Australian Journal of Basic and Applied Sciences, 5(12): 1830-1838, 2011 ISSN 1991-8178

Corresponding Authors: Mohammad Reza Asadi Asad Abad, Department of Mechanical Engineering, Islamic Azad University, Buinzahra branch, Qazwin, Iran. E-mail: Asadi_reza2007@yahoo.com

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Dynamic Load Analysis and Optimization of Connecting Rod of Samand Engine

1Mohammad Reza Asadi Asad Abad, 1Mohammad Ranjbarkohan and 1Behnam Nilforooshan Dardashti

1Department of Mechanical Engineering, Islamic Azad University, Buinzahra branch, Qazwin, Iran.

Abstract: Samand is one of the numerous vehicles in Iran. Also engine of this vehicle is national engine of Iran. According to these and high expensive repair and replacement of slider-crank mechanism parts and their effect on the other parts like cylinder block and piston it is necessary doing a complete research about slider-crank mechanism of Samand engine. In this regard, this paper presents the cinematic and kinetic analyses of the crank mechanism, stress and fatigue analysis and finally optimization of connecting rod of Samand engine. For this purpose the slider-crank mechanism was simulated in MSC/ADAMS/Engine software and forces acting on different parts of crank mechanism were extracted after that connecting rod was simulated in ANSYS software, critical loads were exerted on it and stress and fatigue analysis was done. Finally according to results optimization of connecting rod was done. Key words: Samand, Engine, Slider-crank Mechanism, Kinematics and Kinetic Analysis, Stress and

Fatigue.

INTRODUCTION

The automobile engine connecting rod is a high volume production, critical component. It connects reciprocating piston to rotating crankshaft, transmitting the thrust of the piston to the crankshaft. Every vehicle that uses an internal combustion engine requires at least one connecting rod depending upon the number of cylinders in the engine. Beside these points Samand engine is national engine of Iran and also Samand is one of numerous vehicles of Iran.

According to above reasons, it is only logical that optimization of the connecting rod for its weight or volume will result in large-scale savings. It can also achieve the objective of reducing the weight of the engine component, thus reducing inertia loads, reducing engine weight and improving engine performance and fuel economy.

The connecting rod is subjected to a complex state of loading. It undergoes high cyclic loads of the order of 108 to 109 cycles, which range from high compressive loads due to combustion, to high tensile loads due to inertia. Therefore, durability of this component is of critical importance. Due to these factors, the connecting rod has been the topic of research for different aspects such as production technology, materials, performance simulation, fatigue, etc. For the current study, it was necessary to investigate finite element modeling techniques, optimization techniques, developments in production technology, new materials, fatigue modeling, and manufacturing cost analysis. This brief literature survey reviews some of these aspects.

Webster et al. (1983) performed three dimensional finite element analysis of a high-speed diesel engine connecting rod. For this analysis they used the maximum compressive load which was measured experimentally, and the maximum tensile load which is essentially the inertia load of the piston assembly mass. The load distributions on the piston pin end and crank end were determined experimentally. They modeled the connecting rod cap separately, and also modeled the bolt pretension using beam elements and multi point constraint equations (Webster, W. D., 1983).

In a study reported by Repgen (1998), based on fatigue tests carried out on identical components made of powder metal and C-70 steel (fracture splitting steel), he notes that the fatigue strength of the forged steel part is 21% higher than that of the powder metal component. He also notes that using the fracture splitting technology results in a 25% cost reduction over the conventional steel forging process. These factors suggest that a fracture splitting material would be the material of choice for steel forged connecting rods. He also mentions two other steels that are being tested, a modified micro-alloyed steel and a modified carbon steel. Other issues discussed by Repgen are the necessity to avoid jig spots along the parting line of the rod and the cap, need of consistency in the chemical composition and manufacturing process to reduce variance in microstructure and production of near net shape rough part (Repgen, B., 1998).

El-Sayed and Lund (1990) presented a method to consider fatigue life as a constraint in optimal design of structures. They also demonstrated the concept on a SAE key hole specimen. In this approach a routine calculates the life and in addition to the stress limit, limits are imposed on the life of the component as calculated using FEA results (El-Sayed, M.E.M., and Lund, E.H., 1990).

