MBD for Piston Assembly

AIM

To perform MBD simulation on a Piston Assembly.

OBJECTIVE

To model all the parts(Piston,Wrist pin,Crank,Connecting Rod) and then assemble them in SolidWorks and to carry out motion analysis test on the motion of the setup in accordance with the various parameters given below:-

Case	Wrist Pin Offset `	Crank Speed (Rpm)
1	"0" mm	2000
2	10 mm Positive	2000
3	10 mm Negative	2000

THEORY

A slider-crank linkage is a four-link mechanism with three revolute joints and one prismatic, or sliding, joint. The rotation of the crank drives the linear movement the slider, or the expansion of gases against a sliding piston in a cylinder can drive the rotation of the crank.Such a mechanism is widely used in engines to convert the reciprocating motion of piston into rotational motion at the crank.There are two types of slider-cranks: in-line and offset.In our experiment we’ll be studying a special kind of offset crank mechanism.

Wrist Pin Offset - The position of the Wrist Pin is going to be varied as followed, but the centre of the axis of the Piston Head and the Crank are coincident to each other i.e. they are on the same plane in all the three mentioned cases.

STEPS

• First, the components are modelled as per known dimensions
• Then the model is assembled using appropriate mates as shown. The last mate is used only to align the crank and is suppressed later.
• With the Solidworks motion add-in enabled, click on motion study 1 and then click on motion analysis.
• Add a motor to the axis of the crank as shown. Set constant speed to 2000 rpm.
• Go to settings and set frame rate to 12000 frames per second to get output reading for each degree. Also enable precise contacts for accurate analysis.
• Set key point time to 0.4 seconds for faster calculation time.
• Hit calculate and wait for the analysis to complete.
• Insert plot for linear displacement for the piston by selecting it’s top face under magnitude.
• Do the above processes for +10 mm and -10 mm wrist pin offset as well to get 3 total cases.

MOTION ANALYSIS AND RESULTS

Case 1: Offset is 0mm.(image)

A plot of linear displacement with time

Animation:https://youtu.be/v-LkQgEdx2w

Case 2: Offset is 10mm.

A plot of linear displacement with time

Animation: https://youtu.be/BoPTbfhX5Rk

Case 3: Offset is -10mm.

A plot of linear displacement with time

Animation https://youtu.be/AjOF1uW5dmI

OBSERVATIONS

The 3 graphs are superimposed and the following observations are made. Graph is zoomed in for observation purpose.

1. The max displacement of zero offset case is slightly more than that of +- 10 mm offset.
2. The +10 mm offset case lags behind the 0 mm offset case for the same displacement.
3. The -10 mm offset case leads the 0 mm offset case for the same displacement.

CONCLUSIONS

1. With offset, the stroke of piston is reduced. This can be used to slightly alter the piston kinematics in an engine.
2. The horizontal lag has no significance since the time period is still the same.
3. Offsetting might prevent the mechanism from locking in a toggle position since the force on connecting rod will never be in line with it. This can make the engine self-starting.