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xx建筑大学毕业论文外文文献及译文
本科毕业论文
外文文献及译文
文献、资料题目： Measurements and Predictions
of Steady-State and Transient Stress Distributions
in a Diesel Engine Cylinder Head
文献、资料来源：SAE
文献、资料发表日期：1999.4
院 （部）：机电工程学院
专 业：
班 级：
姓 名：
学 号：
指导教师：
完成日期：
1
外文文献：
Measurements and Predictions of Steady-State and Transient Stress Distributions in a Diesel Engine Cylinder Head
ABSTRACT
A combined experimental and analytical approach was followed in this work to study stress distributions and causes of failure in diesel cylinder heads under steady-state and transient operation. Experimental studies were conducted first to measure temperatures, heat fluxes and stresses under a series of steady-state operating conditions. Furthermore, by placing high temperature strain gages within the thermal penetration depth of the cylinder head, the effect of thermal shock loading under rapid transients was studied. A comparison of our steady-state and transient measurements suggests that the steady-state temperature gradients and the level of temperatures are the primary causes of thermal fatigue in cast-iron cylinder heads. Subsequently, a finite element analysis was conducted to predict the detailed steady-state temperature and stress distributions within the cylinder head. A comparison of the predicted steady-state temperatures and stresses compared well with our measurements. Furthermore, the predicted location of the crack initiation point correlated well with experimental observations. This suggests that a validated steady-state FEM stress analysis can play a very effective role in the rapid prototyping of cast-iron cylinder heads.
INTRODUCTION
Heavy-duty diesel engine cylinder heads experience severe thermal and mechanical loading, under both steady-state and transient engine operation. Consequently, cylinder head design is very sophisticated as it needs to house complex cooling passages for ensuring compliance with thermal stresses, while providing sufficient mechanical strength to withstand combustion pressures, and yet accommodating intake and exhaust valves and ports, and the fuel injector. As a result of design, weight and manufacturing compromises, cylinder heads often fail in operation due to cracks that are initiated due to thermal fatigue in regions where cooling is limited, such as in the narrow bridge between valves, or around the exhaust valve seat.
A number of studies have so far been conducted to develop analytical methodologies suitable for rapid design and virtual prototyping of cylinder heads. The finite element method has been the foundation of many of the analyses that predict the thermal and stress fields within the cylinder head. However, the accuracy of such analyses critically depends on our understanding of the problem, and the accuracy of the boundary conditions used in the formulation. Thermal stresses are induced by any of the following causes：
Temperature gradients under steady-state operation, including the effects of cyclic temperature changes in the combustion chamber wall
An increase in the mean temperature of a component, which affects the expansion and distortion characteristics, thus inducing stresses
Thermal shock loading resulting from a sudden change in speed or load during transients, which change the rate of heat flux from the gas to the cylinder head.
Due to the inherent difficulties in measuring stress fields near the critical regions on the firedeck surface, especially under transient conditions, limited sets of measurements that can shed light on the problem have been reported .A numerical study of thermal shock calculations by Keribar and Morel has shown that thermal waves propagate into components during engine transients, with the steepness of the front depending on material thermal properties. While for a ceramic component severe shock loads can cause surface compressive stresses to overshoot final steady-state values, the effect was not pronounced in higher conductivity materials. In order to validate this analytical finding, and attribute appropriately causes of failure in cast-iron cylinder heads, a combined experimental and analytical approach is followed here to study stress distributions under steady-state and transient operation.
Experimental studies are conducted first to measure temperatures, heat fluxes and stresses under a series of steady-state and transient operating conditions. Both biaxial and uni-axial high temperature strain gages have been inserted within the thermal penetration depth of a diesel engine cylinder head. The strain gage insertion beneath the surface of the firedeck ensures the durability and reliability of the sensor. At the same time, the placement within the thermal penetration depth allows for studying the effect of thermal shock loading under rapid transients, and for contrasting those measurements with corresponding steady-state magnitudes. Subsequently, a finite element analysis is conducted to predict the steady-state temperature and stress distributions within the cylinder head. Predictions are compared with measurements, and the potential of the method to predict high stress regions that could lead to crack initiation is explored.