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In this study, cinematic and kinetic analysis of crank mechanism also stress and fatigue analysis and finally optimization of connecting rod of Samand engine was done. Methods:

There is different ways for kinematics and kinetic analysis for example this job can be done by Newton's lows and different computer's software (J.E. Shigley, 2001; Meriam, J.L., 1998). In this project MSC. ADAMS/ Engine software was used to kinematics and kinetic analysis of slider-crank mechanism. For obtaining to this purpose crank mechanism was simulated in ADAMS/ Engine software. Figure 1 shows dynamic model of Samand engine in ADAMS/Engine software.

. Fig. 1: Dynamic Model of Engine in ADAMS/Engine.

Combustion chamber pressure curve was measured in Iran Khodro's power test lab. These experimental

data have been shown in Figure 2 by diagram. Data of these curves were exerted on piston in modeled mechanism in ADAMS/Engine software.

Fig. 2: Combustion Pressure in Different RPM and Load.

For stress analysis of connecting rod it was modeled and meshed in ANSYS (Ver. 9) software. Solid 92

element was considered to carry analyzing. This element is three dimensional with 10 nods. Also, this element related to Solid 72 is better specially, in problems with curve bounds had more accuracy, but it increases time need to solve problems (Jahed Motlagh, H., 2003). Figures 3-A and 3-B show modeled and meshed connecting rod in ANSYS software. Material qualification of C70S6 steel (used for this connecting rod) has been shown in table 1 (Anonymous, 2008). Also microstructure of C70S6 steel has been shown in figure 4.

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Fig. 3: A Modeled connecting rod in ANSYS ver9 software.

Fig. 3: B Meshed connecting rod in ANSYS ver9 software.

Fig. 4: Microstructure of C70S6 steel.

Table 1: Material qualification of C70S6 steel.

C (wt%)

Si (wt%)

Mn (wt%)

P (wt%)

S (wt%)

0.2%PS (N/mm2)

UTS (N/mm2)

EI %

R/A %

0.68/0.75 0.15/0.35 0.50/0.60 0.045 0.060/0.070 580 1000 12 15

To calculate stress, forces was exerted on corresponding parts in modeled connecting rod in ANSYS

software’s medium considering following notes: 1. Inertia forces were evenly exerted on pin end inner level (Kolchin, A., 1984). The value of these forces

was calculated using following formula:

)/(2

2mNlr

FP

sm

ii (1)

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Where iP is force per unit area (N/m2), sl is pin end width (m), iF is inertia force and mr is pin end mean

radius (m). 2. The force resulted from combustion pressure were sinusicaly exerted on pin end inner level (Kolchin, A.,

1984). The value of this force was calculated using following formula:

sin)2

(sm

gg lr

FP )/( 2mN (2)

Where gP is force per unit area (N/m2) and gF is force resulted from combustion (N).

3. The force resulted from falsifying of pin end’s linier and also from friction between linier and piston pin were evenly exerted on pin end inner level all situations. These forces cause pressure stress in linier and tensile stress in connecting rod. This pressure was calculated using following formula (Kolchin, A., 1984).

b

sisasisu

S

sisusisusu

totb

E

Udddd

E

Uddddd

P)/()())(( 22222222

(3)

Where tot is sum of initial diameter differences and diameter differences resulted from friction (m), sud is

pin end’s outer diameter (m), sid is pin end’s inner diameter (m), U is Poisson ratio and bs EE , is elasticity

module of connecting rod and linier (Pa). The value of pressure using above formula for MF-285 was obtained as 26.4 MPa, that this pressure was evenly exerted on pin end level (Kolchin, A., 1984).

4. To obtain stress resulted from preloading in crank end the force resulted from preloading each screw must be evenly exerted on backrest level of screws (Shenoy, P.S., 2005).

Results: Kinematics:

Piston and connecting rod acceleration are main results of kinematics analysis of crank mechanism. All accelerations increased which is cleared by increasing engine velocity. Because the connecting rod horizontal acceleration has major role on torque so it is very important. In figure 5, 6 and 7 piston and connecting rod acceleration have been shown. Because of low horizontal displacement it's observed that connecting rod vertical acceleration is nearly similar to piston acceleration.

