Hindawi Publishing Corporation

Advances in Mechanical Engineering

Volume 2014, Article ID 862853, 7 pages
http://dx.doi.org/10.1155/2014/862853

Research Article
Failure Analysis of a Modern High Performance
Diesel Engine Cylinder Head

Bingbin Guo, Weizheng Zhang, and Xiaosong Wang

School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China

Correspondence should be addressed to Weizheng Zhang; zhangwz@bit.edu.cn

Received 25 April 2014; Accepted 12 May 2014; Published 27 May 2014

Academic Editor: Filippo Berto

Copyright © 2014 Bingbin Guo et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This paper presents a failure analysis on a modern high performance diesel engine cylinder head made of gray cast iron. Cracks
appeared intensively at the intersection of two exhaust passages in the cylinder head. The metallurgical examination was conducted
in the crack origin zone and other zones. Meanwhile, the load state of the failure part of the cylinder head was determined through
the Finite Element Analysis. The results showed that both the point of the maximum temperature and the point of the maximum
thermal-mechanical coupling stress were not in the crack position. The excessive load was not the main cause of the failure. The large
cooling rate in the casting process created an abnormal graphite zone that existed below the surface of the exhaust passage (about
1.1 mm depth), which led to the fracture of the cylinder head. In the fractured area, there were a large number of casting defects (dip
sand, voids, etc.) and inferior graphite structure (type D, type E) which caused stress concentration. Moreover, high temperature
gas entered the cracks, which caused material corrosion, material oxidization, and crack propagation. Finally, premature fracture
of the cylinder head took place.

1. Introduction 2. Failure Description of the Cylinder Head

In the durability assessment test of a V-shaped 8-cylinder Each cylinder of the diesel engine has four valves (two intake
diesel engine which had been strengthened, the cracking fail- valves, two exhaust valves). During the durability assessment
ure of the cylinder head occurred frequently. Compared with test, the unexpected cracking failure of the cylinder head
the prototype diesel engine (the rated speed is 2100 r/min, the happened frequently. Details of the failures and times of
cylinder diameter is 132 mm, and the single-cylinder power is occurrence are given in Table 1. The cracking situation of
55 kW), the rated speed of the improved diesel engine reaches the cylinder head was shown in Figure 1. Cracks appeared
2500 r/min and the single-cylinder power is up to 92 kW. intensively at the intersection of two exhaust passages in the
The arrangement of the cylinder head in this diesel engine cylinder head and extended to cooling watercourse from the
is stand-alone (A cylinder head corresponds to a cylinder), surface to the depth direction, which made the coolant leak
and the material of the failed cylinder head is gray cast iron and even led to the failure of the cylinder head.
(HT250) which is mainly composed of iron matrix and flake
graphite. 3. Fracture Surface Investigation
In this paper, a detailed metallurgical investigation and
a fractographic observation on the failed cylinder head were 3.1. Optical Inspection and Stereomicroscopy. The sample was
conducted. The load state of the failure part of the cylinder taken from the crack position on the cylinder head of
head was determined through the Finite Element Analysis. the engine and washed by acetone solvent with ultrasonic
Finally, the possible failure causes were assessed. wave, and the observation on the surface was conducted by

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Table 1: Description of failure types in the testing programme.

Element Test engine designation Cycle hours at failure (hours) Mode of failure
Engine A 150 Cracked cylinder head
Engine A 189 Piston ring failure
Engine A 268 Cracked cylinder head
Durability assessment tests
Engine A 398 Turbocharger failure
Engine B 262 Cracked cylinder head
Engine B 419 Cracked cylinder head

