MBD SIMULATION ON A PISTON ASSEMBLY

1.AIM :

TO PERFORM MBD SIMULATION ON A PISTON ASSEMBLY USING SOLIDWORKS

2.OBJECTIVE :

1. To model all the parts of a piston assembly using Solidworks.
2. To perform Motion Analysis of the Piston Assembly in No Offset, Positive Offset and Negative Offset condition of the Piston.
3. To plot Linear Displacement of the Piston Head in the three cases mentioned.

3.INTRODUCTION :

A piston assembly consist of a piston head, wrist pin/gudgeon pin, connecting rod, end cap and crank.

A piston assembly works on the priciple of slider-crank mechnism which has three revolute joints and one prismatic, or sliding, joint. The rotation of the crank drives the linear movement the slider(piston), or the expansion of gases against a sliding piston in a cylinder can lead to 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.

Here motion analysis will be conducted for both the results.

In offset case the position of the Wrist Pin is going to be varied, 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 cases.

4.MODEL AND ASSEMBLY :

Initially all the parts are made using Solidworks and finally using mates the piston assembly was made.

PISTON HEAD

WRIST PIN/GUDGEON PIN

CONNECTING ROD

END CAP

CRANK

PISTON ASSEMBLY

5.MOTION ANALYSIS :

Motion analysis for the piston assembly can be done using the SOLIDWORKS Motion feature available as an add-on in the solidworks, innside this feature the option motion analysis can be used to perform the operation.

To rotate the crank a constant 2000 RPM motor (clockwise direction) is assigned to that component, further a solid body contact is established between the components using the contact option, also the material steel is chosen for all the parts.

In settings the frames per second is set to 12000 and precise contact is enabled, this helps to note very precise data of linear displacement of piston head.

The moiton analysis study is conducted for 3 cases as shown below

SL. NO	WRIST PIN OFFSET	CRANK SPEED (RPM)
1	0 mm	2000
2	10 mm positive	2000
3	10 mm negative	2000

For case 1 the 0mm offset is as follows

For case 2 the 10mm positive offset is as follows

For case 3 the 10mm negative offset is as follows

The animation for all the cases can be viewed below :-

CASE 1

CASE 2

CASE 3

6.RESULTS :

The plots for Liinear displacement of piston head as a function of time for 3 cases are given below:-

CASE 1 (0mm offset)

As seen from the graph the piston head travels in vertical direction in reciprocating manner with a linear displacement of around 10mm, the peaks of curve shows the piston at top dead centre (TDC) and the trough of the curve shows piston head at bottom dead centre (BDC).

Due to highh frames per second the values are precise and almost same at every stroke, this gives a very smooth curve.

CASE 2 (10mm positive offset)

As seen from the graph the piston head travels in vertical direction in reciprocating manner with a linear displacement less than the 0mm offset case, which is due to positive offset of wrist pin, the peaks of curve shows the piston at top dead centre (TDC) and the trough of the curve shows piston head at bottom dead centre (BDC).

The offsetting might help to prevent the locking of mechanism as the force of connecting rod will never be inline with wrist pin.

Due to highh frames per second the values are precise and almost same at every stroke, this gives a very smooth curve.

CASE 3 (10mm negative offset)

As seen from the graph the piston head travels in vertical direction in reciprocating manner with a linear displacement less than the 0mm offset case, which is due to negative offset of wrist pin, the peaks of curve shows the piston at top dead centre (TDC) and the trough of the curve shows piston head at bottom dead centre (BDC).

The offsetting might help to prevent the locking of mechanism as the force of connecting rod will never be inline with wrist pin.

Due to highh frames per second the values are precise and almost same at every stroke, this gives a very smooth curve.

Superimposing all 3 graphs in one

As seen from the graph the piston head travels in vertical direction in reciprocating manner with a linear displacement less than the 0mm offset case in caase of positive and negative offset of wrist pin, the peaks of curve shows the piston at top dead centre (TDC) and the trough of the curve shows piston head at bottom dead centre (BDC).

The positive offset (case 2) case lags behind the 0mm (case 1) case and the negative offset (case 3) case leads the 0mm (case 1) case.

Link to solidworks files,excel sheets and plots:-

https://drive.google.com/drive/folders/1yc4LwVAoJicBfU_jpjTTceIwsuv9egO0?usp=sharing

7.CONCLUSION :

1. The motion analysis of the piston assembly for all 3 cases was completed.
2. Motion analysis helps to simulate the real time perfomance and give various results and plots to better understand the working.
3. The offsetting of wrist pin helps to reduce the stroke which can be used to alter the piston kinematics and engine dimensions.
4. The linear displacement of piston head is slightly more in no offset case than the other 2 offset case.
5. Keeping the wrist pin at an offset to one side leads the connecting rod to not being in vertical position in at extreme position which allows the crank to rotate with much less resistance, giving engine more power and speed due to less losses.