MBD simulation of a Piston Assembly in Solidworks

Aim:

To perform MBD simulation on a Piston Assembly.

Objective:

• To model and assemble the Piston Assembly.
• To perform motion analysis at different wrist pin offset values.
• To analyse the resulting plots.

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.

There are two types of slider-cranks: in-line and offset.

1. In-line: An in-line slider-crank has its slider positioned so the line of travel of the hinged joint of the slider passes through the base joint of the crank. This creates a symmetric slider movement back and forth as the crank rotates.
2. Offset: If the line of travel of the hinged joint of the slider does not pass through the base pivot of the crank, the slider movement is not symmetric. It moves faster in one direction than the other. This is called a quick-return mechanism.

Such a mechanism is widely used in engines to convert the reciprocating motion of piston into rotational motion at the crank.

In our experiment we’ll be studying a special kind of offset crank mechanism.

Setup Explanation:

1. First, the components are modelled as per known dimensions as shown

2. Then the model is assembled using appropriate mates as shown. The last mate is used only to align the crank and is suppressed later.

3. With the Solidworks motion add-in enabled, click on motion study 1 and then click on motion analysis.

4. Add a motor to the axis of the crank as shown. Set constant speed to 2000 rpm.

5. Set contact between each pair of solid bodies as per logic and set material to steel (dry).

6. 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.

7. Set key point time to 0.4 seconds for faster calculation time.

8. Hit calculate and wait for the analysis to complete.

9. Insert plot for linear displacement for the piston by selecting it’s top face under magnitude.

10. Do the above processes for +10 mm and -10 mm wrist pin offset as well to get 3 total cases.

Results:

Case 1:

• Wrist pin offset is set to zero by keeping the piston hole in the centre as shown. The part file of the piston is opened and edited for this.
• Animation: https://youtu.be/DShSuIKrUq8
• Result: The following curve is obtained

Case 2:

• Wrist pin offset is set to +10 mm by shifting the piston hole as shown. The part file of the piston is opened and edited for this.

• Animation: https://youtu.be/w4sMKp5v5z8
• Results: The following curve is obtained

Case 3:

• Wrist pin offset is set to +10 mm by shifting the piston hole as shown. The part file of the piston is opened and edited for this.

• Animation: https://youtu.be/XE4lXOCTnug
• Results: The following curve is obtained

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.

File Link: https://drive.google.com/drive/folders/1x9mti-ACI3BrCXYxaVj46UvkOLbc9b1I?usp=sharing