MBD Simulation on a Piston Assembly

MOTION STUDY AND ANALYSIS OF PISTON ASSEMBLY

AIM – Create 3D models of crank, connecting rod, end cap, wrist pin, and piston head and perform motion analysis using Solidworks.

OBJECTIVES-

1. Create 3D models of crank, connecting rod, end cap, wrist pin, and piston head both with and without offset.
2. Perform motion analysis by rotating crank at 2000 rpm for following three cases-

Sr. No.	Wrist Pin offset	Crank speed ( rpm)
1	0 mm	2000
2	10 mm positive	2000
3	10 mm negative	2000

3. Obtain the linear displacement of piston head in all three cases.

INTRODUCTION-

The slider crank mechanism is used to convert rotary motion of crank into translational motion of slider or vice-versa. Reciprocating piston assembly in internal combustion engine and reciprocating pump are prime examples of it.

In internal combustion engine the gases produced on combustion of air fuel mixture pushes piston down and its sliding motion is converted into rotary motion of camshaft.

There are two types of slider crank mechanisms –

1. INLINE: Its slider is positioned as such line of travel of hinged joint passes through crank joint of cam. Time of ascent is equal to time of descent.
2. OFFSET: Its slider is not positioned in way as such line of travel of hinged joint would pass through crank joint of cam. Tome of ascent and descent are not equal.

This simulation is done to analyze the displacement of piston head when cam is rotated at a particular value of rpm.

APPLICATIONS:

1. Reciprocating engine
2. Rotary engine
3. Oscillating cylinder engine
4. Hand pump

PROCEDURE:

1. Create following parts in Solidworks, using following drawings.
2. Crank
3. Connecting Rod
4. End cap
5. Gudgeon pin
6. Piston head ( no offset)
7. Piston head ( Positive offset and negative offset)

2. Now click on NEW> ASSEMBLY. For case 1 with no offset let us start with piston head with symmetrical hole at its centre.
3. Provide proper mates with assembly’s Front, Top and Right planes and also between temporary axes of components.
4. Enable Solidworks Motion from Tools > Add in. Click on Motion Study palette. Select Motion Analysis from Drop Down Menu.
5. Click on motor icon. By default, selection is Rotary Motor. Select axis of crank in component. In Motion select Constant Speed with 2000 RPM in clockwise direction.
6. Click on Contact and make following contacts: - between crank and end cap, between connecting rod and gudgeon pin and finally between gudgeon pin and piston head.
7. Click on gravity and select Y direction.
8. Click on Motion property manager and set Frame per seconds to 12000. Select use precise contact option. Now click on Calculate.
9. Plot the Linear displacement of piston head. Export it to spreadsheet.
10. Now for case 2 replace piston head without offset with piston head with offset and repeat steps 3-10.

11. For negative offset rotate the piston head 180° about line passing perpendicular to its top face. Repeat steps 3-10.
12. Now, copy data from all three workbooks to a single workbook and make a comparison plot.

RESULTS –

Case 1 No offset

Position of piston head at top dead centre = 303.267mm

Position of piston head at bottom dead centre = 203.271mm

So, displacement of piston = 100mm

Case2 Positive offset (10 mm)

Position of piston head at top dead centre = 303.206mm

Position of piston head at bottom dead centre = 203.203mm

So, displacement of piston = 100mm

Case 3 Negative offset (10 mm)

Position of piston head at top dead centre = 303.206mm

Position of piston head at bottom dead centre = 203.199mm

So, displacement of piston = 100mm

COMPARISION OF LINEAR DISPLACEMENT

CONCLUSIONS & EXPLANATIONS:

1. From plots and position data imported in excel we can see that piston will execute simple harmonic motion.
2. We can see that position coordinates of top face is highest in case of 0mm offset, it can be easily inferred as connecting rod becomes vertical as piston reaches top dead centre.
3. Time taken to reach top dead centre to bottom dead centre (forward stroke) and time taken to reach bottom dead centre to top dead centre (return stroke) is equal.
4. However, this time varies in case of gudgeon pin offset.
5. In positive offset the return stroke is quicker than forward stroke.
6. In negative offset the forward stroke is quicker than return stroke.