Generator
Content
Engineering Failure Analysis 35 (2013) 499–507
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis
journal homepage: www.elsevier.com/locate/engfailanal
Analysis of a failed Detroit Diesel series 149 generator
C.D. Munro ⇑
Defence Research and Development Canada – Atlantic, Dockyard Laboratory (Atlantic), CFB Halifax, Bldg. D-20, PO Box 99000 Stn Forces, Halifax,
Nova Scotia B3K 5X5, Canada
a r t i c l e
i n f o
Article history:
Available online 7 June 2013
Keywords:
Engine failures
Valve failures
Erosion
Corrosion
a b s t r a c t
Several components of a diesel generator failed dramatically after only nine months in service. Operators noticed the generator, a two-stroke Detroit Diesel model 16V149TIB, producing abnormal noises and smoke. Upon inspection the piston crown, cylinder head,
fuel injector, and exhaust valves from the No. 1 cylinder were found to be extremely damaged, as was the exhaust turbine from the turbocharger assembly. Examinations of the
recovered parts were conducted through visual and chemical analysis, fractography, and
metallography. Various fracture mechanisms, such as thermal cracking, intergranular fracture, and high-cycle fatigue, were observed for the different materials and parts involved. It
was determined that the damage sustained by the engine could be explained as the result
of severe and undetected erosion–corrosion (‘guttering’) of one of the No. 1 cylinder
exhaust valves, which caused the valve head to fracture and enter the combustion chamber. Possible causes of this valve guttering are discussed, and recommendations are offered
to help avoid similar catastrophic failures in the future.
Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
1. Introduction
A Detroit Diesel series 149 (DD149) generator failed after only nine months in service. Defence Research and Development Canada – Atlantic was requested by the equipment owner to conduct an analysis of the engine components and a survey of the operational logs in order to determine the root cause of the failure. The ensuing failure investigation was
noteworthy due to the extent of the damage to the engine components, and the variety of fracture mechanisms encountered.
In this paper, the observations and conclusions of the failure analysis are provided as a case study, and recommendations are
offered to help avoid similar failures in the future.
2. Background
The DD149s are two-stroke diesel engines commonly used in mining, marine, construction, and other industries. These
engines can have up to 20 cylinders, and are so named because of their 149 in.3 of cylinder volume. DD149 engines employ
the ‘‘pot head’’ design, in which the cylinder heads fit inside the cylinders. A schematic of one of the DD149 cylinder heads is
shown in Fig. 1.
Since the DD149 is a two-stroke engine, all four of the valves that pass through each cylinder head are exhaust valves.
These valves are actuated in pairs by the camshaft and the exhaust from pairs of valves passes through cavities in the cylinder head before exiting through separate exhaust ports. In other words, the exhaust streams from the two pairs of valves
on each cylinder do not mix until they exit the cylinder head.
⇑ Tel.: +1 9024272601.
E-mail address: [email protected]
1350-6307/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.engfailanal.2013.05.009
500
C.D. Munro / Engineering Failure Analysis 35 (2013) 499–507
Fig. 1. Schematic of a cylinder head from a DD149 engine [1].
The particular generator model that failed after nine months in service consisted of 16 cylinders in a ‘V’ configuration and
was equipped with four turbochargers. These turbochargers were separated, so that each was powered by the exhaust gases
from four cylinders. This generator was also fitted with 16 pyrometers in order to monitor the temperatures of exhaust gases
emitted from each cylinder.
From installation the failed DD149 generator saw steady, though not continuous, use. Initially, operation of the generator
was uneventful, although there were intermittent problems with the pyrometers used to measure exhaust gas temperatures.
The connections of these pyrometers were prone to vibrating loose so that at times they did not give any readings at all.
Although attempts were made to address the problem, repairs were not entirely successful and some loose connections
persisted.
Six months into use of the generator operators began recording abnormal exhaust gas temperatures coming from cylinder
No. 1. The exhaust at this location was showing as 100 °F hotter than usual. However, since the cylinder No. 1 pyrometer
was one that was prone to failure, operators did not trust these abnormal readings. So, although the temperature measurements continued to be recorded by the operators, they were not considered to be accurate and were disregarded.
Roughly three months later (or after nine months in total of generator operation) the temperature reading from cylinder
No. 1 changed again, this time abruptly dropping by almost 200 °F so that it was running significantly colder than normal.
Again, these readings were not trusted and so were ignored. Just a few days later operators noticed a grinding noise and
excessive white smoke and oil leakage coming from the turbocharger serving cylinder No. 1. The generator was then shut
down, and extensive damage to the turbocharger and cylinder No. 1 components was discovered.
3. Methods and results
3.1. Visual analysis
When the turbocharger serving cylinder No. 1 was disassembled, extensive damage to its exhaust turbine was found. The
turbine blades, which were fabricated from an aluminum alloy, were worn away due to what appeared to be impact. Several
large pieces of metallic debris were also recovered from the exhaust side of the turbocharger housing.
Tracing the exhaust pathway upstream from the failed turbocharger, technicians began finding exhaust valve heads. The
first of which, Head A shown in Fig. 2, was found near the turbocharger housing, where the exhaust enters the turbocharger.
Further up the exhaust stream another valve head, Head B of Fig. 2, was found in the exhaust manifold. By comparing these
Head A
10 mm
Head B
New
Head
Fig. 2. Valve heads recovered from the exhaust pathway of DD149 generator. A new valve head is shown for comparison.
C.D. Munro / Engineering Failure Analysis 35 (2013) 499–507
501
Fig. 3. Recovered valve Head B detail. A region of material loss is visible in the circled area.
valve heads to a new, unused component (Fig. 2) it was clear that the recovered valve heads had each experienced at least
two chordal fractures and significant impact damage. Additionally, on Head B a distinct region of material loss was visible.
