MBD Simulation on a Piston Assembly

Aim: To model and simulate piston assembly for motion analysis of Piston in three different conditions

Theory: The piston assembly is an example of slider crank mechanism which is most commonly used mechanism when a need of transmitting rotary motion to reciprocatory and vice versa. Input link is changed to get different mechanism. some examples includes IC engine, Heat pump etc.

In an IC engine as the fuel burns it pushes down the piston enclosed in the cylinder, piston transmits the power to conecting rod via gudgeon pin and connecting rod finally transmits power and motion to crankshaft which drives the tyres of an automobile.

Main components of Piston assembly are piston, piston rings, gudgeon pins, connecting rod, crank shafts, bearings etc.

In a simple single piston assembly center of the axis of the piston head, crank and gudgeon pin lies in the same plane.

In our case we will give input to crank which will transmit the sliding motion to the piston.

Let us set up the general motion study settings.

Precise contact with 12000 FPS is take to encounter every degree rotation per second and every simulation is in run for 0.2 seconds.

Sl.No	Wrist Pin Offset `	Crank Speed (Rpm)
1	"0" mm	2000
2	10 mm Positive	2000
3	10 mm Negative	2000

Case 1

0 mm offset

Animation link:

https://drive.google.com/file/d/1r__9sB5Mzgpa08ixIdYpDpgPA6jcZY6V/view?usp=sharing

Case 2

Firstly to obtain 10mm positive offset for wrist or gudgen pin, the existing sketch for the slot in the piston head is changed as follows. As the the axis of the piston head and the crank are coincident to each other there is no need to change anything.

Animation Link:

https://drive.google.com/file/d/12XgAJAlGPR5k3rJxKvN_l3IyJcCwaygR/view?usp=sharing

Case 3

Firstly to obtain 10mm negative offset for wrist or gudgen pin, the existing sketch for the slot in the piston head is changed as follows. As the the axis of the piston head and the crank are coincident to each other there is no need to change anything.

Animation Link:

https://drive.google.com/file/d/1j0Mud9grU5b4tkn3lz4ZiongkreNsFJg/view?usp=sharing

Now, Let us discuss the results.

As the piston is connected by crank shaft with the help of connecting rod, we get a fine simple harmonic motion as connecting rod is in general motion (Translation+Rotation) in all the cases

In the above plot red curves indicates Case 1, Green curves indicates case 2 and blue curves indicates case 3. It is clearly visible that the number of rotations are the lowest in zero offset, highest in positive offset and in between for negative offset method.

we can conclude that positive offset will give more output with same input but in this case side thrust will generate due to asymmetry which will increase the cost of cylinder walls, piston rings etc.