DESIGN AND MBD SIMULATION OF IC ENGINE VALVE TRAIN AND FEA ANALYSIS OF ROCKER ARM
Mohd Azhar Merchant updated on Jun 18, 2020

DESIGN AND MBD SIMULATION OF IC ENGINE VALVE TRAIN AND FEA ANALYSIS OF ROCKER ARM:

AIM:

To model and assemble components of IC Engine Valve Train and to perform Motion analysis and to plot the various lifts and contact forces using SOLIDWORKS MOTION and to do FEA analysis of Rocker Arm.

DESCRIPTION:

A valvetrain or valve train is a mechanical system that controls the operation of the intake and exhaust valves in an internal combustion engine. The intake valves control the flow of air/fuel mixture (or air alone for direct-injected engines) into the combustion chamber, while the exhaust valves control the flow of spent exhaust gasses out of the combustion chamber once combustion is completed.

SIGNIFICANCE OF THIS SIMULATION:

By performing the motion analysis of the Valvetrain it helps to determine the displacement of the valve by knowing the cam lift from the cam profile which will help us to control the flow of the air/fuel mixture into the combustion chamber in order to design the engine and to determine the various contact forces occurring during the motion and ways to reduce it.

OBJECTIVES:

1) Model and assemble Parts in SOLIDWORKS.

2) To run the motion analysis simulation using the below parameters

 Sl.No CAM Lift (mm) Speed (RPM) Material 1 3.5 1500 Cast Carbon Steel 2 6 1500 Cast Carbon Steel

And to Obtain the plots for

1. Valve Lift.

2. Push Rod Lift.

3. The contact force between

Cam and Push Rod

Pushrod and Rocker Arm

Rocker Arm and Valve

3) To perform FEA Analysis of Rocker Arm at the point where the Cam and Valve are having the maximum lift and contact force.

Components:

Engine valves are used in engines to allow air or air-fuel mixture to enter into the cylinder (through inlet valve) and also to push exhaust gases (through outlet valve) from the cylinder at a specific time during the engine cycle. Typical components are listed below in order from the crankshaft to the valves.

Camshaft

Camshaft is a shaft in which cams are provided as integral part of the shaft. Separate cam is provided for each valve (inlet and exhaust). Cam has its path profile having a high spot known as nose and the lowest point termed as base. These two points nose and base corresponds to fully opened and closing of valves respectively. The cam profile is provided to have a smooth rise (opening) and fall (closing) of valves. The cam is a rotating element that generates the reciprocating motion to the follower which is known as push rod in a plane at right angle to the cam axis. Camshaft is also used to drive fuel pumps. camshaft is made up of forged alloy steel which is case hardened.

Multiple Cams mounted on Camshaft

Cams used are different for 2 Cases:

CASE 1:

When we require cam lift of 3.5mm

We have our lower circle Radius R1=12.5mm

And Upper circle Radius R2= 5mm

Cam Lift = (L – R1) + R2

3.5= (L – 12.5) + 5,

Distance between the two centres L = 11mm.

Below is our cam profile which is obtained by drawing two circles of the given radius and then providing a distance between the two centers and after completion of the sketch, it is extruded along the midplane by 10mm.

CASE 2:

When we require cam lift of 6mm

We have our lower circle Radius R1=12.5mm

And Upper circle Radius R2= 5mm

Cam Lift = (L – R1) + R2

6= (L – 12.5) + 5,

Distance between the two centers L = 13.5mm.

Below is our cam profile which is obtained by drawing two circles of the given radius and then providing a distance between the two centers and after completion of the sketch, it is extruded along the midplane by 10mm.

Pushrod:

Pushrods are long, slender metal rods that are used in overhead valve engines to transfer motion from the camshaft (located in the engine block) to the valves (located in the cylinder head). The reciprocating motion of the push rod actuates the rocker arm which moves in an arc about its pivot. The bottom end of a pushrod is fitted with a lifter, upon which the camshaft makes contact. The camshaft lobe moves the lifter upwards, which moves the pushrod. The top end of the lifter pushes on the rocker arm, which opens the valve.