This region is shown circled in Fig. 3. It was clear that this material loss was not due to fracture, based on the area’s curved
surfaces and well-defined edges. Furthermore, this region had a cracked, black surface, suggesting significant oxidation occurred at this location.
Since the failed turbocharger served cylinders Nos. 1–4, all of these cylinders were disassembled and closely inspected for
damage. Cylinders Nos. 2–4 exhibited no noticeable damage. However, several components from cylinder No. 1 were severely damaged. The underside of the No. 1 cylinder head, pictured in Fig. 4, showed serious impact damage. Likewise there
was significant impact damage to the top of the piston crown, to the point that there were three holes clearly punched
through it. The fuel injector (not shown in Fig. 4) was also destroyed, apparently by impact damage.
In Fig. 4, the No. 1 cylinder exhaust valves are shown still in their guides. In this image one can see that three of the exhaust valve heads (numbered 1–3 in the figure) were fractured from their stems. The remaining valve stem (valve 4) exhib-
4
1
3
2
25 mm
Fig. 4. Underside of No. 1 cylinder head. Fractured valve stems are numbered, still in their guides. The circled area highlights a region of distinct material
loss on the No. 2 valve seat insert.
Valves 1 and 2
Valves 3 and 4
Fig. 5. Side view of No. 1 cylinder head showing the state of its two exhaust pathways.
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C.D. Munro / Engineering Failure Analysis 35 (2013) 499–507
ited a chordal fracture and was driven up into its seat. There was no damage to any of the valve springs that would otherwise
have affected proper valve closure.
Although three exhaust valves from the No. 1 cylinder were fractured, only two valve heads were found in the engine.
There were, though, a number of large pieces of debris recovered from the exhaust side of the failed turbocharger as well
as from the crankcase. Some of these large pieces were chemically identified as being from the exhaust valves. However,
these pieces were all heavily abraded and misshapen due to impact, such that their fracture surfaces retained no useable
features.
The four valve stems were removed from their guides and cleaned in a hot detergent solution. After cleaning, large regions
of corrosion damage were revealed on valve stems 1 and 2, near their respective fracture surfaces. By comparison, valve
stems 3 and 4 exhibited no such corrosion. Perhaps related to this observation, the exhaust port serving valves 1 and 2
was found to be much cleaner than that serving valves 3 and 4 (Fig. 5). As shown in the figure, the latter port was coated
with a thick layer of incomplete combustion product.
Like the underside of the cylinder head, the exhaust valve seat inserts from the No. 1 cylinder also exhibited considerable
impact damage. However, there was a region of distinct material loss on one of the valve seat inserts. This region is shown
circled in Fig. 4 on the valve seat corresponding to valve stem 2. The surface of this region was darker in colour than the rest
of the valve seat, and was covered in many fine cracks.
3.2. Chemical analysis
The debris recovered from the turbocharger housing and the exhaust valve heads, Head A and Head B, were cleaned and
analyzed for chemical composition using energy-dispersive X-ray spectroscopy (EDS). The heads and debris proved to be
similar in composition, corresponding to a nickel-based precipitation hardened superalloy known as Pyromet 31. This
austenitic alloy is often used for diesel exhaust valves and is well suited to this application [2].
The regions of material loss on Head B and the No. 2 valve seat insert were also analyzed via scanning electron microscopy
(SEM) and EDS. Both surfaces were found to be covered with a cracked oxide layer and exhibited similar surface chemistry.
Present on these oxide surfaces were elements commonly found in fuels and oil additives: magnesium, phosphorous, sulphur, calcium, and zinc. The concentrations of these elements present in the oxidized surface are given in Table 1. As shown,
calcium was present in the highest concentration of the impurity elements.
3.3. Fractography and metallography
The fracture surfaces of the three valve stems shown in Fig. 4 were analyzed visually and via SEM. The fracture surface of
valve stem 1, Fig. 6, was largely undamaged after fracture. On the macroscale it appeared to be a predominantly brittle fracture, although there were small shear lips present. Under the SEM (Fig. 7), the fracture of stem 1 proved to be generally trans-
Table 1
Average concentration (wt.%) of impurity elements on guttered region of Head B, as detected by EDS.
Mg
P
S
Ca
Zn
0.8
0.8
0.5
6.9
1.7
Shear Lip
Smearing
2 mm
Fig. 6. Valve stem 1 fracture surface macrograph.
C.D. Munro / Engineering Failure Analysis 35 (2013) 499–507
503
Fig. 7. Electron micrograph of valve stem 1 fracture surface (450 original magnification).
granular, but some intergranular fracture was apparent. There was also some secondary cracking perpendicular to the fracture surface, and this cracking was mostly intergranular.
The fracture surface of valve stem 2, which shared an exhaust cavity with valve stem 1, was also largely undamaged after
fracture. It also showed a very brittle fracture on the macroscale with little in the way of plastic deformation, and therefore
resembled the surface shown in Fig. 6. Unlike stem 1, however, shear lips were absent from the fracture surface. A considerable amount of secondary cracking was visible even under relatively low magnification (Fig. 8). Under the SEM (Fig. 9), the
fracture had a strong intergranular character, and the secondary cracking appeared to be intergranular as well. Ductile tearing was also visible on this fracture surface, but to a lesser degree than the intergranular character. A cross-section of the
fracture surface (Fig. 10) provided corroborating evidence of a predominantly intergranular fracture. It also showed that
there was notable surface corrosion on the valve stem, and that there were many cracks beginning at areas of corrosion.
