Static Analysis of Crankcase and Crankshaft of Single Cylinder Four Stroke Diesel Engine

Transcription

1 Static Analysis of Crankcase and Crankshaft of Single Cylinder Four Stroke Diesel Engine Kakade Pratik 1 Post Graduate Student kakadepratik@gmail.com Pasarkar M. D. 2 Assistant Professor mdpasarkar@gmail.com Thakur A. G. 3 Professor ajay_raja34@yahoo.com Abstract A live case study of premature failure of crankcase is selected as per the requirement of user. It is observed that crankcase fails when operating at its nominal speed of 2600 rpm. Further investigation of crankcase pointed out that the crack has appeared at one of its tap hole on the flywheel end. This paper focuses on critical survey of literature and thereby use of methodologies to find out the critical area through static analysis. As the case referred by the user, the area where crack is initiated at operating condition is critically examined and then static analysis was performed to check the area critically. The analysis result validates the point of high stress where the crack was initiated. After viewing the results, modifications were suggested in the crankcase, so as to eliminate the susceptibility of crack initiation and propagation. Analysis of crankshaft has also been performed and its analysis proves to be safe in the given working conditions. Keywords- Crankcase, crankshaft, stress concentration point, nodal rigid bodies (NRB). ***** I. INTRODUCTION One of the leading manufacturer and exporter of single cylinder four stroke diesel engines from western Maharashtra region was facing problem in the crankcase. The company has the capacity to assemble, test, refit, finish and pack 36,000 engines per annum with flexibility to increase production further to meet market demand arising in the company s product range. Company basically deals with casting of crankcase and related machining which contains drilling tap holes, external facing etc. With the increase in demand for the engines, company has been producing it with speed higher than the conventional range. But with the production of engines with higher speeds, few of these models faced crack failure at the customer s working field. This crack failure has been basically initiated near one of the tap holes of crankcase at flywheel bearing end. Thus to counter this problem static analysis is performed on both the crankcase and the crankshaft to check whether the model is safe in the static loading condition and to find various stress concentration points in the model. The names of the engine manufacturers and power plant owners have been omitted to preserve the anonymity of the parts involved. II. LITERATURE SURVEY Crankshaft is one of the most important moving parts in internal combustion engine. Crankshaft is a large component with a complex geometry in the engine, which converts the reciprocating displacement of the piston into a rotary motion. This study was conducted on a single cylinder 4- stroke diesel engine. It must be strong enough to take the downward force during power stroke without excessive bending. So the reliability and life of internal combustion engine depend on the strength of the crankshaft largely [1]. And as the engine runs, the power impulses hit the crankshaft in one place and then another. The torsional vibration appears when a power impulse hits a crankpin toward the front of the engine and the power stroke ends. If not controlled, it can break the crankshaft. Jian Meng et al. [2] analyzed crankshaft model and crank throw were created by Pro/ENGINEER software and then imported to ANSYS software. The crankshaft deformation was mainly bending deformation under the lower frequency. And the maximum deformation was located at the link between main bearing journal, crankpin and crank cheeks. Gu Yingkui et al. [3] researched a three-dimensional model of a diesel engine crankshaft was established by using PRO/E software. Using ANSYS analysis tool, it shows that the high stress region mainly concentrates in the knuckles of the crank arm & the main journal and the crank arm & connecting rod journal,which is the area most easily broken. Xiaorong Zhou et al. [4] described the stress concentration in static analysis of the crankshaft model. The stress concentration is mainly occurred in the fillet of spindle neck and the stress of the crankpin fillet is also relatively large. Based on the stress analysis, calculating the fatigue strength of the crankshaft will be able to achieve the design requirements. From the natural frequencies value, it is known that the chance of crankshaft resonant is unlike. This paper deals with the dynamic analysis of the whole crankshaft. Farzin H. Montazersadgh et al. [5] investigated first dynamic load analysis of the crankshaft. Results from the FE model are then presented which includes identification of the critically stressed location, variation of stresses over an entire cycle, and a discussion of the effects of engine speed as well as torsion load on stresses. 1

