外文翻譯--穩(wěn)態(tài)和瞬態(tài)工況下柴油機(jī)氣缸蓋應(yīng)力分布的實(shí)際測(cè)量與理論預(yù)測(cè)（英文）

1、11999-01-0973Measurements and Predictions of Steady-State and Transient Stress Distributions in a Diesel Engine Cylinder HeadKyo Seung Lee and Dennis N. Assanis The University of MichiganJinho Lee and Kwang Min Chun Yons

2、ei Univ.Copyright © 1999 Society of Automotive Engineers, Inc.ABSTRACTA combined experimental and analytical approach was followed in this work to study stress distributions and causes of failure in diesel cylinder

3、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 condi- tions. Furthermore, by placin

4、g 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 measuremen

5、ts suggests that the steady- state temperature gradients and the level of temperatures are the primary causes of thermal fatigue in cast-iron cyl- inder heads. Subsequently, a finite element analysis was conducted to pre

6、dict the detailed steady-state tempera- ture and stress distributions within the cylinder head. A comparison of the predicted steady-state temperatures and stresses compared well with our measurements. Furthermore, the p

7、redicted location of the crack initiation point correlated well with experimental observations. This suggests that a validated steady-state FEM stress analy- sis can play a very effective role in the rapid prototyping of

8、 cast-iron cylinder heads.INTRODUCTIONHeavy-duty diesel engine cylinder heads experience severe thermal and mechanical loading, under both steady-state and transient engine operation. Conse- quently, cylinder head design

9、 is very sophisticated as it needs to house complex cooling passages for ensuring compliance with thermal stresses, while providing suffi- cient mechanical strength to withstand combustion pres- sures, and yet accommodat

10、ing 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

11、in regions where cooling is limited,such as in the narrow bridge between valves, or around the exhaust valve seat [1-3]. A number of studies have so far been conducted to develop analytical methodologies suitable for rap

12、id design and virtual prototyping of cylinder heads [3-9]. 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 accur

13、acy of such analyses critically depends on our understanding of the problem, and the accuracy of the boundary condi- tions used in the formulation. Thermal stresses are induced by any of the following causes [10]: ? Temp

14、erature 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 compo- nent, which affects the expansion and di

15、stortion char- acteristics, 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 cylin- der headDue to th

16、e inherent difficulties in measuring stress fields near the critical regions on the firedeck surface, espe- cially under transient conditions, limited sets of measure- ments that can shed light on the problem have been r

17、eported [6, 11]. A numerical study of thermal shock cal- culations by Keribar and Morel [9] has shown that ther- mal waves propagate into components during engine transients, with the steepness of the front depending on

18、material thermal properties. While for a ceramic compo- nent severe shock loads can cause surface compressive stresses to overshoot final steady-state values, the effect was not pronounced in higher conductivity material

19、s. 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 u

20、nder steady-state and transient operation. 3Two types of high temperature strain gages (120 ? and 350 ?) were used. The specifications of the sensors made by Micromeasurement Co. are described in Table 2. According to th

21、e manufacturer, the response time of the strain gages was 300 kHz (3.33 µs). In case of the 120 ? strain gage, the strain gage was coated with high temperature resistance bond after attachment to the inside surface

22、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 re- coated and re-heated at 150°C for 3 more hours. In case of the 35

23、0 ? strain gage, heating was applied for a grand total of 4 hours at a temperature of 175°C. Figure 5 shows a picture of the finished high tempera- ture, strain gage plug assembly. Following construction of the inst

24、rumentation 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 experimenta

25、lly measured strain versus temperature char- acteristic 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 stra

26、in gageThe highest component temperatures, and hence ther- mally-induced stresses are experienced at the combus- tion chamber surface. While it is desirable to measure stresses on the surface, sensors mounted flush with

27、the surface have a very short life. In order to ensure the dura- bility 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

28、thermal transientsoriginating at the gas-side surface [12]. 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 ex

29、haust valve seats of cylin- der #2 and #4 (see Fig. 6 ). Figure 6. Schematic diagram of strain gage positionThe stain gages inserted in cylinder #4 were of the bi- axial type, measuring strain in the x and y directions,

30、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

31、 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 a

32、cquired every 0.01 seconds, while load was cycled between 0 and 100% for several engine speeds.Figure 7. Stress measurement directionsTEMPERATURE AND HEAT FLUX MEASUREMENTS – Figures 8 and 9 show the steady-state tempera

33、tures measured at the four locations within the firedeck of cylin- ders #2 and #6, respectively. In all cases, the measured temperatures increase linearly with respect to engine speed. Increasing speed allows less time f

34、or 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 ho

35、le 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

36、 lowest temperature as it is exposed to significant forced cooling from the adjacent coolant passage and from the induced fresh air.Table 2. Specifications of high temperature strain gagesGage Type WA-06- 062TT-120 WA-06

37、- 60WT-350Resistance in ? 120.0 ± 0.4 % 350.0 ± 0.4 %Lot Number D-A38AD73 K44FD121Gage Factor At 75°F 2.01 ± 0.5 % 2.07 ± 1.0 %RangeCont. Use -75 to 205°C -269 to 290°CShort Use -195