These cracks, however, did not always propagate in an intergranular manner and were relatively short (<200 lm). Examples
of this type of cracking beginning at an area of corrosion are labelled in Figs. 10 and 11.
The fracture surface of stem 3 did not resemble that of stem 1 or stem 2. This surface was severely damaged due to impact/abrasion after the fact, destroying any fractographic features on the macroscale. However there were small areas of the
surface that were undamaged and thus could be inspected using SEM. Upon inspection the fracture was almost entirely
transgranular, with regularly-spaced ridges on the surface lying roughly parallel to one another (Fig. 12). These ridges were
much more prominent than slip bands, and appeared to bend around inclusions. They were even present in recessed areas of
the fracture surface, where rubbing would not be expected. Furthermore, ridges were aligned in the same orientation at different locations on the fracture surface. For these reasons, it is likely that these lines were fatigue striations, indicating the
progression of a fatigue crack under cyclic loading. These striations were measured and found to be spaced roughly 1.5 lm
Fig. 8. Electron micrograph of valve stem 2 (100 original magnification). Arrows indicate shallow secondary cracking.
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C.D. Munro / Engineering Failure Analysis 35 (2013) 499–507
Fig. 9. Electron micrograph of valve stem 2 fracture surface (300 original magnification).
Fracture Surface
Corrosion
Cracking
Fig. 10. Cross-section of valve stem 2 fracture surface.
Cracking
Corrosion
Fig. 11. Cross-section of valve stem 2, showing cracking beginning in region of surface corrosion.
apart at the centre of the valve stem. Based on this striation spacing the stem must have been under high cyclic stress, as it is
estimated that fracture would have occurred in less than ten thousand cycles after initiation. At an engine speed of 1800 rpm
fracture would have therefore occurred in a matter of minutes. Unfortunately the fracture surface was otherwise too damaged to determine where and how the fatigue crack started, but the fractographic evidence shows that stem 3 fractured under cyclic loading at high stresses.
C.D. Munro / Engineering Failure Analysis 35 (2013) 499–507
505
Fig. 12. Electron micrograph of valve stem 3 fracture surface (700 original magnification).
Fig. 13. Electron micrograph of Head B fracture surface (500 original magnification).
As visible in Fig. 2, the head–stem fracture surface on Head A had suffered severe impact after fracture. There were no
regions of the original fracture surface that remained to be examined even by SEM. The head–stem fracture surface of Head
B, as seen in Fig. 3, also experienced extensive damage, making it difficult to see any fractographic features on the macroscale. However, there were small regions of this fracture surface that were undamaged and could be inspected via SEM. This
revealed that the fracture had a strong intergranular character (Fig. 13), and was therefore similar to the fracture surface of
valve stem 2. Unfortunately, an exhaustive attempt to match fractographic features between Head B and stem 2 could not be
conducted, due to time constraints.
4. Discussion
4.1. Sequence of failure
The failure of the turbocharger serving cylinders Nos. 1–4 was the event that alerted operators of the DD149 generator to
the wider damage. However, the metal particles recovered from the failed turbocharger were of the same alloy as the exhaust valves, and severely damaged exhaust valve heads were found upstream in the exhaust pathway leading to the turbocharger. These observations suggest that the failure of the turbocharger was merely a secondary consequence of
fracture of the three cylinder No. 1 valve stems. Therefore, in order to understand the wider failure it is necessary to examine
what caused the fracture of the exhaust valve stems in the first place.
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C.D. Munro / Engineering Failure Analysis 35 (2013) 499–507
4.2. Erosion–corrosion of exhaust valve
The regions of material loss on Head B and valve seat insert No. 2 likely hold the key to the fracture of the valve stems.
Properly functioning exhaust valves and seats would be expected to exhibit some minor oxidization during their operating
life, but certainly not to the extent, and with the degree of material loss, seen on the recovered parts. This type of excessive
damage does occur though, and is referred to as valve ‘guttering’. Guttering initiates when a valve fails to seat correctly, due
to, for example, valve distortion or the interference of combustion product deposits. This creates an opening through which
hot, pressurized exhaust gases can leak, leading to material loss in the valves and valve seats via an erosive–corrosive mechanism [3]. An example of a guttered valve taken from a DD149 engine is shown in Fig. 14. It is clear that the appearance of the
burnt area in this example is very similar to the circled region of Fig. 3. The guttering of Head B, however, appears to be much
more extensive than in the example figure, as the guttered region on Head B extends almost all the way to the valve stem.
Unmitigated exhaust valve guttering, such as evidenced on Head B, would have had several effects on the operation of the
DD149 generator. First, one exhaust cavity (if only one valve was guttered) inside the affected cylinder head would be permanently opened to the cylinder, even during the combustion portion of the power cycle. This would raise both the temperature of exhaust gases coming from the cylinder and the temperature of the exhaust valves sharing the affected port.
Furthermore, these valve stems would be bathed in a stream of corrosive exhaust gas, leading to increased valve corrosion
and perhaps impairing the mechanical properties of the valves. Finally, the guttered valve head itself would have been weakened due to excessive, unbalanced material loss.
The observations made during this analysis support not only the presence of valve guttering at cylinder No. 1, but also
that this guttering occurred on valve 2. First, the pyrometer measuring the exhaust gas temperature at cylinder No. 1 showed
elevated temperatures in the months leading up to the failure. Second, the exhaust port serving valves 1 and 2 was scoured
clean of incomplete combustion product, indicating that it was one of these valves that was open to the combustion chamber. Third, the stems of valves 1 and 2 exhibited notable corrosion, while those of stems 3 and 4 did not. Finally, the only
evidence of valve guttering on any of the valve seat inserts was found on seat insert 2, and this region exhibited the same
surface chemistry as the guttered region on Head B. Therefore, although it cannot be said for certain that valve Head B came
from valve stem 2, the observations strongly support this conclusion.