EXPERIMENTAL MEASUREMENTS
EXPERIMENTAL SETUP – Temperature, heat flux and stress measurements were acquired on a six-cylinder, naturally-aspirated, direct-injection, Hyundai diesel engine, primarily used in bus applications. The primary specifications of the engine are reported in Table 1.
TEMPERATURE AND HEAT FLUX SENSORS – A total of 8 steady-state temperature and heat flux probes were installed around the intake and exhaust valve seats of cylinders #2 and #6. The probe tips were mounted at a depth of 1.0 mm beneath the firedeck surface, at the locations shown in Figs. 1 and 2. A schematic diagram showing the construction of the temperature and heat flux probe is shown in Fig. 3. The probe was made of Ktype thermocouples. A near-surface (1.0 mm beneath the firedeck) and an in-depth junction (4.0 mm beneath the surface) make it possible to calculate heat flux. To enhance the sensitivity of the junctions, a thin (1mm thickness), circular copper plate was welded at the tip of the sensor. Temperature and heat flux data were acquired every 1 second, under full load, over a speed range from 1000 rpm to 2500 rpm, every 500 rpm.
Table 1. Specification of the test engine
Displacement Volume
7545 cc
Bore X Stroke
118 X 115 mm
Compression Ratio
17.5
Ignition Order
1-5-3-6-2-4
Maximum Torque
475 N·m @1500 rpm
Maximum Power
123 kW @ 2200 rpm
Figure 1. Temperature and heat flux measuring points on the firedeck
HIGH TEMPERATURE ST RAIN GAGES – For measuring stress within engine cylinder heads, especially near the gas-side surface, strain gages with high temperature durability are needed. A special procedure has been developed in this work for constructing a strain gage sensor plug (see Fig. 4) that is suitable for such measurements. The details of the sensor selection, attachment in the instrumentation plug, and verification of its operation are described next.
Figure 2. Location of sensor on the firedeck
Figure 3. Schematic diagram of temperature and heat flux sensor
Figure 4. Schematic diagram illustrating strain gage sensor and critical dimensions
Two types of high temperature strain gages (120and 350) were used. The specifications of the sensors made by Micromeasurement Co. are described in Table 2. According to the manufacturer, the response time of the strain gages was 300 kHz (3.33 s). In case of the 120 strain gage, the strain gage was coated with high temperature resistance bond after attachment to the inside surface of a cup-shaped plug. Then, the strain gage was heated in a microwave oven for 3 hours. After cooling to ambient temperature, the strain gage was recoated and re-heated at 150°C for 3 more hours. In case of the 350strain gage, heating was applied for a grand total of 4 hours at a temperature of 175°C.
Table 2. Specifications of high temperature strain gages
Gage Type
WA-06-
062TT-120
WA-06-
60WT-350
Resistance in
120.0 ± 0.4 %
350.0 ± 0.4 %
Lot Number
D-A38AD73
K44FD121
Gage Factor
At 75°F
2.01 ± 0.5 %
2.07 ± 1.0 %
Range
Cont.
Use
-75 to 205°C
-269 to 290°C
Short
Use
-195 to 260°C
370°C
Figure 5 shows a picture of the finished high temperature, strain gage plug assembly. Following construction of the instrumentation plug, its sensing behavior was explored. The plug temperature was varied by exposing it to a torch, and recorded via an attached thermocouple. Corresponding strain readings were also recorded. The experimentally measured strain versus temperature characteristic was compared to the one published by the manufacturer, and used as the basis for validating the sensor plug behavior.
Figure 5. A photograph of high temperature strain gage
The highest component temperatures, and hence thermally- induced stresses are experienced at the combustion chamber surface. While it is desirable to measure stresses on the surface, sensors mounted flush with the surface have a very short life. In order to ensure the durability and reliability of the strain gage sensor plug, it was inserted 1.5 mm beneath the surface. This location was still within the penetration depth of thermal transients originating at the gas-side surface. Thus, it allowed studying the effect of thermal shock loading under rapid transients. A total of 4 strain gage sensor plugs were inserted near the intake and exhaust valve seats of cylinder #2 and #4 (see Fig. 6 ).