2 III. DESIGN CALCULATIONS FOR CRANKCASE AND Where, CRANKSHAFT P z = Gas force acting in Z direction, The configuration of the diesel engine for the crankcase and R f = Reaction at flywheel end bearing, crankshaft is tabulated in Table I. R g = Reaction at gearbox end bearing. Below calculations give the values of the reaction TABLE I. SPECIFICATION OF ENGINE forces. 2600rpm 8HP Reaction at flywheel end: R f = [(P z x distance between gear box and crank Cylinder Bore 87.5mm center)] distance between two bearings Stroke 80 mm = [( x ) ] Swept volume Cylinder compression ratio Crankcase material Crankshaft material Modulus of elasticity 481 cc 16.7 Grey Cast iron (FG260) Spheroidal graphite cast iron (SG700/2) MPa Poisson s ratio 0.27 Distance between Gear Box and Crank centre Distance between flywheel and Crank centre Distance between two bearings Gas force on piston TABLE II. Tensile strength m m m N MATERIAL PROPERTY Modulus of Allowable Stress FG E (Tensile) SG700/ E (Tensile) = N Reaction at gear box end: R g = P z R f = = N IV. DESIGN METHODOLOGY FOR CRANKCASE Initially 3D model of crankcase was created in CATIA. This model was then exported to HYPERMESH for meshing. The meshed file with the required boundary conditions was exported to NASTRAN. Post processing was done in HYPERVIEW. A. Procedure for Static Analysis Following procedure was adopted for static analysis of crankcase. 1) First the 3D model of crankcase was generated in CATIA as shown in Figure 2 and saved as STEP file. This file was imported in Hypermesh for meshing. Courtesy: Indian standard grey iron castings specification (Fourth revision) Iron castings with spheroidal or nodular graphite specification (Third revision) A. Forcce Calculation Gas force acting on the piston is available from the manufacturer. The crankshaft is considered as a simply supported beam, supported at the bearings, with point load acting at the centre of crankpin as shown in Figure 1. The reaction forces at the bearings are calculated as below. Figure 2. 3D model of crankcase 2) The model was meshed with the global element size 5mm and minimum size 1mm. properties were added. The model was tetra meshed as shown in Figure 3. The mesh properties are tabulated in Table III. TABLE III. PROPERTES OF CRANKCASE Figure 1. Forces acting on crankshaft Elements Nodes Density (kg/m3) Tensile strength Modulus Of Allowable Stress FG E (Tensile) 2

3 It was found that the point of high stress i.e. stress concentration point closely ressembled the actual crack location, as shown in Figure 6, thus proving the prime cause of crack initiation and propagation. Figure 3. Meshed model of crankcase 3) Boundary conditions are applied according to the actual conditions. Crankcase is constrained at the bottom and forces are applied to the upper half of bearing support with the help of NRBs as shown in Figure 4. Figure 6. Actual model of crankcase with crack 5) To eliminate the chances of crack the stress concentration point should be shifted away from the drill hole. This is done by adding material between the ribs on the rear end of the drill hole near crack. The cut-section of the meshed model where the material is added is shown in Figure 7. Figure 4. Crankcase with boundary conditions 4) The meshed model with boundary conditions was exported to NASTRAN. The result file obtained was post processed in HYPERVIEW. Von-misses stress was obtained as shown in Figure 5. Figure 7. Cut-section of modified crankcase 6) The modified crankcase was meshed and boundary conditions were applied.the analysis was performed same as before. Von-misses stress was obtained as shown in Figure 8. Table IV displays the comparison of stresses in both models. TABLE IV. STRESS COMPARISON OF CRANKCASES Figure 5. Von-misses stress in crankcase Model Original Crankcase Modified Crankcase Max Von-misses Stress (MPa)