4.3. Valve guttering and fractography
If guttering of valve 2 is acknowledged, as the observations suggest, then this helps to explain the differences in fracture
surfaces seen on the valve stems. Valve stem 2 exhibited a predominantly intergranular fracture, indicating that the grain
boundaries of this stem had been weakened. Studies of Pyromet 31 valves have shown that the abnormal exposure to exhaust gases as a result of valve guttering can promote intergranular attack of this alloy [3]. This attack has been found to
occur more readily when lubricating oils containing calcium-based, rather than magnesium-based, detergent additives
are used. Relatively high concentrations of calcium were detected on the guttered surfaces of Head B and valve seat 2, showing that the particularly damaging calcium-based detergents were used in the failed generator. Thus guttering of valve 2
would have exposed its stem to a steady stream of exhaust gases that preferentially attacked and weakened the grain boundaries of the alloy. So, when fracture of the stem did occur, it generally proceeded in an intergranular manner.
Valve stem 1 shared an exhaust port with valve stem 2, and so it was also subjected, albeit to a lesser degree, to the corrosive action of the exhaust gas. For this reason its fracture surface exhibited a slight intergranular character. Meanwhile,
valves 3 and 4 did not share the affected exhaust port, and so these valves were not weakened by corrosive action. Valve
stem 3 therefore only fractured after being fatigued due to thousands of cycles of high stress loading, and valve stem 4
did not fracture at all when its head was driven up into its seat. In this way, the mechanism of guttering at valve 2 corresponds well with the manner in which fracture of the exhaust valves occurred.
5 mm
Fig. 14. Example of a guttered valve taken from a DD149 engine.
C.D. Munro / Engineering Failure Analysis 35 (2013) 499–507
507
Although the fractographic evidence indicates that valve 2 was subjected to intergranular attack, the question still remains as to what caused the valve head to finally fracture from its stem. The fracture surface was not entirely intergranular
so some loading must have been present to cause fracture. The most plausible explanation is that the normal valve closing
forces, which can be significant both in magnitude and in loading rate, caused a fracture in the weakened and unbalanced
Head B. It is likely, given the degree of guttering and the necessary associated material loss, that the first fracture to occur
was a chordal fracture of this valve head. This having occurred, one or more large chunks of material would have subsequently fallen into the combustion chamber, to be pulverized between the piston crown and cylinder head and to cause further fracture of the exhaust valves.
4.4. Causes of valve guttering
Given the degree of damage to the cylinder No. 2 valves and seat inserts it was impossible to detect any evidence as to
what initiated the guttering. However, there are a few recognized causes of exhaust valve erosion–corrosion that may be
applicable in this situation.
Valve guttering is, in simplest terms, caused by the incomplete closing of a valve against its seat. This creates a radial leakage path through which exhaust gases can begin to flow and, in some instances, erode/corrode the surrounding material. The
contact surfaces between valves and seats are fabricated with precise tolerances, and so anything that interferes with these
mating surfaces can lead to leakage. Gross valve distortion, either permanent or thermally-induced during operation, can
understandably disrupt valve sealing, but more commonly the initial cause of leakage is more subtle.
Studies into the characteristics of ash deposits, or ‘scales’, on the sealing faces of valves and seats have shown that these
play an important role in the guttering process [3]. Such scales are ubiquitous, consisting of, for example, sulphates, phosphates and oxides of inorganic fuel and oil constituents. Not only do these scales interfere with the tight geometric fit of
valves and seats, but they also restrict heat flow out of the valves. Around 75–80% of the heat absorbed by exhaust valves
exits through the contact between valve and valve seat [4]. Valves can therefore be insulated by excessive scale, raising their
temperature and making them more susceptible to distortion or damage. Finally, uneven flaking or spalling of the scale itself
under the closing action of valves can create sufficient localized leakage paths to initiate valve guttering.
5. Conclusions and recommendations
Based on evidence found on one of the recovered valve heads and a valve seat insert from the No. 1 cylinder, extensive
guttering of a diesel exhaust valve from a failed DD149 generator had occurred. The initial cause of guttering could not be
determined, due to the resulting damage to the valve head and seat. However, records of engine temperatures support the
existence of guttering as they showed elevated exhaust temperatures at the affected cylinder beginning roughly six months
into operation of the generator. After three more months of operation the guttered valve was so weakened due to the erosive–corrosive action of the exhaust gas that it fractured, likely under normal valve opening and closing forces. Debris from
this fracture travelled to different areas within the engine causing significant secondary damage.
Two recommendations can be drawn from this failure. First, pyrometer readings should not be ignored by operators and
technicians. It should not be assumed that a pyrometer is malfunctioning when it gives an abnormal reading. Elevated exhaust temperatures should instead be investigated as soon as possible, as they may help to avoid catastrophic failure. Second, it may be worthwhile to avoid using calcium-containing oil detergents in this type of engine, as these have shown to
promote guttering of Pyromet 31 valves. Magnesium-based detergents may be a better option, as these have led to fewer
instances of guttering in controlled engine tests [3].
References
[1]
[2]
[3]
[4]
Detroit Diesel series 149 service manual, Detroit Diesel Corporation, Detroit, 1993.
Woldman NE, Frick JP. Woldman’s engineering alloys. 9th ed. Materials Park: ASM International; 2001.
Scott CG, Riga AT, Hong H. The erosion–corrosion of nickel-base diesel engine exhaust valves. Wear 1995;181–183:485–94.
