MBD Simulation on a Piston Assembly

Aim:
To model piston assembly and to do the motion analysis using solidworks on the piston crank assembly under three different offset conditions.

Objective:

The Main Objective is to Model all the parts and then assemble them in SolidWorks and do motion analysis of Piston kinematics in three different conditions as follows.

SI. NO.	WRIST PIN OFFSET	CRANK SPEED (RPM)
1.	0 mm	2000
2.	10 mm Positive	2000
3.	10 mm Negative	2000

1. To model and assemble internal combustion piston assembly
2. Then to assemble the Piston crank Assembly with proper constraints.
3. To perform motion analysis at different wrist pin offset values in Solidworks.
4. To study the effect of wristpin offset on the linear displacement of the piston.
5. Finally to analyze the resulting plots and to superimpose the resulted plots. 

Introduction:

A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, and pneumatic cylinders, among other similar mechanisms. 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 a pump, the function is reversed and force is transferred from the crankshaft to the piston for the purpose of compressing or ejecting the fluid in the cylinder.

Piston motion analysis is the study of the reciprocating motion of the piston in the cylinder and the motion of the assembly attached i.e. connecting rod, end cap, crankshaft & wrist pin. First of all, for the assembly of a connecting rod, crankshaft, end cap, wrist pin, and the piston itself, we will prepare the geometric model of all the necessary components.

Procedure :

1. Model and assemble internal combustion piston assembly
2. Then assemble the Piston crank Assembly with proper constraints.
3. Providing motor at the crank with 2000 rpm
4. Perform motion analysis at different wrist pin offset values in Solidworks.
5. plot linear displacement vs time for a piston for all cases.
6. Finally to analyze the resulting plots and to superimpose the resulted plots.

Parts Design:

As per the dimensions provided in the video the parts of the piston crank are drawn as follows.

• Piston Head 

• Gudgeon Pin (Wrist pin)

• Connecting rod

• End Cap

• Crank

Assembly:

Case 1: Motion Analysis for "0" mm Offset:

Case 2: Motion Analysis for 10 mm positive Offset

Case 3: Motion Analysis for 10 mm Negative Offset

Superimposed graph: 

Here linear displacement graph from each case is exported to excel and then superimposed there.

Observation: 

1. From the plots obtained, we can see that Linear Displacement increases as we move from negative wrist pin offset to positive wrist pin offset i.e. the magnitude of the Linear displacement increases.
2. Also from the graph obtained after superimposing it, we can see that for a given time-frequency of rotation of piston with wrist pin offset negative is higher compared to one with zero offsets and pin with zero offsets has a higher frequency than one with a positive offset.

CONCLUSION:

1. Piston pin offset refers to the piston wrist pin being a bit offset to one side of the piston i.e. for it to not be in the centre of the piston.For instane 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.
2.  When there isn't any offset the piston displacement is maximum and when we provide the offset there is a reduction in the piston displacement which is obvious since the piston has to travel a bit less distance because of offsetting. This in turn allows the crank to rotate with much less resistance giving the engine more power and speed.
3. The concept of offsetting the position of the wrist pin from the centre of the piston is quite beneficial because if there had not been any offset then the connecting rod had to be exactly straight at TDC and at BDC. The problem in this is that the crankshaft has to put an extra effort to make the piston reciprocate whenever it is at the two extreme ends and its own motion is hampered.
4. This results in a tremendous amount of stress on the connecting rod and also loss of power as the crankshaft has to do some work in order to move the piston along with some loss in rpm produced by the engine.
5. This can be reduced by offsetting the wrist pin position, this will allow the connecting rod not to be straight at BDC or TDC, it will reciprocate following an arc.