microscope and scanning electron microscope (SEM). The

microstructure was evaluated according to the Chinese stan-
dard [1], and the result was shown in Figure 3. From Figure 3,
it could be concluded that in most zones, the microstructure
was composed of the flake graphite (type A) [1, 2], fine
pearlite, and a small amount of ferrite. However, the length
of the graphite plate was over the specified range.
Most importantly, as shown in Figure 4 (the crack
Exhaust Exhaust region), the abnormal graphite zone (about 1.1 mm depth)
passage passage was found below the exhaust passage surface. It was shown
in Figure 5 that type A graphite (homogeneous distribution)
and type B graphite (aggregate of chrysanthemum-shaped,
small, and curled graphitic flakes) coexisted in the area
beyond the depth of 1.1 mm, while Figure 6 indicated that
Figure 1: Failed cylinder head. type D graphite (the small and curled graphitic flakes in
interdendritic spaces distributed with no direction) and type
E graphite (the small and curled graphitic flakes between den-
×30 Smoked drite secondary branches distributed directionally) coexisted
black spots
in the abnormal graphite zone within the depth of 1.1 mm.
Exhaust side Crack origin Exhaust Usually type D and type E graphite were formed with large
side cooling rate of liquid iron in the casting process. Therefore, in
the exhaust side, the existence of type D and type E graphite
Crack propagation in array resulted from the large cooling rate, and it was
direction easy to initiate crack under the mechanic load and thermal
load.
Meanwhile, as shown in Figure 7, near the main crack
Red rust
of the exhaust side, another two cracks were found, which
came from the casting process. Two cracks were derived
Cooling water side 73 𝜇m
from casting defects. Crack A with the length of 300 𝜇m
Figure 2: The cracking trace. propagated along the tiny graphitic flakes in array, and oxide
existed in the crack (Figure 7(a)). Crack B with the length of
4.2 mm was initiated from the burnt-on sand in the surface
and propagated along the flake graphite which arrayed with
stereoscopic microscope. The result (the cracking trace) was
crystal in the austenite dendrite (Figure 7(b)). The tip of the
shown in Figure 2.
crack B was oxidized with the 10 𝜇m oxidized layer, and
From a macropoint of view, the fracture surface was
much oxide existed in the crack (Figure 8). The expansion
relatively flat and contained a lot of oxides. Cracks in the
of oxide and the oxidized erosion in the crack tip furthered
exhaust side contained black smoke stains which were gen-
the crack propagation [3]. In addition, as shown in Figure 9,
erated by the engine exhaust, since it deposed and infiltrated
the amount of casting defects with large size was observed
into the crack surface for a period of time. The fracture zone
near the origin place of the crack. Obviously, if this casting
embodied the characteristics of the fatigue crack propagation.
material worked under a thermal-mechanical load, these
According to the trace of the crack, it could be conducted that
defects would generate a high concentration stress, which
the cracking source was initiated at the intersection of two
would promote microcracks propagation. As a result, the
exhaust passages, and then it grew and deepened along the
cylinder head would fracture prematurely.
surface and extended to the cooling watercourse.

3.2. Microstructure. The microstructure observations on the 3.3. Chemical Composition of the Cylinder Head Material. The
samples taken from the fractured end were conducted by chemical composition of the failed cylinder head material was

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Advances in Mechanical Engineering 3

(a) (a)

(b) (b)

Figure 3: Typical microstructure of the cylinder head material: (a) Figure 5: Microstructure of the normal graphite: (a) low magnifi-
low magnification; (b) high magnification. cation; (b) high magnification.

had the same proportion. In addition, compared with piece B,

piece A contained more O (oxygen), which might be induced
by oxidation of the microcracks on the surface.