-25000

-20000

-15000

-10000

-5000

0

5000

10000

15000

0 90 180 270 360 450 540 630 720

Crank angle(deg)

acce

lera

tio

n (

M/S

2)

1000RPM 3000RPM 4500RPM 6000RPM

Fig. 5: Piston Acceleration in Different rotational speeds. Kinetic:

Kinetic analysis was done for different rotational speeds. These speeds were 1000 RPM (slow rotational speed), 3000 RPM (middle rotational speed), 4500 RPM (maximum torque rotational speed) and 6000 RPM (maximum power rotational speed of engine). Figure 8 shows force on the piston due to ignition pressure in mentioned speeds.

In figure 9, 10, 11, 12 and 13 the total force on connecting rod, pin journal bearing force (vertical and horizontal), crankshaft torque and flywheel torque have been shown in mentioned speeds. From these figures we can get the following:

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1. Flywheel and clutch design: by average value and fluctuation ratio (flywheel design parameter) could design the suitable flywheel. Major of this fluctuation is absorbed by flywheel. The clutch could be design after flywheel designation and consideration of output torque and etc.

2. Resonance phenomena: Resonance phenomena are a main factor of damaging. The applying torque to the crankshaft is motivation factor to engine and even transmission system (after the fluctuation absorbed by flywheel). By using frequency and amplitude of fluctuation and natural frequency of parts like cylinder block and crankshaft (with flywheel), the design can be optimized for prevention from resonance.

3. Stress analysis: as the engine is damaged, stress analysis of different situation for some parts is essential. The fatigue analysis of moving part like connecting rod and crankshaft could be done by using the diagrams.

-20000

-15000

-10000

-5000

0

5000

10000

15000

20000

0 90 180 270 360 450 540 630 720

Crank angle(deg)

Acc

eler

atio

n (

M/S

2 )1000RPM 3000RPM 4500RPM 6000RPM

Fig. 6: Connecting rod Vertical Acceleration in Different rotational speeds.

-20000

-15000

-10000

-5000

0

5000

10000

15000

20000

0 90 180 270 360 450 540 630 720

Crank angle(deg)

Acc

eler

atio

n(M

/S2 )

1000RPM 3000RPM 4500RPM 6000RPM

Fig. 7: Connecting rod Horizontal Acceleration in Different rotational speeds.

0

5000

10000

15000

20000

25000

30000

35000

40000

0 90 180 270 360 450 540 630 720

crank angle(deg)

forc

e(N

)

1000RPM 3000RPM 4500RPM 6000RPM

Fig. 8: force on the piston due to ignition pressure in Different rotational speeds.

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-20000

-10000

0

10000

20000

30000

40000

0 100 200 300 400 500 600 700 800

crank angle(deg)

Forc

e(N

)

1000RPM 3000RPM 4500RPM 6000RPM

Fig. 9: Exerted forces on connecting rod in different speeds.

-60000

-50000

-40000

-30000

-20000

-10000

0

10000

20000

30000

0 90 180 270 360 450 540 630 720

crank angle(deg)

forc

e(N

)

1000rpm 3000rpm 4500rpm 6000rpm

Fig. 10: Pin Journal bearing Vertical Force in Different rotational speeds.

-8000

-6000

-4000

-2000

0

2000

4000

6000

8000

0 90 180 270 360 450 540 630 720

crank angle(deg)

Fo

rce(

N)

1000RPM 3000RPM 4500RPM 6000RPM

Fig. 11: Pin Journal bearing horizontal Force in Different rotational speeds.

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-2500

-2000

-1500

-1000

-500

0

500

1000

1500

2000

0 90 180 270 360 450 540 630 720

crank angle(deg)

torq

ue(

N.M

)

1000RPM 3000RPM 4500RPM 6000RPM

Fig. 12: Crankshaft torque in Different rotational speeds.

-1000

-800

-600

-400

-200

0

200

400

600

800

1000

0 90 180 270 360 450 540 630 720

Crank angle(deg)

torq

ue

(N.M

)

Flywheel crankshaft

Fig. 13: Comparison crankshaft torque and fly wheel torque in 4500 RPM rotational speed.

Stress:

As shown in figure 9 the maximum pressure force exerted on connecting rod is 37122 (N) and it occurs at 4800 RPM rotational speed (in 374 degree of crank angle) and the maximum tensile force is 9278 (N) and it occurs at 6000 RPM rotational speed (in 181 degree of crank angle). For calculating pressure and tensile stresses above forces were used.

