Arjstme 03 006

Advanced Research Journals of Science and Technology

ADVANCED RESEARCH JOURNALS OF SCIENCE AND TECHNOLOGY

(ARJST)

FATIGUE LIFE PREDICTION OF A FREE-PISTON ENGINE MOUNTING USING FINITE ELEMENT METHOD

2349-1845

K.Sudhavani 1, SK.Bazani 2, 1 Research Scholar, Department of Mechanical Engineering,Eswar College of Engineering, Narasaraopet, Guntur,India.

2 Assistant professor , Department of Mechanical Engineering, Eswar College of Engineering, Narasaraopet, Guntur,India.

Abstract The present study details the fatigue life prediction of a new free piston linear generator engine mounting using finite element method. The objective of the work is to assess the critical fatigue locations of the component due to cyclic loading conditions. The effect of mean stress on the fatigue life has also been investigated.

Materials SAE 1045-450-QT and SAE 1045-595-QT are considered to represent the free piston linear generator engine mounting. The finite element modelling and analysis was carried out by computer-aided design software (Uni Graphics) and ANSYS Fatigue module respectively, in addition to this, fatigue life prediction for free piston linear generator engine mounting was also carried out. Total-life approach and Crack initiation approach have been applied to predict the fatigue life of the free-piston linear engine mounting.

The results shows the contour plots of fatigue life and damage histogram at the most damaged case. The comparison between the total–life approach and crack initiation approach were also been investigated. From the results, it can be concluded that Marrow mean stress correction method gives most conservative (exclusively for less life) results for crack initiation method. It can be concluded that material SAE 1045-595-QT gives constantly higher life than material SAE1045-450-QT for all loading conditions under both methods. *Corresponding Author: K.Sudhavani, Research Scholar, Department of Mechanical Engineering, Eswar College of Engineering, Narasaraopet, Guntur,India.

Published: January 04, 2016 Review Type: peer reviewed Volume: III, Issue : I Citation: K.Sudhavani,Research Scholar (2016) FATIGUE LIFE PREDICTION OF A FREE-PISTON ENGINE MOUNTING USING FINITE ELEMENT METHOD

magnetic field (containing coils), and an electromagnetic force (EMF) will be induced in the coils if the movement of the rod causes a disturbance of the field. The main principle of the free-piston generator is producing electricity directly from the linear motion of the pistons. The crank shaft, is eliminated as it is normally required in conventional hybrid concepts.

The disappearance of the crankshaft has better aspects.

The friction losses associated with the crankshaft, the conventional connecting rod, and their accessories are eliminated. As piston is no longer under the influence of an angular loading, piston friction is reduced. As the number of moving parts is reduced to one the system also becomes more robust

INTRODUCTION The hybrid vehicle concept is environmentally friendly, highly efficient, and is gaining popularity by the day. This is the main reason for why most vehicle manufacturer are investing in the emerging hybrid vehicles market. This leads to a heavy competition among vehicle manufacturers, they in turn stresses the engineers and researchers working with alternative vehicles and to get better and newer vehicles. The main important requirements are high specific performance, increased system efficiency, reduced number of system components, etc.

Out of all free-piston generator concept is one of the best and relatively new (and still emerging) hybrid vehicle concepts that could give good solutions to some of these requirements with an electrical generator. This is shown in Fig.1.1. The rod which acts as a prime mover for the generator that connects the two oppositely placed combustion chambers.

The connecting rod has an oscillating motion, in the two chambers cause due to the reciprocating, ignition and compression processes. Now, if this rod is placed in a

The integrated engine generator

As the engine compression ratio is now no longer fixed, at least theoretically, multi-fuel operation is enabled.

Achieving a variable compression operation for the same fuel type is difficult. A modular design approach with several distributed units would also become possible, offering redundancy and improved reliability, allowing an application in military or other operation critical vehicles.

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Uncertainties associated with this concept are the thermal insulation between the combustion chamber and the generator portions, the excessive and repetitive forces that act on the moving rod, high expectations upon reductions in system costs and improvements in the specific performance. Much of these uncertainties are expected to vanish in the near future, with the maturity of the free-piston concept technology.

Free-piston engine basics Free-piston engine basics due to the breadth of the freepiston term, many engine configurations will fall under this category. The free-piston term is most commonly used to distinguish a linear engine from a rotating crankshaft engine. The piston is ‘free’ because its motion is not restricted by the position of a rotating crankshaft, as known from conventional engines, but only determined by the interaction between the gas and load forces acting upon it.

This gives the free-piston engine some distinct characteristics, including Variable stroke length and The need for active control of piston motion.

power generating magnetic flux within the machine. The efficiency of electric machinery is, however, generally very high.