4. Discussion of Cause and Prevention

1.1 mm
4.1. The Load Analysis of the Cracking Parts. Temperature and
stress in the crack position of the failure cylinder head were
determined with Finite Element Analysis. A finite element
model of the cylinder head is constructed, as shown in
Figure 10. The finite element model has 157868 elements and
253966 nodes. The assembled finite element model, including
cylinder head, cylinder bolts, cylinder sleeve, gasket, and
Figure 4: Abnormal graphite zone near the crack origin region.
engine block, is constructed in order to precisely simulate
the real contact and loading conditions in the stress analysis,
as shown in Figure 11. Contact condition is assigned to the
determined by spectroscopy chemical analysis method. The interfaces between components. A friction coefficient of 0.15
results were shown in Table 2. Piece A was taken from the is defined to the contact surface between cylinder head and
abnormal graphite zone which is nearby the source of the gasket.
crack and about 1.1 mm far from the exhaust duct, while piece With steady state thermal analysis, the temperature
B was taken from the normal graphite zone. It can be seen field of diesel engine cylinder head was obtained based on
that the proportion of C (carbon) in the abnormal graphite ABAQUS software. As shown in Figure 12, a partial high tem-
zone was lower than in the normal graphite zone, but other perature area appeared at the intersection of the two exhaust
elements (except oxygen) like Si in these two zones roughly passages in the cylinder head where there existed the highest

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4 Advances in Mechanical Engineering

Table 2: Chemical composition of the gray cast iron used in the failed cylinder head (wt%).

C Si Mn S P Cr O Fe
Piece A 4.09 2.07 1.03 0.12 0.08 0.38 3.21 Balance
Piece B 5.24 2.05 1.01 0.07 0.08 0.44 0.08 Balance

(a)
(a)

(b)
(b)
Figure 7: Two cracks are derived from casting defects: (a) voids; (b)
Figure 6: Microstructure of the abnormal graphite: (a) low magni- dip sand.
fication; (b) high magnification.

temperature (381.6∘ C). However, the highest temperature did

not occur at the cracking location (Figure 12).
Based on the temperature field which had been obtained
above, the explosion pressure and the bolt pretightening force
were considered in the further analysis. The result was shown
in Figure 13 which was the thermal and mechanical coupling
stress (von Mises stress) diagram at the cracking parts of
the cylinder head. From Figure 13, it could be seen that a
local high stress area (the maximum pressure is 273.9 MPa,
a compressive stress) existed at the intersection of the two
exhaust ducts. However, just like the temperature distribution
shown in Figure 12, the maximum pressure also did not
appear in the crack position.

4.2. Compression Properties of Material of the Cylinder Head.

The crush tests of gray cast iron (with normal graphite Figure 8: Oxide existing in the crack.
morphology) were conducted to obtain the compressive
properties of the material of the cylinder head under different

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Advances in Mechanical Engineering 5

(a)

Figure 10: Finite element model of the cylinder head.

(b)

(c)

Figure 11: The assembled finite element model.

that compared with the specified one, the length of the

graphite was generally longer. However, this defect would not
(d)
lead to the premature fracture of the cylinder head [2, 4]. The
Finite Element Analysis results showed that, at the junction of
Figure 9: Casting defects. exhaust ducts, both the point of the maximum temperature
and the point of the maximum stress were not in the crack
position. At the crack position, the load on the material was
lower than the compression yield strength with the same
temperatures. As shown in Figure 14, compressive stress- temperature, which meant that the high load was not the
strain curves at 20∘ C, 400∘ C, and 500∘ C were obtained. main reason that caused the failure of the cylinder head. The
The compressive yield limit was about 500 MPa at 400∘ C, large cooling rate in the casting process created an abnormal
which was far greater than the thermal and mechanical cou- graphite zone that existed below the exhaust passage surface
pling stress (273.9 MPa). Thus, under normal circumstances, (about 1.1 mm depth), which led to the fracture of the cylinder
the junction of the exhaust passages had a high safety factor, head. In this area, there was a large number of casting defects
which meant that it would not crack. (dip sand, voids, etc.) which caused the stress concentration
points. Therefore, fatigue cracks were initiated in this region
4.3. Analysis on the Failure Causes and Prevention of Future more easily. High temperature gas among cracks caused
Failures. From the results in Sections 2–4, it could be inferred corrosion or oxidation furthering the crack propagation.

