MBD Simulation on a Piston Assembly

AIM: To design a 3D model of a piston assembly and simulate the piston kinematics for the different wrist pin offset value.

THEORY:

A piston is a moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a connecting rod.

In a pump, the function is reversed and force is transferred from the crankshaft to the piston to compress or eject the fluid in the cylinder.

In 2 stroke engines, the piston also acts as a valve by covering and uncovering ports in the cylinder.

OBJECTIVES:

1. To study the motion of a piston assembly for zero offsets, positive offset and negative offset of the wrist pin.

2. To obtain the curve of linear displacement of the piston head for all the three cases.

PROCEDURE OF DESIGNING:

1. Calculate and note down all necessary measurements before implementing in SOLIDWORKS.

2. Design the parts separately using the part design option and design the piston of zero offsets.

3. Assemble all the parts and add the necessary mates to make the assembly Single Degree of Freedom.

4. Analyse the motion of the assembly and obtain the necessary plots and export into Excel.

5. Edit the piston part and change the offset value to positive and negative and update the assembly respectively.

6. Analyse the motion of the assembly and obtain the necessary plots and export into Excel.

7. Superimpose all the three graphs into a single graph and compare the results.

DESIGNS:

1. Crank

2. Connecting Rod

3. End cap

4. Gudgeon Pin

5. Piston (Zero Offset)

6. Piston Positive Offset

7. Piston Negative Offset

8. Piston Assembly of Zero Offset

9. Piston Assembly of Positive Offset

10. Piston Assembly of Negative Offset

SIMULATIONS:

1. Zero Offset

Animation- https://youtu.be/efKY737fCq4

2. Positive Offset

Animation- https://youtu.be/IZV3JpsESEM

3. Negative Offset

Animation- https://youtu.be/nE9lVMac6O8

4. Superimposed Result

CAD Files- https://drive.google.com/drive/folders/10Gjx_pT8s8AMLcgOxsC_Kw2kjo6vUy5l?usp=sharing

RESULTS:

From the above curves, it is observed that the linear displacement of the piston head is approximately equal in all three cases.

ADVANTAGES OF OFFSET:

1. When the piston pin is in the centre of the piston, now when the piston reaches top or bottom dead centre then, in this kind of setup the rod is straight up & down placing a huge load on the rod. The crank has to rotate past TDC or BDC in order to get the piston moving again. This robs a lot of both power and RPMs from the engine.

2. Now with an offset, the piston wrist pin is a little offset to one side of the piston therefore the rod is not straight up and down when the piston is at TDC or BDC. This, in turn, allows the crank to rotate with much less resistance giving the engine more power and speed.

3. Having offset pin relieves the stress that is caused in the rod when it reaches the top or bottom dead centre. For the rod does not have to slam the piston up and then back down. The rod travels more in an arc.