The load force of a permanent magnet electric machine coupled to a purely resistive load will be proportional to the translator speed, although other designs or the implementation of power electronics may allow variations on this. Air compressors were the original free-piston load devices but are not necessarily better suited for this purpose than the other two.

The variable stroke of the free-piston engine may lead to poor volumetric efficiency of the air compressor when operating at varying load levels. If operating with atmospheric inlet pressure, a large compressor cylinder is needed resulting in a large and heavy construction. The load profile of an air compressor is like that of a gas filled bounce chamberin the compression phase and with an approximately constant load force when the discharge valves are open towards the end of the stroke.

Other important features of the free-piston engine are potential reductions in frictional losses and possibilities to optimize engine operation using the variable compression ratio.

Free-piston loads The free-piston engine requires a linear load, and for the overall system to be efficient the load must provide efficient energy conversion. The rotating power source, such as internal combustion engines and turbines, has been the standard for many years within electric power generation but also rotating hydraulic and pneumatic machinery are highly developedTechnologies. A challenge for free-piston engine developers is to find linear equivalents of these machines with comparable performance.

The mechanical requirements for free-piston engine load devices are high since the load is coupled directly to the mover, and the load will be subjected to high acceleration forces. Secondary effects from the high accelerations such as cavitation in hydraulic cylinders must also be considered. Furthermore, the load device may be subjected to heat transfer from the engine cylinders. Known freepiston engine loads include electric generators, hydraulic pumps and air compressors. The dynamic properties of these differ widely.

Important factors when determining the feasibility of a linear load for a free-piston engine are: Moving mass, physical size, efficiency and load force profile.

The following characteristics are typical for the mentioned load devices.

Hydraulic pumps typically work against a high discharge pressure. Combined with the incompressible working fluid, this allows a small unit with very low moving mass.

The efficiency of such units is generally high and high operational flexibility has been demonstrated Using electronically controlled hydraulic control systems with fastacting valves in free-piston engines.

Electric generators can be relatively compact in size but often suffer from a high moving mass due to magnets or back iron in the mover, required to supply or direct the

The typical load characteristics of free-piston engine

FINITE ELEMENT BASED FATIGUE ANALYSIS Despite the fact that most engineers and designers are aware of fatigue and that a vast amount of experimental data has been generated on the fatigue properties of various metallic and non-metallic materials, fatigue failures of engineering components are still common. A number of factors influence the fatigue life of a component in service, viz., (i) complex stress cycles, (ii) engineering design, (iii) manufacturing and inspection, (iv) service conditions (v) material of construction. The use of calculations and simulations is a key feature of the modern design process.

Several properties such as stress, strength, stiffness, durability, handling, and ride comfort and crash resistance are to be achieved.

Fatigue has become progressively more prevalent as technology has developed a greater amount of equipment, such as automobiles, aircraft, compressors, pumps, turbines, etc., subject to repeated loading and vibration, until today it is often stated that fatigue accounts for at least 90 percent of all service failures due to mechanical causes. A fatigue failure is particularly insidious because it occurs without any obvious warning. The failure usually occurs at a point of stress concentration such as sharp corner or notch or at a metallurgical stress concentration like an inclusion.

The basic factors are necessary to cause fatigue failure.

These are (1) a maximum tensile stress of sufficiently high value, (2) a large enough variation or fluctuation in the applied stress, and (3) sufficiently large number of cycles of the applied stress. In addition, there are a host

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of other variables such as stress concentration, corrosion, temperature, overload, metallurgical structure, residual stresses, and combined stresses, which tend to alter the conditions for fatigue. At the outset it will be advantageous to define briefly the general types of fluctuating stresses which can cause fatigue.

The typical fatigue stress cycles are broadly divided into completely reversed cycle of stress of sinusoidal form.

This is an idealized situation which is produced by an R.R. Moore rotating- beam fatigue machine and which is approached in service by a rotating shaft operating at constant speed without overloads. For this type of stress cycle the maximum and minimum stresses are equal.

Minimum stress is the lowest algebraic stress in cycle.

Tensilestress is considered positive, and compressive stress is negative. Repeated stress cycles in which the maximum stress and minimum stress are not equal also exist. A fluctuating stress can be considered to be made up of two components, a mean or steady, stress σm and an alternating or variable stress σa. We also consider the range of stress σr.

A free piston generator integrates a combustion engine and a linear electrical machine into a single unit without a crank shaft. This provides an unconventional solution for a series of hybrid vehicles and emergency power units.

Blarigan(2000) developed the free piston alternator in the 30kW range. Here a non- conventional combustion technique known as homogeneous charge compression ignition (HCCI). The schematic diagram of free piston linear engine is shown in fig1. The absence of the crankshaft has benefits in the reliability, efficiency, fuel consumption and environmental emissions. Use of free piston generators to produce electricity with the Stirling engines has been around for quite some times. Applications with the internal combustion engines are relatively new. Arshad et al. (2004) investigated.