Lewis R, Dwyer-Joyce RS. Automotive engine valve recession. London: Professional Engineering Publishing Limited; 2002.
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis
journal homepage: www.elsevier.com/locate/engfailanal
Analysis of a failed Detroit Diesel series 149 generator
C.D. Munro ⇑
Defence Research and Development Canada – Atlantic, Dockyard Laboratory (Atlantic), CFB Halifax, Bldg. D-20, PO Box 99000 Stn Forces, Halifax,
Nova Scotia B3K 5X5, Canada
a r t i c l e
i n f o
Article history:
Available online 7 June 2013
Keywords:
Engine failures
Valve failures
Erosion
Corrosion
a b s t r a c t
Several components of a diesel generator failed dramatically after only nine months in service. Operators noticed the generator, a two-stroke Detroit Diesel model 16V149TIB, producing abnormal noises and smoke. Upon inspection the piston crown, cylinder head,
fuel injector, and exhaust valves from the No. 1 cylinder were found to be extremely damaged, as was the exhaust turbine from the turbocharger assembly. Examinations of the
recovered parts were conducted through visual and chemical analysis, fractography, and
metallography. Various fracture mechanisms, such as thermal cracking, intergranular fracture, and high-cycle fatigue, were observed for the different materials and parts involved. It
was determined that the damage sustained by the engine could be explained as the result
of severe and undetected erosion–corrosion (‘guttering’) of one of the No. 1 cylinder
exhaust valves, which caused the valve head to fracture and enter the combustion chamber. Possible causes of this valve guttering are discussed, and recommendations are offered
to help avoid similar catastrophic failures in the future.
Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
1. Introduction
A Detroit Diesel series 149 (DD149) generator failed after only nine months in service. Defence Research and Development Canada – Atlantic was requested by the equipment owner to conduct an analysis of the engine components and a survey of the operational logs in order to determine the root cause of the failure. The ensuing failure investigation was
noteworthy due to the extent of the damage to the engine components, and the variety of fracture mechanisms encountered.
In this paper, the observations and conclusions of the failure analysis are provided as a case study, and recommendations are
offered to help avoid similar failures in the future.
2. Background
The DD149s are two-stroke diesel engines commonly used in mining, marine, construction, and other industries. These
engines can have up to 20 cylinders, and are so named because of their 149 in.3 of cylinder volume. DD149 engines employ
the ‘‘pot head’’ design, in which the cylinder heads fit inside the cylinders. A schematic of one of the DD149 cylinder heads is
shown in Fig. 1.
Since the DD149 is a two-stroke engine, all four of the valves that pass through each cylinder head are exhaust valves.
These valves are actuated in pairs by the camshaft and the exhaust from pairs of valves passes through cavities in the cylinder head before exiting through separate exhaust ports. In other words, the exhaust streams from the two pairs of valves
on each cylinder do not mix until they exit the cylinder head.
⇑ Tel.: +1 9024272601.
E-mail address: [email protected]
1350-6307/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.engfailanal.2013.05.009
500
C.D. Munro / Engineering Failure Analysis 35 (2013) 499–507
Fig. 1. Schematic of a cylinder head from a DD149 engine [1].
The particular generator model that failed after nine months in service consisted of 16 cylinders in a ‘V’ configuration and
was equipped with four turbochargers. These turbochargers were separated, so that each was powered by the exhaust gases
from four cylinders. This generator was also fitted with 16 pyrometers in order to monitor the temperatures of exhaust gases
emitted from each cylinder.
From installation the failed DD149 generator saw steady, though not continuous, use. Initially, operation of the generator
was uneventful, although there were intermittent problems with the pyrometers used to measure exhaust gas temperatures.
The connections of these pyrometers were prone to vibrating loose so that at times they did not give any readings at all.
Although attempts were made to address the problem, repairs were not entirely successful and some loose connections
persisted.
Six months into use of the generator operators began recording abnormal exhaust gas temperatures coming from cylinder
No. 1. The exhaust at this location was showing as 100 °F hotter than usual. However, since the cylinder No. 1 pyrometer
was one that was prone to failure, operators did not trust these abnormal readings. So, although the temperature measurements continued to be recorded by the operators, they were not considered to be accurate and were disregarded.
Roughly three months later (or after nine months in total of generator operation) the temperature reading from cylinder
No. 1 changed again, this time abruptly dropping by almost 200 °F so that it was running significantly colder than normal.
Again, these readings were not trusted and so were ignored. Just a few days later operators noticed a grinding noise and
excessive white smoke and oil leakage coming from the turbocharger serving cylinder No. 1. The generator was then shut
down, and extensive damage to the turbocharger and cylinder No. 1 components was discovered.
3. Methods and results
3.1. Visual analysis
When the turbocharger serving cylinder No. 1 was disassembled, extensive damage to its exhaust turbine was found. The
turbine blades, which were fabricated from an aluminum alloy, were worn away due to what appeared to be impact. Several
large pieces of metallic debris were also recovered from the exhaust side of the turbocharger housing.
Tracing the exhaust pathway upstream from the failed turbocharger, technicians began finding exhaust valve heads. The
first of which, Head A shown in Fig. 2, was found near the turbocharger housing, where the exhaust enters the turbocharger.
Further up the exhaust stream another valve head, Head B of Fig. 2, was found in the exhaust manifold. By comparing these
Head A
10 mm
Head B
New
Head
Fig. 2. Valve heads recovered from the exhaust pathway of DD149 generator. A new valve head is shown for comparison.