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6 Advances in Mechanical Engineering

NT11 NT11
+3.816e + 02 +3.816e + 02
Cracking site
+3.553e + 02 +3.553e + 02
+3.290e + 02 +3.290e + 02
+3.027e + 02 +3.027e + 02
+2.765e + 02 +2.765e + 02 Cracking site
+2.502e + 02 +2.502e + 02
+2.239e + 02 +2.239e + 02
+1.976e + 02 +1.976e + 02
+1.713e + 02 +1.713e + 02
+1.451e + 02 +1.451e + 02
+1.188e + 02 +1.188e + 02
+9.249e + 01 +9.249e + 01
+6.620e + 01 +6.620e + 01
(a) (b)

Figure 12: Temperature field: (a) a sectional view; (b) exhaust crossing.

S, Mises (avg.: 75%) ×102

+2.739e + 02 12
+2.515e + 02
Cracking site
+2.292e + 02 10
+2.068e + 02
+1.844e + 02 8
+1.621e + 02
Stress (MPa)

+1.397e + 02 6
+1.174e + 02
+9.501e + 01
4
+7.265e + 01
+5.029e + 01
2
+2.793e + 01
+5.574e + 00
0
Figure 13: Thermal-mechanical coupling stress field.
0.0 0.1 0.2 0.3 0.4
Strain
20∘ C, 1 400∘ C, 2
20∘ C, 2 500∘ C, 1
400∘ C, 1 500∘ C, 2

Figure 14: The compressive stress-strain curve of the material.

The crack was initiated from the exhaust passage surface of
the cylinder head and propagated toward the edge of the
cylinder head under the mechanical and thermal load. 5. Conclusions
Based on the above reasons, the next generation of
modern high performance diesel needed new technologies In order to analyze the failure causes of a modern high
to improve the cylinder head components. On one hand, a performance diesel cylinder head, the detailed metallurgical
new casting technology should be applied to avoid abnormal investigation and the fractographic observation on the failed
graphite morphology and the casting defects resulting from cylinder head were conducted. The load state of the failure
excessive cooling rate and uneven cooling at the exhaust part of the cylinder head was determined through the Finite
surface, and then the quality of the material used on the Element Analysis, and then the possible failure causes were
exhaust ducts could be improved. On the other hand, a assessed. The following conclusions were drawn.
technical requirement of the casting cylinder head needed
to be reenacted, which meant that a new standard of the (1) The crack was initiated from the exhaust passage
acceptance to the quality of the surface at the intersection of surface of the cylinder head and propagated toward
two exhaust passages in the cylinder head at casting process the edge of the cylinder head under the mechanical
should be adopted. and thermal load.

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Advances in Mechanical Engineering 7

(2) The Finite Element Analysis results showed that

the leading crack did not go through the highest
temperature area and the highest stress area. So the
excessive load was not the main cause of the failure.
(3) The large cooling rate in the casting process created
an abnormal graphite zone that existed below the
exhaust passage surface (about 1.1 mm depth), which
led to the fracture of the cylinder head. In this area,
there were a large number of casting defects (dip sand,
voids, etc.) which caused stress concentration points.
Therefore, it was easier for the fatigue cracks to be
initiated in this region.
(4) High temperature gas entered the cracks and caused
corrosion and oxidation, which furthered the crack
propagation.

Conflict of Interests
The authors declare that there is no conflict of interests
regarding the publication of this paper.

References
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[2] X. Xu and Z. Yu, “Failure analysis of a diesel engine cylinder
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2006.
[3] T. Seifert and H. Riedel, “Mechanism-based thermomechanical
fatigue life prediction of cast iron. Part I: models,” International
Journal of Fatigue, vol. 32, no. 8, pp. 1358–1367, 2010.
[4] F. J. Espadafor, J. B. Villanueva, and M. T. Garcı́a, “Analysis
of a diesel generator crankshaft failure,” Engineering Failure
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