Mechanical and cyclic properties the materials Properties

Materials SAE1045-450-QT

SAE 1045-595-QT

Yield strength(Mpa)

1515.00

1860.00

Ultimate tensile strength(Mpa)

1584.00

2239.00

Elastic modulus(Mpa)

207000

207000

Fatigue strength coefficient(Sf)

1686.00

3047.00

Fatigue strength exponent(b)

-0.06

-0.10

Fatigue ductility exponent(c)

-0.83

-0.79

Fatigue ductility coefficient(εf’)

0.97

0.13

Cyclic strain hardening exponent(n’)

0.09

0.10

Cyclic strength coefficient(k )

1874.00

3498.00

Shows comparison between the two materials with respect to S-N behaviour. It can be seen that these curves exhibit different life behaviour depending on the stress range experienced. From the fig.3.6, it is observed that in the long life area (high cycle fatigue), the different is lower while in the short life area (low cycle fatigue), the difference is higher.

Fatigue analysis prediction strategy The fatigue analysis is used to compute the fatigue life at one location in a structure. For multiple locations the process is repeated using geometry information applicable for each location. Necessary inputs for the fatigue analysis are shown in fig 3.1. The three inputs information boxes are descriptions of the material properties, loading history and local geometry.

Material Data

Stress-Life (S-N) plot

The material data is one of the major input, which is the definite of how a material behaves under the cyclic loading conditions it typically experiences during services operation. Cyclic material properties are used to calculate elastic-plastic stress strain response and the rate at which fatigue cycle. The materials parameters required depend on the analysis methodology being used. Normally these parameters are measured experimentally, and available in various hand books. Two different materials were used for this component.SAE1045-450-QT and SAE1045-595-QT.The mechanical and cyclic properties of materials SAE1045-450-QT and SAE1045-595-QT are shown in table given below Cyclic stress-strain life Plot for SAE1045-450-QT

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of mounting were shown in the following figures.

Geometry model of free piston engine mounting strain-life plot

Loading Data Loading is another major input for the finite element based fatigue analysis. Unlike static stress, which is analyzed with calculations for a single stress state, fatigue damage occurs when stress at a point changes over time.

There are essentially four classes of fatigue loading, with the ANSYS Fatigue Module currently supporting the first three: • Constant amplitude, proportional loading • Constant amplitude, non-proportional loading • Non-constant amplitude, proportional loading • Non-constant amplitude, non-proportional loading

RESULTS AND DISCUSSIONS The linear static finite element analysis was performed using ANSYS FEM Module. The equivalent von Misses stress contours and critical locations are shown in Fig The bolts holes and fillet areas were found to be areas of high stresses. The von-misses equivalent stresses are used for subsequent fatigue life analysis and comparisons. From the analysis results, the maximum von misses stress of 122.28 MPa was obtained.The fatigue life of the free piston engine mounting is obtained using variable amplitude loading conditions by mean of SAETRN and SAEBRAKT data set. The fatigue life prediction results of mounting were shown in the following figure for corresponding to SAETRN load history. It can be observed that the predicted fatigue life at most critical location near the bolt edge is 4.7826e+005 seconds when using SAE1045450-QT material with no mean stress. The fatigue equivalent unit is 3000 cpm (cycles per min) of time history.

The three-dimensional cycle histogram and corresponding damage histogram for materials SAE1045-450-QT using SAETRN loading histories is shown in the figures 4.3 and 4.5 given below. Fig: 4.3 shows the results of the rain flow cycle count for the component.

Finite element analysis In the two stroke free-piston engine system design, the mounting structure is among the most critical parts. Numerical techniques are necessary to simulate the physical behavior and to evaluate the structural integrity of the different designs. The objective of the current study are to calculate the fatigue life for a mounting of linear engine using total life and crack initiation methods, to investigate the effect of mean stress on fatigue life and the probabilistic nature of fatigue on the S-N curve via the design criteria. Table shows the Geometry information about free piston engine mounting.

Three-dimensional model of linear generator engine mounting was developed and analysis was carried out using ANSYS workbench software. A parabolic tetrahedron element was used for the solid mesh using patch conforming method is employed. Convergence of stresses was observed, as the mesh size was successively refined. A total of 34924 elements and 61425 nodes were generated at 0.68mm element length. The constraints were applied on the bolt-hole for all six degree of freedom. Geometry, loading and boundary conditions used for the FE analysis

It can be seen that a lot of cycles with a low stress range and fewer with a high range. The height of each tower represents the number of cycles at that particular stress range and mean. Each tower is used to obtain damage on the S-N curve and damage is summed over all towers. Fig 5.5 shows that lower stress ranges produced zero damage.