C.D. Munro / Engineering Failure Analysis 35 (2013) 499–507
501
Fig. 3. Recovered valve Head B detail. A region of material loss is visible in the circled area.
valve heads to a new, unused component (Fig. 2) it was clear that the recovered valve heads had each experienced at least
two chordal fractures and significant impact damage. Additionally, on Head B a distinct region of material loss was visible.
This region is shown circled in Fig. 3. It was clear that this material loss was not due to fracture, based on the area’s curved
surfaces and well-defined edges. Furthermore, this region had a cracked, black surface, suggesting significant oxidation occurred at this location.
Since the failed turbocharger served cylinders Nos. 1–4, all of these cylinders were disassembled and closely inspected for
damage. Cylinders Nos. 2–4 exhibited no noticeable damage. However, several components from cylinder No. 1 were severely damaged. The underside of the No. 1 cylinder head, pictured in Fig. 4, showed serious impact damage. Likewise there
was significant impact damage to the top of the piston crown, to the point that there were three holes clearly punched
through it. The fuel injector (not shown in Fig. 4) was also destroyed, apparently by impact damage.
In Fig. 4, the No. 1 cylinder exhaust valves are shown still in their guides. In this image one can see that three of the exhaust valve heads (numbered 1–3 in the figure) were fractured from their stems. The remaining valve stem (valve 4) exhib-
4
1
3
2
25 mm
Fig. 4. Underside of No. 1 cylinder head. Fractured valve stems are numbered, still in their guides. The circled area highlights a region of distinct material
loss on the No. 2 valve seat insert.
Valves 1 and 2
Valves 3 and 4
Fig. 5. Side view of No. 1 cylinder head showing the state of its two exhaust pathways.
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C.D. Munro / Engineering Failure Analysis 35 (2013) 499–507
ited a chordal fracture and was driven up into its seat. There was no damage to any of the valve springs that would otherwise
have affected proper valve closure.
Although three exhaust valves from the No. 1 cylinder were fractured, only two valve heads were found in the engine.
There were, though, a number of large pieces of debris recovered from the exhaust side of the failed turbocharger as well
as from the crankcase. Some of these large pieces were chemically identified as being from the exhaust valves. However,
these pieces were all heavily abraded and misshapen due to impact, such that their fracture surfaces retained no useable
features.
The four valve stems were removed from their guides and cleaned in a hot detergent solution. After cleaning, large regions
of corrosion damage were revealed on valve stems 1 and 2, near their respective fracture surfaces. By comparison, valve
stems 3 and 4 exhibited no such corrosion. Perhaps related to this observation, the exhaust port serving valves 1 and 2
was found to be much cleaner than that serving valves 3 and 4 (Fig. 5). As shown in the figure, the latter port was coated
with a thick layer of incomplete combustion product.
Like the underside of the cylinder head, the exhaust valve seat inserts from the No. 1 cylinder also exhibited considerable
impact damage. However, there was a region of distinct material loss on one of the valve seat inserts. This region is shown
circled in Fig. 4 on the valve seat corresponding to valve stem 2. The surface of this region was darker in colour than the rest
of the valve seat, and was covered in many fine cracks.
3.2. Chemical analysis
The debris recovered from the turbocharger housing and the exhaust valve heads, Head A and Head B, were cleaned and
analyzed for chemical composition using energy-dispersive X-ray spectroscopy (EDS). The heads and debris proved to be
similar in composition, corresponding to a nickel-based precipitation hardened superalloy known as Pyromet 31. This
austenitic alloy is often used for diesel exhaust valves and is well suited to this application [2].
The regions of material loss on Head B and the No. 2 valve seat insert were also analyzed via scanning electron microscopy
(SEM) and EDS. Both surfaces were found to be covered with a cracked oxide layer and exhibited similar surface chemistry.
Present on these oxide surfaces were elements commonly found in fuels and oil additives: magnesium, phosphorous, sulphur, calcium, and zinc. The concentrations of these elements present in the oxidized surface are given in Table 1. As shown,
calcium was present in the highest concentration of the impurity elements.
3.3. Fractography and metallography
The fracture surfaces of the three valve stems shown in Fig. 4 were analyzed visually and via SEM. The fracture surface of
valve stem 1, Fig. 6, was largely undamaged after fracture. On the macroscale it appeared to be a predominantly brittle fracture, although there were small shear lips present. Under the SEM (Fig. 7), the fracture of stem 1 proved to be generally trans-
Table 1
Average concentration (wt.%) of impurity elements on guttered region of Head B, as detected by EDS.
Mg
P
S
Ca
Zn
0.8
0.8
0.5
6.9
1.7
Shear Lip
Smearing
2 mm
Fig. 6. Valve stem 1 fracture surface macrograph.
C.D. Munro / Engineering Failure Analysis 35 (2013) 499–507
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Fig. 7. Electron micrograph of valve stem 1 fracture surface (450 original magnification).
granular, but some intergranular fracture was apparent. There was also some secondary cracking perpendicular to the fracture surface, and this cracking was mostly intergranular.
The fracture surface of valve stem 2, which shared an exhaust cavity with valve stem 1, was also largely undamaged after
fracture. It also showed a very brittle fracture on the macroscale with little in the way of plastic deformation, and therefore
resembled the surface shown in Fig. 6. Unlike stem 1, however, shear lips were absent from the fracture surface. A considerable amount of secondary cracking was visible even under relatively low magnification (Fig. 8). Under the SEM (Fig. 9), the
fracture had a strong intergranular character, and the secondary cracking appeared to be intergranular as well. Ductile tearing was also visible on this fracture surface, but to a lesser degree than the intergranular character. A cross-section of the
fracture surface (Fig. 10) provided corroborating evidence of a predominantly intergranular fracture. It also showed that
there was notable surface corrosion on the valve stem, and that there were many cracks beginning at areas of corrosion.
