MBD Simulation on a Piston Assembly

Aim: To create piston assembly models with – Zero offset, Negative Offset and Positive Offset of wrist pin and to run motion analysis and plot linear displacement curve on each of these models.

Introduction:

Piston-Connecting rod mechanism is a setup that is being used mostly on I.C Engines and also being used in industries. So, the parts involved are piston, piston rings, connecting rod and crankshaft. In the context of automotive applications, piston is placed inside a cylinder of the engine block and it is connected to connecting rod through smaller end and the big end of the connecting rod is attached to the crank-pin of the connecting rod. The idea behind this arrangement is to convert reciprocating motion of the piston into a rotary motion of the crankshaft. The crankshaft in turn drives the wheels of the automobile.

Steps Taken in CAD Models:

1. SolidWorks was opened and units set to MMGS.
2. For piston, line commands and revolve was used. Depending on the required model the circle command was used to cut at the face of the piston in front plane and a arc command was used in right plane to flip cut the lower portion of the piston.
3. The same process was followed for positive and negative offset and the only thing that was new to create a circular cut in front plane with offset of 10 mm from the mid-portion of the piston.
4. For connecting rod, two concentric circles were created and other two concentric circles at a distance for bigger end. Horizontal constraints applied. With the Line tool the outer circles were joined and appropriate dimensions were applied. Mirror tool was used to mirror the line on the other side. Fillet was applied on appropriate region and then mid-plane extruded.
5. The other or the bigger circle was half trimmed in the edit sketch. The bigger end cap was created on the new part model. The commands used were 3-point arc and circle for extrude cut and extrusion. 3D fillet was applied on sides. The same was done for the connecting rod.
6. Gudgeon pin or wrist pin was created by creating two concentric circles and then extruding the same. Later, chamfer of distance 2.7mm was used on either side.
7. Crankshaft was created by 3-point arc, line and applying vertical and coincident constraints appropriately. It was then extruded. On any one face, with the help of circle command the main shaft was created. On the other side of the crank face, crank-pin was created at specified distance from its center.
8. Again, a 3-point arc was made on the right plane of the crankshaft and made on top and lower section to remove entities. This was done and cut one at a time. Later mirror entities were used to mirror the entire body of the crankshaft about crank-pin face.
9. Assembly was done with first constraining the position of the crankshaft (making sure it was in “float”) followed by importing components: connecting rod and bearing cap. Temporal axis was enabled and the axis were coincident.
10. Piston and wrist pin were imported and appropriate mating constraints was applied.

Motion-Analysis:

1. Animation was changed to motion analysis.
2. Motor was applied in the axis of the crankshaft. 2000 RPM was set.
3. Under solid contacts, material was specified as “Steel-dry”:

• Between bearing cap and crankshaft.
• Between connecting rod and crankshaft.
• Between connecting rod and wrist-pin.
• Between piston and the wrist pin.

4. Under motion analysis setting, the FPS setting changed to 12000 and then “Use precise contact” was clicked.
5. Calculate button was pressed.
6. Result and plot icon, on clicking it and specifying parameters such as “Linear displacement”- “Magnitude”-Selecting the top head of the piston.
7. Graph was plotted.
8. The same was done for positive offset and the negative offset.

Pictures of CAD Model:

Base piston:

Zero Offset Piston:

Positive Offset Piston:Negative Offset Piston:

Connecting Rod:

Big End Cap:

Wrist Pin:

Crankshaft:

Final Assembly:

1. Zero Offset Assembly Model:

2. Positive Offset Assembly Model:

3. Negative Offset Assembly Model:

Graph Oservation and Plots:

1. Zero Offset wrist-pin

2. Positive Offset wrist-pin

3. Negative Offset wrist-pin

Excel Graph:

The simulation datas were exported to spread sheets and graphs were obtained till 0.1 s in each cases.

1. Zero Offset Model:

2. Positive Offset Model:

3. Negative Offset Model:

Superimposed Graph for all three cases:

Inference from the plots:

1. The starting point of linear displacement graphs for negative offset was on the opposite to that of zero offset and positive offset.
2. The peaks on all the 3 cases came in sinosudal form.
3. Based on results obtained from simulation till 0.1 seconds, it was found that linear displacement:

• For Zero Offet = 256.574 mm.
• For Positive Offset = 246.945 mm.
• For Negative Offset = 216.698 mm.

Conclusion:

The maximum linear displacement of the piston was maximum when it was at zero offset followed by positive offset and negative offset.