Here the sketch of one side was made and revolution boss/ base was selected to convert the sketch to 3D

Rocker arm / bucket tappet:

The rocker arm is used to convert the upward movement of push rod to the downward movement of the valve (opening up) and also the downward movement of pushrod to upward movement of valve (closing down). The rocker arm can be made of either hollow or solid material. The rocker arms are pivoted to rocker shaft which also provides lubrication

Here the sketch is made by using circles, lines and fillets were added to the sketch for reducing the stress concentration and sketch was extruded in the midplane by 20mm.

Valve Spring:

Valve spring (helical) is used to keep valve in regular contact with the tappet and the tappet with the cam. These valve springs should be fatigue resistant and are made up of high-grade spring steel wire, hard-drawn steel or chrome-vanadium steel. The valve spring will be added during the motion simulation.

Valve and Valve mount:

Although various types of valves are available but poppet valves are the most commonly used engines valves being used in engines these days. These poppet valves are also known as mushroom valves as its shape resembles to the mushroom having head and stem. Generally, inlet valves are larger in size to enhance the air intake and exhaust valves are made relatively harder than the inlet valves as these are exposed to hot exhaust gases always. Exhaust valves are made up of special material i.e. austenitic steels and precipitation hardening steel which should have high strength to resist tensile loads, wear resistance due to heat, corrosion resistance, low coefficient of thermal expansion and high thermal conductivity. Since, the valve temperature reaches to several hundred (7500C), valve cooling is also essential for which valve stems are sometimes filled with sodium (which has high thermal conductivity).

Here the sketch is made by using circles, lines and fillets were added to the sketch for reducing the stress concentration and revolution boss/ base was selected to convert the sketch to 3D.

ASSEMBELY:

1. FILE>NEW>ASSEMBLY >OK, first drag all the parts to one screen, as we know the first part will always be fixed by default hence right click on that part and select float.
2. Enable all the temporary axis.
3. First bring all the parts to front plane of assemble by applying coincident mate between the front plane on the assembly and front plane of all components.
4. For Rocker arm coincide the axis of the circular hole with the Right plane of the assembly and also the top plane.
5. For cam the case circle temporary axis is selected and the Right plane of the assembly is selected and provided a distance mate of 20mm. and from the top plane the distance mate of 125mm is provided with the axis of base circle.
6. Similarly provide a distance mate of 20mm for the temporary axis of the push rod and the Right plane of the assembly.
7. The temporary axis of the Valve and the Valve mount is made to coincide with each other and the distance mate is provided to the face of the valve mount with the top plane to be 60mm.
8. For both Valve and Valve mount the temporary axis is selected are provided with distance mate of 50mm from the Right plane of the assembly.
9. Using the Move component > physical dynamics feature we can make the parts to touch each other with a very small clarence between them.

MOTION ANALYSIS:

1) Click on the SOLIDWORKS add-in > SOLIDWORKS MOTION, Change the animation to motion analysis, and enable all the temporary axis.

2) Click on add motor and select the temporary axis of the CAM and enter 1500 rpm clockwise and click ok.

3) Click on settings and change the frames per sec to 9000 and enable precise contact. Since if we have a smaller value of fps then there is a chance, we may miss the maximum and minimum values of the valve and cam lifts.

4) Click of the contacts and apply it between 1. cam and push rod 2. push rod and rocker arm  3. rocker arm and valve and choose material as steel dry for all.

5) Choose all the components by dragging and right-click to assign material > Assign Cast Carbon steel.

Material properties of Cast carbon steel is shown below:

6) Now the spring is added between the valve mount and the valve to keep the valve in regular contact with the pushrod and the push rod with the cam. Springs used are different for both the cases since the Cam profile is different in both the cases

for case 1, Spring used is of 13 N/mm and with a free length of 47mm since the spring needs to be pretension, that means the spring will be compressed when the valve is closed.

for case 2, Spring used is of 3 N/mm and with a free length of 36.5mm, since the spring needs to be pretension, that means the spring will be compressed when the valve is closed. In this case, we can see that the spring constant and the free length required is less since the cam lift is more if we add a spring with the higher stiffness and/or greater free length then all the parts in the assembly will fall apart and if lower stiffness and/or lesser free length will result in no opening and closer of the valve.

7) Enable gravity and hit on calculate.