The maximum pressure stress was obtained between pin end and connecting rod linkage. The value of this stress was 297.361 MPa (Fig. 14). The maximum tensile stress was obtained in crank end. The value of this stress was 202.927 MPa (Fig. 15).

According to table 1, ultimate strength ( u ) of C70S6 steel (used for this connecting rod) is 1000 MPa. So

factor of safety (F.S.) will be:

363.3361.297

1000..

all

ustresspressureSF

927.4927.202

1000..

all

ustresstensileSF

Fair factor of safety for mechanical tools is about 2 to 3 (M. Khanali, 2006), so calculated factor of safety is

fair for connecting rod under pressure and tensile loads.

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Fig. 14: Stress distribution in connecting rod, resulted from maximum pressure force considering Van Misses.

Fig. 15: Stress distribution in connecting rod, resulted from maximum tensile force considering Van Misses.

Fatigue:

For doing fatigue analysis and calculating lowest fatigue cycle, various critical nods in different speed and different parts of connecting rod were investigated for fatigue analysis. Among critical analyzed nods lowest fatigue cycle was calculated equal 109 cycles. Nodes whit this fatigue cycle were between pin end and connecting rod linkage. Figure 16 parts A and B show tow sample of analyzed nods for fatigue cycle (these nods located in rod of connecting rod and between pin end and connecting rod linkage).

Fig. 16: A Obtained results for fatigue calculation for a sample node in rod of connecting rod.

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Fig. 16: B Obtained results for fatigue calculation for a sample node between pin end and connecting rod linkage.

Conclusion:

According to this research below results can be drown: 1. Increasing the inertia forces and overcoming to combustion forces in high speed. 2. High loads are applied to engine in high speeds in comparison with other conditions. There is necessity to

stress, fatigue and frailer in these conditions. 3. Defining maximum speed of engine in electronic control unit (ECU) for preventing uncontrollable engine

speeds. 4. The factor of safety for pressure stress was obtained 3.363 and for tensile stress was obtained 4.927. Fair

factor of safety for connecting rods is about 2 to 3 (Kolchin, A., 1984), so calculated factor of safety is fair for connecting rod under pressure and tensile loads.

5. Least fatigue cycle was obtained equal 109 cycle. Common range of fatigue cycle for connecting rods is between 108 to 109 cycles (Kolchin, A., 1984). According to this point obtained fatigue cycle for Samand engine connecting rod is fair.

Calculated stresses and fatigue cycle for Samand connecting rod was fair but according to above results

following proposals can be offered for optimization and better resistance under hard loads. 1. Decreasing the diameter of rod. (stresses in this part of connecting rod were lower of common range and

also fatigue cycle for this part was higher than common range) 2. Lessen friction between piston pin and connecting rod bush. (The force caused by friction between piston

pin and connecting rod bush was high and so finding a method for decreasing this force can decrease critical stresses)

REFERENCES

Anonymous, 2008. "Samand Maintenance and Repayments catalogue". Iran Khodro Co. El-Sayed, M.E.M. and Lund, E.H., 1990. “Structural optimization with fatigue life constraints,”

Engineering Fracture Mechanics, 37(6) 1149-1156. Jahed Motlagh, H., M. Nouban and M.H. Ashraghi, 2003. "Finite Element ANSYS," University of Tehran

Publication, Tehran, 990. Khanali, M., 2006. "Stress analysis of frontal axle of JD 955 combine". M.Sc.thesis. Thran University, 124. Kolchin, A., V. Demidov, 1984. "design of Automotive Engines," MIR Publication. Meriam, J.L., L.G. Kraige, 1998. Engineering Mechanics, 5th Edition, New York, john willey, 712. Repgen, B., 1998. “Optimized Connecting Rods to Enable Higher Engine Performance and Cost

Reduction,” SAE Technical Paper Series, Paper No. 980882. Shenoy, P.S., A. Fatemi, 2005. "Connecting Rod Optimization for Weight and Cost eduction". Journal of

Sound and Vibration, 243(3): 389-402. Shigley, J.E., C.R. Mischke, 2001. "Mechanical Engineering Design", McGraw-Hill, New York. 776. Webster, W.D., R. Coffell and D. Alfaro, 1983. “A Three Dimensional Finite Element Analysis of a High

Speed Diesel Engine Connecting Rod,” SAE Technical Paper Series, Paper No. 831322.