4 Figure 8. Von-misses stress of modified crankcase B. Results and Discssion Both the stress values are below the allowable limit of the material as shown in Table 2. From above analysis it can be stated that the cause of crack was generation of stress concentration point near the drill hole. This stress concentration point was shifted to flywheel end where there is no hole present as shown in Figure 8. The stress values are reduced and hence the modified design can be considered safe. V. DESIGN METHODOLOGY FOR CRANKSHAFT The methodology used for crankshaft is similar to that of crankcase. The model is generated in CATIA and meshed in HYPERMESH. Analysis is performed in NASTRAN and post processing is done in HYPERVIEW. A. Procedure for Static Analysis The procedure adopted here is same as that of crankcase. 1) The 3D model of crankshaft was generated in CATIA as shown in Figure 9 and saved as STEP file. This file was imported in Hypermesh for meshing. Figure 10. Meshed model of crankcase 3) Boundary conditions are applied according to the actual conditions. Crankshaft is constrained at the two bearing ends and the force is applied to upper of the crankpin with the help of NRBs as shown in Figure 11. Figure 11. Crankshaft with boundary conditions 4) The meshed model with boundary conditions was exported to NASTRAN. The result file obtained was post processed in HYPERVIEW. Von-misses stress was obtained as shown in Figure 12. 2) The model was meshed with the global element size 5mm and minimum size 0.786mm. properties were added. The mesh properties are tabulated in Table V. The mesh size is reduced to capture the fillet region precisely as shown in Figure 10. TABLE V. Elements Figure 9. 3D model of crankshaft Nodes PROPERTIES OF CRANKSHAFT Density (kg/m3) Tensile strength Modulus Of SG700/ E+05 Allowable Stress 173 (Tensile) B. Results and Discussion Figure 12. Von-misses stress in crankshaft The maximum value of stress obtained in crankshaft is MPa. This value is more than the allowable limit of 173 MPa. The point of maximum stress is obtained at the fillet region near crank web which is highly prone area for crack initiation. The mesh created near the fillet region is very fine with element length of 0.786mm. This has led to prediction of higher stress value at the fillet region. A little coarser mesh would have drastically reduced the stress value. Thus this 4

5 stress doesn t pose any severe threat to the component and the value can be of great assistance in calculation of fatigue life of the shaft. VI. CONCLUSION 1) Both crankcase and crankshaft were modeled in CATIA and the meshing and analysis was done in HYPERMESH and NASTRAN respectively. 2) The stress concentration point found in crankcase analysis showed close resemblance with the actual model. Thus it can be stated that FEA is reliable tool to reduce time consuming theoretical work with appreciable accuracy. 3) Modified crankcase analysis proved to be safe with respect to maximum allowable stress. Further analysis can be done such as dynamic analysis to support the safety of design with the help of FEA. 4) Analysis of crankshaft showed higher values of stresses at a very small area which can be neglected and the design can be considered safe. There is a huge scope for optimization of crankshaft as very less or no stress is generated in other location. REFERENCES [1] Jaimin Brahmbhatt, Prof. Abhishek Choubey, 2012, Design and Analysis of Crankshaft for Single Cylinder 4-Stroke Deisel Engine, International Journal of Advanced Engineering Research and Studies, Volume 1, Issue 4 E-ISSN [2] Jian Meng, Yongqi Liu, Ruixiang Liu,2011, Finite Element Analysis of 4-Cylinder Diesel Crankshaft, I.J. Image, Graphics and Signal Processing, 5, [3] Gu Yingkui, Zhou Zhibo,2011, Strength Analysis of Diesel Engine Crankshaft Based on PRO/E and ANSYS, Third International Conference on Measuring Technology and Mechatronics Automation [4] Xiaorong Zhou, Ganwei Cai, Zhuan Zhang, Zhongqing Cheng, 2009, Analysis on Dynamic Characteristics of Internal Combustion Engine Crankshaft System, International Conference on Measuring Technology and Mechatronics Automation [5] Farzin H. Montazersadgh and Ali Fatemi, 2007, Dynamic Load and Stress Analysis of a Crankshaft, SAE Technical Paper No , Society of Automotive Engineers 5