These cracks, however, did not always propagate in an intergranular manner and were relatively short (<200 lm). Examples
of this type of cracking beginning at an area of corrosion are labelled in Figs. 10 and 11.
The fracture surface of stem 3 did not resemble that of stem 1 or stem 2. This surface was severely damaged due to impact/abrasion after the fact, destroying any fractographic features on the macroscale. However there were small areas of the
surface that were undamaged and thus could be inspected using SEM. Upon inspection the fracture was almost entirely
transgranular, with regularly-spaced ridges on the surface lying roughly parallel to one another (Fig. 12). These ridges were
much more prominent than slip bands, and appeared to bend around inclusions. They were even present in recessed areas of
the fracture surface, where rubbing would not be expected. Furthermore, ridges were aligned in the same orientation at different locations on the fracture surface. For these reasons, it is likely that these lines were fatigue striations, indicating the
progression of a fatigue crack under cyclic loading. These striations were measured and found to be spaced roughly 1.5 lm
Fig. 8. Electron micrograph of valve stem 2 (100 original magnification). Arrows indicate shallow secondary cracking.
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Fig. 9. Electron micrograph of valve stem 2 fracture surface (300 original magnification).
Fracture Surface
Corrosion
Cracking
Fig. 10. Cross-section of valve stem 2 fracture surface.
Cracking
Corrosion
Fig. 11. Cross-section of valve stem 2, showing cracking beginning in region of surface corrosion.
apart at the centre of the valve stem. Based on this striation spacing the stem must have been under high cyclic stress, as it is
estimated that fracture would have occurred in less than ten thousand cycles after initiation. At an engine speed of 1800 rpm
fracture would have therefore occurred in a matter of minutes. Unfortunately the fracture surface was otherwise too damaged to determine where and how the fatigue crack started, but the fractographic evidence shows that stem 3 fractured under cyclic loading at high stresses.
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Fig. 12. Electron micrograph of valve stem 3 fracture surface (700 original magnification).
Fig. 13. Electron micrograph of Head B fracture surface (500 original magnification).
As visible in Fig. 2, the head–stem fracture surface on Head A had suffered severe impact after fracture. There were no
regions of the original fracture surface that remained to be examined even by SEM. The head–stem fracture surface of Head
B, as seen in Fig. 3, also experienced extensive damage, making it difficult to see any fractographic features on the macroscale. However, there were small regions of this fracture surface that were undamaged and could be inspected via SEM. This
revealed that the fracture had a strong intergranular character (Fig. 13), and was therefore similar to the fracture surface of
valve stem 2. Unfortunately, an exhaustive attempt to match fractographic features between Head B and stem 2 could not be
conducted, due to time constraints.
4. Discussion
4.1. Sequence of failure
The failure of the turbocharger serving cylinders Nos. 1–4 was the event that alerted operators of the DD149 generator to
the wider damage. However, the metal particles recovered from the failed turbocharger were of the same alloy as the exhaust valves, and severely damaged exhaust valve heads were found upstream in the exhaust pathway leading to the turbocharger. These observations suggest that the failure of the turbocharger was merely a secondary consequence of
fracture of the three cylinder No. 1 valve stems. Therefore, in order to understand the wider failure it is necessary to examine
what caused the fracture of the exhaust valve stems in the first place.
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4.2. Erosion–corrosion of exhaust valve
The regions of material loss on Head B and valve seat insert No. 2 likely hold the key to the fracture of the valve stems.
Properly functioning exhaust valves and seats would be expected to exhibit some minor oxidization during their operating
life, but certainly not to the extent, and with the degree of material loss, seen on the recovered parts. This type of excessive
damage does occur though, and is referred to as valve ‘guttering’. Guttering initiates when a valve fails to seat correctly, due
to, for example, valve distortion or the interference of combustion product deposits. This creates an opening through which
hot, pressurized exhaust gases can leak, leading to material loss in the valves and valve seats via an erosive–corrosive mechanism [3]. An example of a guttered valve taken from a DD149 engine is shown in Fig. 14. It is clear that the appearance of the
burnt area in this example is very similar to the circled region of Fig. 3. The guttering of Head B, however, appears to be much
more extensive than in the example figure, as the guttered region on Head B extends almost all the way to the valve stem.
Unmitigated exhaust valve guttering, such as evidenced on Head B, would have had several effects on the operation of the
DD149 generator. First, one exhaust cavity (if only one valve was guttered) inside the affected cylinder head would be permanently opened to the cylinder, even during the combustion portion of the power cycle. This would raise both the temperature of exhaust gases coming from the cylinder and the temperature of the exhaust valves sharing the affected port.
Furthermore, these valve stems would be bathed in a stream of corrosive exhaust gas, leading to increased valve corrosion
and perhaps impairing the mechanical properties of the valves. Finally, the guttered valve head itself would have been weakened due to excessive, unbalanced material loss.
The observations made during this analysis support not only the presence of valve guttering at cylinder No. 1, but also
that this guttering occurred on valve 2. First, the pyrometer measuring the exhaust gas temperature at cylinder No. 1 showed
elevated temperatures in the months leading up to the failure. Second, the exhaust port serving valves 1 and 2 was scoured
clean of incomplete combustion product, indicating that it was one of these valves that was open to the combustion chamber. Third, the stems of valves 1 and 2 exhibited notable corrosion, while those of stems 3 and 4 did not. Finally, the only
evidence of valve guttering on any of the valve seat inserts was found on seat insert 2, and this region exhibited the same
surface chemistry as the guttered region on Head B. Therefore, although it cannot be said for certain that valve Head B came
from valve stem 2, the observations strongly support this conclusion.
