MBD Simulation on a Piston Assembly

AIM

To study motion analysis for piston assembly using Solidworks motion.

INTRODUCTION

Piston assembly, here, includes piston, gudgeon pin, connecting rod and crankshaft. This assembly works on principle of crank-slider mechanism, converting translational motion of piston to rotational motion of crankshaft.

Every IC engine, reciprocating pump uses this assembly to convert translational motion to rotational motion or vice-versa.

Depending on application, the design of components varies. For example, IC engine piston heads have recess, which again varies depending upon type of the engine (2 stroke engine piston have scoop for scavenging purpose).

credits:https://sites.google.com/site/educartionlearnyourcar/engine-block/piston-assembly

Components:

1. Piston - Main component that moves converts the combustion to useful motion. It is a hollow cylinder with the top covered. Piston also makes the cylinder gas tight (usage compression rings of piston). Here the cylinder acts as a guide for the piston. It can be compared to the slider of slider-crank mechanism.
2. Connecting Rod - This component connects the crankshaft and piston. It converts the translational motion to rotational motion, It has two ends, small end where piston is connected and big end where crankshaft is connected.
3. Crankshaft - This is a forged component, The distance between main journal and connecting rod journal of it acts as a crank of the slider crank mechanism. It also contains counterweights for balancing purpose.
4. Gudgeon pin - This cylindrical pin connects the piston and small end of the connecting rod.

Wrist pin offset means the wrist pin or gudgeon pin will be at some offset distance from the piston center line.

Balancing each component is very much essential as it determines the vibrational behaviour of the system.

Using this analysis, we can understand how the linear displacement of piston affects rotational motion. Also how the position of piston with respect to crankshaft affects linear displacement of piston and hence crankshaft.

3D MODEL

Using solidworks part, the sketches were designed and modelled to 3D using extrude or revolve option.

Piston

Sketch:

This sketch revolved then

For compression rings oil ring grooves:

Recess for connecting rod motion and hole for gudgeon oin provided.

Model:

Positive wrist pin offset

Negative wrist pin offset

Zero wrist pin offset

Gudgeon pin

Sketch:

Model:

2.5mm of chamfer given after sketch extrusion.

Connecting Rod

Sketch:

This sketch is extruded and fillet of 2.7 mm given to edges.

Model:

End cap

Sketch:

This sketch is extruded, holes for bots given and at last fillet of 2.7 mm given to edges.

Model:

Crankshaft

Sketch:

This sketch is extruded, main and connecting rod journal shafts added. Then web and counterweights were cur etruded. Atlast mirrored.

Model:

All these components are assembled in solidworks itself

.

MATES:

1. Coincidence between front plane of crankshaft and assembly front plane.
2. Coincidence between main journal axis of crankshaft and assembly top plane.
3. Coincidence between main journal axis of crankshaft and assembly right plane.
4. concentric mate between holes of endcap and connecting rod.
5. Coincidence hole faces of endcap and connecting rod.
6. Coincidence between connecting rod big end axis and connecting rod journal of crankshaft axis.
7. Distance mate of 1mm between inside face of web of crankshaft and outer face of connecting rod.
8. Coincidence between front plane of gudgeon pin and assembly front plane.
9. Coincidence between gudgeon pin axis and connecting rod small end axis.
10. Coincidence between gudgeon pin axis and piston wrist pin provision axis.
11. Coincidence between right plane of piston and assembly rigtht plane.
12. Distance mate of 15 mm between front plane of piston and assembly front plane.

(parts and assemblies attached as zip file)

MOTION ANALYSIS

1. Using solidworks motion motion analysis carried out.
2. Enabled Motion Analysis feature
3. Give motor rotation to crankshaft for constant speed of 2000 RPM.
4. Give contacts as follows:
5. Between crankshaft and end cap
6. Between crankshaft and connectiong rod
7. Between connectiong rod and gudgeon pin
8. Between gudgeon pin and piston
9. Changed frame rates to 12000 with precise contact.
10. Motion analysis calculated and plots were exported.

ANIMATION LINKS:

Zero offset : https://youtu.be/FrP4gnjMIR4

Positive offset : https://youtu.be/LgZr1wyj4fw

Negative offset : https://youtu.be/okwaVVejfXU

RESULT

Totally 3 cases:

Case	Wrist pin offset	Speed (RPM)
1	0	2000
2	10 mm negative	2000
3	10 mm positive	2000

CASE1: Zero offset

CASE2: 10mm negative offset

CASE3: 10mm positive offset

FINAL

CONCLUSION

From the graph it is clear that, compared to all positve wrist pin offset have high frequency(not at great values), that is, more angular velocity whereas negative offset has least.

Another thing is stroke length increases due to offseting(very minute, can be seen in data points, not in graph).

Stress also less in positive offset as there less angularity for connecting rod at BDC/TDC, hence less resistance.

REFERENCE:

http://blog.wiseco.com/what-is-wrist-pin-offset