MBD Simulation on IC Engine Valve Train

Aim: To create a valve train assembly in SOLIDWORKS and perform a motion study over two different cam lift profiles.

Objective: Perform the motion analysis with the design given in the table below

Sr. No	Cam Lift (mm)	Speed (rpm)	Material
1	3.5	1500	Cast Carbon Steel
2	6	1500	Cast Carbon Steel

Also find the Following:

1. Valve Lift.
2. The contact force between
   • Cam and Push Rod
   • Pushrod and Rocker Arm
   • Rocker Arm and Valve

Theory

The valve gear mechanism is a subassembly of the IC engine and its function is to open and close the inlet and exhaust valves at a proper time with respect to the position of the piston and crankshaft. The fuel is admitted into the cylinder when the inlet valve is open. Also, the burnt gases escape when the exhaust valve is open.

The main parts of a valve-gear mechanism are valve, spring, tappet, rocker arm, push rod, cam and camshaft.

First figure above shows the valve gear mechanism for a vertical engine with overhead valves. The Cam (along with the camshaft) is rotated by means of a belt drive from the crankshaft. As the camshaft rotates, the cam pushes the follower and the push rod upwards. The rocker arm is pivoted at its centre by means of a fulcrum pin. When the right end of the rocker arm is pushed up by the push rod, the left end moves downward. This compresses the spring and pushes the valve rod down in the cylinder, thereby causing the valve to open. When the follower moves over the circular portion of the cam, the spring expands and closes the valve. The spring pushes the left end of the rocker arm upward. This causes the right end to move downward and keeps the follower in contact with the cam.

It should be noted that the valve is closed by a helical compression spring. One end of the spring is secured to the valve stem and the other end rests on the cylinder. The spring is initially compressed. The force due to initial compression presses the valve down on its seat. However, the opening of the valve is due to oscillatory motion of the rocker arm.

Designing of Cam

`"Cam lift"=(L-R_1 )+R_2` where `R_1="radius of lower circle",R_2="radius of upper circle" , L="Centre distance between the two circles"`(See third image)

Based on the above Formula and keeping R₁ and R₂ same (i.e.) R₁ = 12.5mm and R₂ = 5mm (Assumed)

For lift of 3.5 mm `L=( 3.5 -5)+12.5=11 mm`

For lift of 6 mm `L=( 6 -5)+12.5=13.5 mm`

Keeping above design parameters in mind, two separate cams were modelled in SolidWorks which are shown.

All the bodies were specified with the materials of Cast carbon Steel.

Designing Cams (All Dimensions are in mm)

Designing Push rod and Rocker Arm (All Dimensions are in mm)

Designing Valve and Valve Mount (All Dimensions are in mm)

While Making all the above parts, the material properties were also changed to Cast Carbon Steel by right-clicking the materials menu in features manager, and then changing it to Cast Carbon Steel.

Making an Assembly (All Dimensions are in mm)

1. All the above parts were assembled together using required mates until the desired motion was achieved.
2. After the desired motion was achieved, the components were manually moved until the entire assembly was not under collided position.
3. Finally, the Move Component command was activated from assembly toolbar and it was set to Free drag and Physical Dimensions
4. Then the valve was moved upwards until it touched the rocker arm and through which the motion was carried towards the cam.
5. At one point after moving the valve, it was seen that there was no more movement in the entire assembly which clearly gave an indication that the contacts were achieved.
6. This made the entire assembly which was available for the motion study.

Creating Motion Study

Using Solid Works Motion Add-in, motion study was created for both the cases with following settings:

• Started with setting up of motor along the cam axis with Constant speed type at 1500rpm (CW).
• Setting up of Solid Body contacts over the entire assembly with material specified as steel (dry).
• Setting up of Spring by selecting the top face of valve seat and bottom face of valve head. To keep the spring in compressed position, the free length of the spring was reduced. Also, the spring stiffness was set to 10 N/mm.
  • In case of lift of 3.5 mm the free length was reduced to 40 mm.
  • In case of lift of 6 mm the free length was reduced to 50 mm.

• Also, the motion study settings were changed to Precise contacts with 9000 frames per second.
• As the set fps were so high the calculations were done only for 0.2 seconds.
• Finally, the plots were generated for the desired quantities which are explained below:

Case 1: Valve Lift of 3.5 mm

• It can be seen from the first plot that the linear displacement of valve was continuously changing between 53.4 mm to 50.5 mm for a complete revolution of cam. Thus, a lift of 2.9 mm was possible through this cam profile.
• One more important thing to note down here was that with 3.5 mm cam lift, 2.9 mm lift in valve was possible, this can be changed by changing the position of the pivot point of the rocker arm.
• Due to high rotation of the cam, there were impact contacts every time there was return in cam, which led to increase in the forces in the form of spikes. During the dwell period, there was not much change in the contact forces as can be seen from the plot and also the animation shown at in the end of the report.
• Similar Trends were seen in the contact forces between push rod-rocker arm and rocker arm-valve pairs as shown below.

Similar Trends were seen in the contact forces between push rod-rocker arm and rocker arm-valve pairs.

Case 1: Valve Lift of 6 mm

• It can be seen from the first plot that the linear displacement of valve was continuously changing between 56 mm to 50 mm for a complete revolution of cam. Thus, a lift of 6 mm was possible through this cam profile.
• One more important thing to note down here was that with 6.5 mm cam lift, 6 mm lift in valve was possible, this can be changed by changing the position of the pivot point of the rocker arm.
• Due to high rotation of the cam, there were impact contacts every time there was return in cam, which led to increase in the forces in the form of spikes. During the dwell period, there was not much change in the contact forces as can be seen from the plot.

Similar Trends were seen in the contact forces between push rod-rocker arm and rocker arm-valve pairs also in this case.

Reason for variation in contact forces between rocker arm and valve compared with magnitude at with contact Forces along X direction

In both the study, it could be seen from the plot and the assembly geometry that in spite of having movement of all the parts along Y direction there are significant amount of forces along X direction which came be seen in both the cases.

One reason was because of converting oscillatory motion of rocker into linear motion of valve which tends to move the valve along XY plane but due to its restrictions in linear movement along Y provided by Valve seat, there were some reaction forces along X direction.

Also because of friction along the contacts, there may be increase in forces along X direction.

Conclusion

Following points can be concluded from above project:

1. The contact forces and valve lift of 3.5 mm lift cam is less than the contact forces and valve lift of 6mm lift cam which is mainly due to increase in spring force that is developed due to increase in cam profile.
2. The cam geometry affects the opening and closing time of valves in IC engines which is very crucial parameter while designing ani IC engine as it would affects the stroke timings and mass of fuel entering into the cylinder.
3. Cams, being higher pairs are subjected to impact loads which were confirmed through the plots and gave an idea that the material must be hard enough to resist these loads.
4. Improper designing of the cam profiles could lead to vibrations in the assembly which is an undesirable quality.

Animation

link https://youtu.be/1WOKHsTcqO0

References

1. Machine Design book by V B Bhandari