteristics can be related to the observed wear state of the engine components.

TABLE 3

Combustion imbalance Lubrication oil restriction Viscosity variation Contamination of lube oil Crankshaft misalignment

Additionally, during the tests, the opportunity is taken of evaluating a range of wear debris monitoring instruments and correlating their response with the observed wear particle characteristics and component failure modes. At Lloyd’s Register, these test data are further analysed using rule induction and other pattern-matching techniques.

During the tests, selective oil samples are taken and ferrographic and spectroscopic analysis techniques are applied to them. This provides precise information for monitoring the running-in wear characteristics of the test engine and for interpreting the results obtained from on-line debris sensors. Another oil sample analysis method, the rotary particle depositor (RPD) technique [9], is also utilized throughout the tests for the purpose of com- paring the two main ferrographic techniques.

In Table 4 the characteristic properties of various wear debris moni- toring techniques are summarized. A combination of techniques is invariably necessary, its exact form depending on the application requirements.

2. Experimental details

2.1. Test engine The diesel test engine at Humberside College is a three-cylinder, four-

stroke Stork Werkspoor Engine, type R153. This engine develops 90 bhp at 1000 rev min-’ .

A vane-type lube oil pump circulates oil from the engine sump firstly through a coarse filter plate and then through a duplex filter. A lube oil pressure control valve built into the oil pump casing maintains the pressure at about 36 Ibf inw2. After the filter, the oil goes through an oil cooler to the main oil gallery running alongside the engine. The temperature of the oil is controlled (maximum 75 “C) by a thermostatic valve which enables an appropriate proportion of the oil flow to bypass the cooler.

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From the main oil gallery the oil passes through bored holes in the cylinder block and in the main bearing bridges to feed the four main bearings and also, via a small-bore pipe, across the engine to feed the camshaft and valve levers.

Oil from the main bearings passes through bored holes in the crank- shaft to feed the big-end bearings and then via the connecting rods to the small-end bearings and pistons. The extra main bearing (flywheel side) feeds oil to the intermediate gear-wheel bearing, and this oil also passes through two radially drilled oil canals in the gear fillet to lubricate the gear wheels.

The lube oil system capacity is 45 1. The lubricant used throughout the tests described in this paper is Mobil RTS 3609, SAE 30 oil, with a viscosity of 101 cSt at 40 “C.

2.2. Experimental determination of test engine wear modes The objective is to monitor both the engine component surface wear

conditions and the respective wear particle generation in relation to the running-in of the engine. Correlations can then be drawn between surface wear progression and particle characteristic trends. Trend analysis serves to identify critical particle characteristics which reflect the wear state of lubricated components. The test engine was observed carefully over the running-in period. Previous work on diesel engines [lo] has suggested that a period of 10 - 15 h is sufficient for effective running-in of wear surfaces, although it is likely that most wear-in is complete within the first few hours [ll].

2.2.1. Oil sampling The results of any oil analysis programme are heavily dependent upon

the standard of oil sampling and the repeatability of this procedure. Since wear particles and contaminants in a lubricating system are seldom uni- formly distributed, sample taking is critical. Oil samples should be removed periodically from the engine lube oil system and subjected to extensive examination. In the case of the Humberside engine, the oil is sampled from the main oil pipe between the oil pump and filter while the engine is running.

The frequency of sampling during the running-in was every hour up to 10 h of running, then every 2 h up to 20 h and again up to 5 h after that. The size of the sample was decided from the requirements of the oil tests and was about 100 ml for each sample.

The analysis techniques employed to examine the condition of the lubricating oil are given in Table 5.

2.2.2. On-line wear debris monitors Since the ultimate objective of this work programme is to develop

an on-board monitoring system, the requirement from the wear ‘debris point of view is for sensors or debris collectors which are permanently installed in the engine lubrication system.

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TABLE 4

Some basic characteristics of a selection of wear debris monitoring techniques

Direct detection

MPD CDM Fulmer Lindley

Particle size range

> 200 /h?I > 100 pm Any size Depends on (but capture particle efficiency hardness poor beiow typically 200 pm) >5grn

7 - 120 Mm

Type of debris detected :

ferrous non-ferrous non-metallic

Detection limit for Fe

Yes Yes NO No)As No No ) Yet

5.0 /Jg individual particles

2.0 j& individual particles

Yt?S No No

1.0 Pfi accumulated debris

Yes Y&S Yes

Typically : 1.0 ppm (by weight in oil system)

Yes Yes Yes

Unknown but should be very sensitive

Particle morphology

Degree of surveillance

- - - - -

Continuous (in-line)

Continuous (in-line)

Continuous (in-line)

Continuous Continuous (on-lie (on-line

bypass) bypass)

Ability to activate alarms

Ease of addition to existing machine

Yes Yes Yes Yes Yes

Poor (plumbing required)

Fairly simple

Poor (plumbing required)

Poor (plumbing required)

Poor (plumbing required)

A variety of devices are currently in use for on-line monitoring of wear debris in lubrication and hydraulic systems. Those employed in this test programme are

Lindley (fluid condition monitor) - debris collector; CDM (continuous debris monitor) - debris collector; QDM (quantitative debris monitor) - debris collector; Fulmer (abrasivity monitor) - debris sensor; MPD (metal particle detector) - debris sensor.

The debris monitors have been installed in an independent circulatory system, shown diagrammatically in Fig. 2.