4.3. Valve guttering and fractography
If guttering of valve 2 is acknowledged, as the observations suggest, then this helps to explain the differences in fracture
surfaces seen on the valve stems. Valve stem 2 exhibited a predominantly intergranular fracture, indicating that the grain
boundaries of this stem had been weakened. Studies of Pyromet 31 valves have shown that the abnormal exposure to exhaust gases as a result of valve guttering can promote intergranular attack of this alloy [3]. This attack has been found to
occur more readily when lubricating oils containing calcium-based, rather than magnesium-based, detergent additives
are used. Relatively high concentrations of calcium were detected on the guttered surfaces of Head B and valve seat 2, showing that the particularly damaging calcium-based detergents were used in the failed generator. Thus guttering of valve 2
would have exposed its stem to a steady stream of exhaust gases that preferentially attacked and weakened the grain boundaries of the alloy. So, when fracture of the stem did occur, it generally proceeded in an intergranular manner.
Valve stem 1 shared an exhaust port with valve stem 2, and so it was also subjected, albeit to a lesser degree, to the corrosive action of the exhaust gas. For this reason its fracture surface exhibited a slight intergranular character. Meanwhile,
valves 3 and 4 did not share the affected exhaust port, and so these valves were not weakened by corrosive action. Valve
stem 3 therefore only fractured after being fatigued due to thousands of cycles of high stress loading, and valve stem 4
did not fracture at all when its head was driven up into its seat. In this way, the mechanism of guttering at valve 2 corresponds well with the manner in which fracture of the exhaust valves occurred.
5 mm
Fig. 14. Example of a guttered valve taken from a DD149 engine.
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Although the fractographic evidence indicates that valve 2 was subjected to intergranular attack, the question still remains as to what caused the valve head to finally fracture from its stem. The fracture surface was not entirely intergranular
so some loading must have been present to cause fracture. The most plausible explanation is that the normal valve closing
forces, which can be significant both in magnitude and in loading rate, caused a fracture in the weakened and unbalanced
Head B. It is likely, given the degree of guttering and the necessary associated material loss, that the first fracture to occur
was a chordal fracture of this valve head. This having occurred, one or more large chunks of material would have subsequently fallen into the combustion chamber, to be pulverized between the piston crown and cylinder head and to cause further fracture of the exhaust valves.
4.4. Causes of valve guttering
Given the degree of damage to the cylinder No. 2 valves and seat inserts it was impossible to detect any evidence as to
what initiated the guttering. However, there are a few recognized causes of exhaust valve erosion–corrosion that may be
applicable in this situation.
Valve guttering is, in simplest terms, caused by the incomplete closing of a valve against its seat. This creates a radial leakage path through which exhaust gases can begin to flow and, in some instances, erode/corrode the surrounding material. The
contact surfaces between valves and seats are fabricated with precise tolerances, and so anything that interferes with these
mating surfaces can lead to leakage. Gross valve distortion, either permanent or thermally-induced during operation, can
understandably disrupt valve sealing, but more commonly the initial cause of leakage is more subtle.
Studies into the characteristics of ash deposits, or ‘scales’, on the sealing faces of valves and seats have shown that these
play an important role in the guttering process [3]. Such scales are ubiquitous, consisting of, for example, sulphates, phosphates and oxides of inorganic fuel and oil constituents. Not only do these scales interfere with the tight geometric fit of
valves and seats, but they also restrict heat flow out of the valves. Around 75–80% of the heat absorbed by exhaust valves
exits through the contact between valve and valve seat [4]. Valves can therefore be insulated by excessive scale, raising their
temperature and making them more susceptible to distortion or damage. Finally, uneven flaking or spalling of the scale itself
under the closing action of valves can create sufficient localized leakage paths to initiate valve guttering.
5. Conclusions and recommendations
Based on evidence found on one of the recovered valve heads and a valve seat insert from the No. 1 cylinder, extensive
guttering of a diesel exhaust valve from a failed DD149 generator had occurred. The initial cause of guttering could not be
determined, due to the resulting damage to the valve head and seat. However, records of engine temperatures support the
existence of guttering as they showed elevated exhaust temperatures at the affected cylinder beginning roughly six months
into operation of the generator. After three more months of operation the guttered valve was so weakened due to the erosive–corrosive action of the exhaust gas that it fractured, likely under normal valve opening and closing forces. Debris from
this fracture travelled to different areas within the engine causing significant secondary damage.
Two recommendations can be drawn from this failure. First, pyrometer readings should not be ignored by operators and
technicians. It should not be assumed that a pyrometer is malfunctioning when it gives an abnormal reading. Elevated exhaust temperatures should instead be investigated as soon as possible, as they may help to avoid catastrophic failure. Second, it may be worthwhile to avoid using calcium-containing oil detergents in this type of engine, as these have shown to
promote guttering of Pyromet 31 valves. Magnesium-based detergents may be a better option, as these have led to fewer
instances of guttering in controlled engine tests [3].
References
[1]
[2]
[3]
[4]
Detroit Diesel series 149 service manual, Detroit Diesel Corporation, Detroit, 1993.
Woldman NE, Frick JP. Woldman’s engineering alloys. 9th ed. Materials Park: ASM International; 2001.
Scott CG, Riga AT, Hong H. The erosion–corrosion of nickel-base diesel engine exhaust valves. Wear 1995;181–183:485–94.
Lewis R, Dwyer-Joyce RS. Automotive engine valve recession. London: Professional Engineering Publishing Limited; 2002.