PLOTS AND RESULTS:

CASE 1: CAM LIFT= 3.5mm

1) Linear Displacement of the push Rod V/s Time:

2) Linear Displacement of Valve V/s Time

3) Contact force b/w Cam and Push Rod V/s Time:

4) Contact force b/w Push Rod and Rocker Arm V/s Time:

5) Contact force b/w Rocker Arm and Valve V/s Time:

6) Contact force b/w Rocker Arm and Valve along X-direction V/s Time:

CASE 2: CAM LIFT= 6mm

1) Linear Displacement of the push Rod V/s Time:

2) Linear Displacement of Valve V/s Time:

3) Contact force b/w Cam and Push Rod V/s Time:

4) Contact force b/w Push Rod and Rocker Arm V/s Time:

5) Contact force b/w Rocker Arm and Valve V/s Time:

6) Contact force b/w Rocker Arm and Valve along X-direction V/s Time:

FEA ANALYSIS OF ROCKER ARM:

1) Since the Rocker arm is having continuous fluctuating motion and is in continuous loads on both the side, from one side it is applied with a continuous spring force and from the other side it experiences stresses from the pushrod hence we will be doing the FEA ANALYSIS of the rocker arm by exporting the motion load In the solver deck to perform FEA Simulation in order to determine the stresses and deformation on the rocker arm.

2) From the contact force plots, we can see that at the maximum lift of the Cam and maximum Valve lift the Contact forces acting on the rocker arm are maximum at that point, and during first rotation of cam the maximum contact forces occurs at 0.31333 sec. so we will be finding the stresses and the deformation at that point.

3) After doing the motion analysis, we will click on the SOLIDWORKS Simulation and click on simulation setup which helps us to setup the structural simulation using motion loads (which exports all the loads to do the FEA Analysis)

4) After clicking on the simulation setup new window will get opened which will ask to select the component so we will select the rocker arm and the add time step of 0.031333sec for Case 1 and 0.03177sec for Case 2 and click on add time in advanced options leave the mesh quality to default. Now we will hit on simulation results, now at this point, it will take the mesh and determine the nodal displacements at all the nodes by inversion of the Stiffness matrix ad determine the nodal displacements. [K].[U]=[F]

5) Now when it says calculated. in the Postprocessing step, we can now determine the stress and deformation by clicking on the stress plot and deformation plots.

CASE 1:

STRESS CONTOUR:

at 0.031333sec time step

We can see that the maximum stress on the Rocker Arm is 0.589MPa which is less compared to (yield strength/fos) = (248/2)=124Mpa

Hence the component is safe.

DEFORMATION CONTOUR:

CASE 2:

STRESS CONTOUR:

at 0.03177sec time step

We can see that the maximum stress on the Rocker Arm is 0.583MPa which is less compared to (yield strength/fos) = (248/2)=124Mpa

Hence the component is safe.

DEFORMATION CONTOUR:

CONCLUSION:

1) From the Linear displacement plots of both Push Rod and the Valve, we can see that in 1st case displacement of the Pushrod is 3.6mm and Valve displacement/lift is 2mm, for the 2nd case displacement of the Pushrod is 6mm and Valve displacement/lift is 4mm,

2) From the Contact force plots b/w the cam and push rod and also the contact force plots b/w the pushrod and rocker arm in both cases, we can see that both the graphs behave the same and we can see that the maximum contact force occurs when the cam is in the vertical position and the push rod is in the Extreme position. Contact forces can be reduced by using spherical contact type pushrod.

3) From the Contact force plot between Rocker Arm and Valve for magnitude plot we can see that it attains a maximum value at maximum valve lift and as the cam rotates it comes to a constant value equal to k.x (spring constant*spring compressed length) For 1st case, we have 13(N/mm)*12(mm)=156N constant value of contact force acting and for 2nd case, we have 3(N/mm)*1.65(mm)=4.95N of force acting constantly on the rocker arm.

4) The Contact force plot b/w Rocker arm and Valve magnitude when compared to the plot in X-direction, we can see that in the latter case, it takes into account the contact force in x-direction due to friction and hence it has the -ve value initially as the pushrod rises as the rocker arm rubs against the valve face in one direction and +ve value as the pushrod falls the rocker arm rubs against the valve face in other direction. The magnitude graph does not take into account the -ve sign, it takes  magnitude of both y-direction and x-direction contact forces hence we can see only +ve values in the graph.

5) As seen from the FEA results is that the stress and deformation are well under the limits and the component is safe.

REFERENCES:

http://ecoursesonline.iasri.res.in/mod/page/view.php?id=672

https://www.wikipedia.org/

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