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BHRA

Oil sample and analysis Debris collection

Soap Ferrograph y RPD PQ Magnetic plugs

3 - 1000 pm

Yes Yes Yes

0.2 ccg/L

-

Continuous (on-line bypass )

Yes

Poor (plumbing required)

>5pm (can be improved by acid digestion)

Yes Yes Yes

YeS Some Some

0.03 ppm 0.05 ppm (by weight (by weight in oil in oil sample) sample)

-

Periodic based on sampling frequency

No

Simple

>lpm

Excellent

No

Simple

>1/.4m

Yes Some Some

Good using PQ

Excellent

No

Simple

Depends on As for CDM debris capture technique no limits in oil sample

Y&3 Yes No ) As No No) Yet No

0.9 /Jg Good (estimated) using

PQ

- Good

Periodic based on inspection frequency

No Yes

Simple Poor (plumbing required)

3. Results

Figure 3 shows that there was a running-in period lasting 5 - 6 h at the beginning of the tests, as indicated by the sharp rise in the particle count. After about 6 h of running-in, the severity of the wear steadied. The running- in tests were each considered complete when the rate of increase in the particle count had become very low and consequently there was no further sharp rise in the slope of the graph. This was confirmed by laboratory analysis of lubricant samples taken at various times during the tests. Table 6 shows the results of sample analysis with a listing of a number of elements. Figure 4 shows that after an initial jump early in the tests associated with

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TABLE 5

Wear particle characterization

Spectrometric analysis Ferrography (DR and analytical) Optical microscopy Scanning electron microscopy Energy-dispersive X-ray analysis Particle counting

Lubricant conditions

Kinematic viscosity (cSt at 40 “C) Fuel dilution (vol.%) Total dilution (vol.%) Water (vol.%) Total base number (mg KOH)

j Sump

Fig. 2. Schematic view of the wear debris detector’s positioning in the lubricating oil system.

the running-in period indicated by the on-line counter results (Fig. 3), the slope settles.

The debris-monitoring results were also supplemented with data from other sensors mounted in the engine components. These sensors included thermocouples and proximity probes in the bearing cells which provided additional information for deciding when the running-in period was com- plete. In the initial stages of the tests, the bearing temperatures were 10 - 15 “C! higher and somewhat erratic whereas after about 5 h of running the temperatures dropped to the expected operating values and were steadier.

The tests were conducted over a period of days, but the engine was stopped for the night, which caused the wear particles to settle down at the bottom of the engine oil sump. The metal particle detector (MPD) employed for particle counting was left showing the count last recorded when the engine was stopped. When the engine was restarted there was

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z 1 1 2 3 4 5 6 7 6 5 10 1, 12 13 1‘ 15 16 17 18 19HRs

Fig. 3.

TABLE 6

Elements Sample numbers

New oil 1 2 3 4 5 6 7 8 9 10 11 12 13

Fe 0 12 14 16 13 17 17 17 20 19 18 21 22 19 Al 1 2 2 3 2 3 3 3 3 3 3 3 4 3 Pb 1 6 6 7 7 7 7 7 8 8 9 10 10 9 Si 7 11 13 16 17 19 21 21 23 25 25 26 26 27 B 0 38 40 51 49 54 61 61 63 62 59 59 61 60 cu 0 2244555566777

30

+ + + 5.4 ++

.

200

an initial increase in the slope, caused by oil movement, which settled down after a short time. The stop-start points are indicated on the graph (Fig. 3).

With the fluid condition monitor (FCM), the running-in progress was monitored in terms of the number of particles per 100 ml of lubricant at two levels of filter cartridges covering the spectrum of very small particles (7 and 14 pm). The particle counts are also converted to IS0 format. The general trend of the results from the FCM supports the particle counter (MPD) and sample analysis results.

4. Discussion and conclusions

Throughout the experimental programme, the debris-monitoring techniques and individual monitors are being evaluated with respect to their suitability for the effective condition monitoring of diesel engines. Each monitor has a detecting ability for varying sizes of mainly ferrous particles in the lube oil. The main factor in assessing these monitors is their ability to detect the particles created during the various stages of diesel engine operation, forewarning of any likelihood of breakdowns.

In the running-in phase of the engine, which is the subject of this paper, two of the on-line monitors, together with oil sample analysis, have successfully detected the initial high wear rates. The remaining monitors and analysis methods failed to provide any significant help in identifying the span of running-in. However, as the work programme enters its further stages concerning the introduction of various fault conditions into the system, all monitoring techniques are expected to play important roles as the nature and size of wear particles and contamination change.

The main difficulty in the application of debris analysis techniques for engine condition assessment is in discriminating between different failure modes. The initial results presented in this paper indicate that com- bining data from a number of on-line debris monitors could provide unique signatures for many important fault conditions. Advanced data processing techniques can be used to design such relationships. These can be translated into the appropriate rules for diagnostic and predictive expert systems.

Acknowledgments

This work is carried out as part of a project being undertaken by a ponsortium led by Lloyd’s Register of Shipping (U.K.), with Humberside College (U.K.), Newcastle University (U.K.) and Marconi (U.K.) as its other members.

The work is funded by the Department of Trade and Industry (U.K.) and the Science and Engineering Research Council (U.K.) and this is grate- fully acknowledged.

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The authors would also like to acknowledge the material assistance of various associate member firms providing the on-line debris monitors, and Mobil Oil (U.K.) for supplying lubricants.

References

7

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11

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