PISTON KINEMATICS IN DIFFERENT POSITIONS OF WRIST PINS (using SOLIDWORKS)

AIM:

To design and simulate the piston kinematics for different positions of wrist pin, i.e. for positive, negative and zero offset using SOLIDWORKS.

OBJECTIVES:

• To understand the basic components of a piston and their workings
• To understand the advantages and disadvantages of offsetting the wrist pin.
• To understand the method of design and to motion analysis using SOLIDWORKS.

INTRODUCTION:

A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the 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 piston rod and/or connecting rod. In some engines, the piston also acts as a valve by covering and uncovering ports in the cylinder.

Advantages of offsetting the wrist pin:

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 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 pin, 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.

Having offset pin relieves the stress that is caused in the rod when it reaches 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.

PROCEDURE OF COMPLETION:

1. All the necessary measurements and calculations were done before-hand and implemented in SOLIDWORKS for the normal piston.
2. Then, another two piston heads were created with offset wrist pins, i.e. 10mm in right side of the centre and in left side of the centre respectively.
3. Then, motion analysis was done using 2000 RPM and 12000 frames per second for better analysis.
4. Linear displacement of the piston head was plotted from all the cases and superimposed on each other using MS-Excel for better visualization of data.

DESIGN AND PLOTTING OF THE LINEAR DISPLACEMENT:

• Zero offset:

The linear displacement graph is as follows:

The simulation can be seen through the following link:

https://youtu.be/CDpOZTVZdkw

• Positive offset by 10mm:

The linear displacement of the piston head is as follows:

The simulation can be seen through the following link:

https://youtu.be/IlmsTRcqvtY

• Negative offset by 10mm:

The linear displacement of the piston head is as follows:

The simulation can be seen through the following link:

https://youtu.be/YECcpRPo3sM

The data in a single plot can be seen as follows:

PARTS AND ASSEMBLIES OF DIFFERENT CONFIGURATIONS OF THE PISTON:

The various parts and assemblies alongwith the data that is imported in the excel spreadsheet can be seen through the following link:

https://drive.google.com/drive/folders/11nxuOrC9_yKF1H1MULjBHnwGTP_JZkTU

OBSERVATIONS AND CONCLUSION:

We can conclude that:

1. The maximum linear displacement is the least in the positive offset and highest in the zero offset case.
2. The amplitude of the linear displacement of the negative offset is between the zero offset and the positive offset.
3. The maximum linear displacement is highest in zero offset because the connecting rod gets ample amount of space to push it above as the wrist pin is centred around the piston head's plane which is not in the case of the positive offset or the negative offset.
4. This in turns increases the friction and the drag as it hits the piston head up and down as it reaches the BDC and TDC.
5. The magnitude of linear displacement is less in the offset pins as the rod moves in an arc.