It is also showed that the high stress ranges were found to give the most of the damage and a fairly wide damage distribution at the higher ranges which mean that it cannot point to a single event causing damage. Most realistic service situations involve nonzero mean stresses, it is, therefore, very important to know the influence that mean stress has on the fatigue process so that the fully reversed (zero mean stress) laboratory data are usefully employed in the assignment of real situations.Four types of mean stress correction method are considered in this study i.e.

Goodman and Gerber correction methods for total-life approach and SWT and Marrow methods for crack initiation approach. The predicted fatigue life at most critical locations are tabulated in table 4.5 and 4.6 respectively, using different materials and approaches.

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FUTURE SCOPE In the present study the model is considered as a single unit but, it is not possible to manufacture in this manner.

Generally these are assembled using weld joints. Depending upon the weld geometry the fatigue locations may shift from bolt holes to the weld areas. These areas to be analyzed further.

REFERENCES Von Misses stresses distribution contours

[1] Achten PAJ. A review of free piston engine concepts, SAE Paper 941776, 1994.

[2] Tikkanen S, Lammila M, Herranen M, Vilenius, M. First cycles of the dual hydraulic free piston engine, SAE Paper 2000-01-2546, 2000.

[3] Fleming JD, Bayer RJ. Diesel combustion phenomena as studied in free piston gasifiers, SAE Paper 630449, 1963.

[4] Baruah PC. A free-piston engine hydraulic pump for an automotive propulsion system, SAE Paper 880658, 1988.

Fatigue life of free piston engine mounting in stress life method.

CONCLUSIONS

[5] Uludogan A, Foster DE, Reitz RD. Modeling the effect of engine speed on the combustion process and emissions in a DI Diesel engine, SAE Paper 962056, 1996.

A computational numerical model for the fatigue life assessment for mounting of the linear generator engine is presented in this study. Through the study, several conclusions can be drawn with regard to the fatigue life of a component when subjected to complex variable amplitude loading conditions.

[6] Aichlmayr, H.T., 2002.design consideration, modeling and analysis of micro-homogenous charge ignition combustion free-piston engine.Ph.D Thesis, university of Minnesota, USA.

The fatigue life was estimated based on Palmgren-Miner rule is non-conservative SWT correction and Morrows methods, and damage rule can be applied to improve the estimation.

It can be seen that when using the loading sequences are predominantly tensile in the nature; the life of mounting in Goodman approach is 1.46x105 sec which is more conservative.

It can be seen that when using the loading sequences are predominantly zero mean (SAEBRAKT), the value of life of the mounting is 2.5x105 sec in Gerber mean stress correction which has found to be more sensitive.

It can be concluded that the influence of mean stress correction is more sensitive to tensile mean stress for total life approach.

It is also seen that the two mean stress methods give lives less than that achieved using no mean stress correction.

It is concluded for crack initiation approach that when the loading is predominantly tensile in nature, the life of the component in SWT approach is 36.023x105sec which is more sensitive and is therefore recommended.

[7] Blarigan,P.V.,2000.Advanced internal combustion engine research.Procedings of 2000 DOE Hydrogen Prog. rev,:NREL/CP-570-28890,pp;1-19.

[8] Hibi A, Ito T. Fundamental test results of a hydraulic free piston internal combustion engine, Proc. Inst. Mech.

Eng., 2004:218:1149–1157.

[9] Flynn G Jr. Observations on 25,000 hours of freepiston-engine operation, SAE Transactions 1957:65:508– 515.

[10] Braun AT, Schweitzer PH. The Braun linear engine, SAE paper 730185, 1973.

[11] Achten PAJ, van den Oever JPJ, Potma J, Vael GEM. Horse power with brains: The design of the Chiron free piston engine, SAE Paper 2000-01-2545, 2000.

[12] Johansen TA, Egeland O, Johannesen EA, Kvamsdal R. Dynamics and control of a free-piston diesel engine, ASME J. Dynamic Systems, Measurement and Control, 2003:125:468–474.

[13] Clark N, Nandkumar S, Atkinson C, Atkinson R, McDaniel T, Petreanu, S et al. Modelling and development of a linear engine, ASME Spring Conference, Internal Combustion Engine Division,1998:30(2):49–57.

When using the time histories has zero mean (SAEBRAKT) then all three methods have been given approximately the same results.

[14] Nasar S.A. &Boldea I., Linear Electric Motors, New Jersey, Prentice-Hall Inc.,1987.

It can be also seen that SAE 1045-595-QT is 30% higher life than SAE 1045-450-QT for all cases.

[15] Galileo Research Inc (USA) web site, www.galileoresearch.com ///15

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Author

K.Sudhavani, Research Scholar, Department of Mechanical Engineering, Eswar College of Engineering, Narasaraopet, Guntur,India.

Sk.Bazani, Assistant professor, Department of Mechanical Engineering, Eswar College of Engineering, Narasaraopet, Guntur,India.

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