Figure 6. Schematic diagram of strain gage position
The stain gages inserted in cylinder #4 were of the biaxial type, measuring strain in the x and y directions, as defined in Fig. 7. The strain gages inserted in cylinder #2 were of the uni-axial type, measuring strain in a 45° axis. Since strain gage signals can be highly affected by even minute lead wire movement, care was exercised to attach them firmly to the engine head. Steady-state stresses were measured as speed was varied from 1000 rpm to 2000 rpm, in increments of 250 rpm, under full load. Transient stress measurements were also acquired every 0.01 seconds, while load was cycled between 0 and 100% for several engine speeds.
Figure 7. Stress measurement directions
TEMPERATURE AND HEAT FLUX MEASUREMENTS – Figures 8 and 9 show the steady-state temperatures measured at the four locations within the firedeck of cylinders #2 and #6, respectively. In all cases, the measured temperatures increase linearly with respect to engine speed. Increasing speed allows less time for heat transfer to the coolant between combustion events. The highest temperature values are recorded at location B, followed by those at A, C, and D. It should be noted that location B is between the injector nozzle hole and the exhaust valve. Since there is no coolant passage near that region, this explains why location B reaches the highest temperature of the four locations investigated. On the other hand, location D experiences the lowest temperature as it is exposed to significant forced cooling from the adjacent coolant passage and from the induced fresh air.
Figure 8. Steady-state wall temperatures in cylinder #2 over a range of speeds
Figure 9. Steady-state wall temperatures in cylinder #6 over a range of speeds
Figures 10 and 11 show the corresponding steady-state heat fluxes computed at the same locations within cylinders #2 and #6, respectively. Again, heat flux increases linearly with engine speed. The heat flux magnitudes are higher for positions B and C, located around the exhaust valve seat, compared to those at A and D, located around the intake valve seat. Note that as speed is increasing, different locations experience different rates of increase of heat flux, a fact that is attributed to differences in turbulent gas motion and coolant flow patterns. When the heat flux rates in cylinders #2 and #6 are compared (see Figs. 10 and 11), it can be noticed that the former experiences higher heat flux rates than the latter. This is attributed to the fact that the coolant flows first around cylinder #2; by the time it reaches cylinder #6, the coolant has picked up some heat and its temperature gradually rises, thus reducing the potential for heat transfer from cylinder #6. As a result of the higher heat fluxes, the firedeck temperatures around cylinder #2 are lower (by about 10 °C) than those around cylinder #6 that is located on the coolant outlet side.
STRESS MEASUREMENTS – In order to be able to isolate the effect of pre-loading on the total stress measurements recorded in a fired engine by the various strain gages, stress measurements were taken during the engine assembly process. Measurements were taken at the four strain gages following the tightening of each head bolt. The initial stress variation is shown in Fig. 12. While the tightening of different bolts produced different amounts of tension and compression at the measurement location, no consistent pattern was revealed by the measurements. However, it is important to notice that pre-loading produced a negligible stress (within ±5MPa) at the measurement locations, irrespective of directions.
Figure 10. Steady-state heat fluxes in cylinder #2 over a range of speeds
Figure 11. Steady-state heat fluxes in cylinder #6 over a range of speeds
Figure 12. Effect of bolt tightening on pre-loading stress
Figure 13 shows the steady-state stresses recorded by the bi-axial strain gages at intake and exhaust valve locations of cylinder #4, as well as the stresses recorded by the uni-axial strain gages at intake and exhaust valve locations of cylinder #2. The measurements were taken over a range of engine speeds and at full load, i.e. conditions that would produce the maximum stress at each speed. It must be noted that as engine speed is increased towards the maximum torque speed (1500 rpm), the measured stresses increase (or decrease) at a faster rate. Beyond the maximum torque speed, the stress variation is slight. Both tensile and compressive stresses appear simultaneously at the different locations, as shown in Fig. 13, thus indicating the complex character of the stress field.
Figure 13. Steady-state stress variation with respect to engine speed at full load
The measured stress near the exhaust valve seat has negative values (x, y directions), indicating a compression effect. This could have been produced from a tendency of high temperature regions (such non-adequately cooled regions around the injector and the valve bridge) to expand, while subjected to mechanical constraints. As a result, the rest of the firedeck expands more in relative terms, as suggested by the tensile stresses experienced at the other measurement locations. It must also be noted that,
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