MBD Simulation on a Piston Assembly
OBJECTIVE:
To model an IC engine piston assembly and carry out motion analysis of the model. The model has three variants with each case having different wrist pin offsets, 0mm, 10mm positive offset and 10mm negative offset. The model analysis is to be carried out for all three cases. The speed of rotation is 2000rpm and direction of rotation is clockwise for all cases. Obtain the Linear displacement of the Piston Head in all the three cases, export the graphs into Excel and superimpose all the three graphs into a single graph.
SOLUTION:
A piston assembly, also called as reciprocating engine is a mechanism in which a linear motion is converted into rotary motion or vice versa. Common applications of piston includes IC engines, hydraulic pump, compressor, and so on.
The assembly has 5 different parts:
- Crank
The crank is the rotating part in the assembly.
- Connecting rod and End cap
The connecting rod is the member connecting the crank and the piston, one end of the rod is attached to the crank and the other to the piston head.
- Wrist pin
The wrist pin holds together the piston head and the connecting rod.
- Piston head
The piston head undergoes translation motion.
- Assembly
In this study, the linear displacement of piston head is analysed in three different cases.
Case1: Wrist pin axis is in the same plane as crank axis
Case2: Wrist pin axis has a positive offset of 10mm to the crank axis
Case3: Wrist pin axis has a negative offset of 10mm to the crank axis
The center of the axis of the piston head and the crank were maintained coincident to each other and they are on the same plane in all the three cases.
For carrying out the multibody dynamics simulation, the crank was given a rotary motion with a speed of 2000rpm in the clockwise direction. Solid body contacts were defined between each adjacent bodies and the material was assigned as Steel. The gravity action was turned on in the ‘y’ direction. The fps was set to 7200 and the precise contacts was enabled.
The linear displacement of the piston head was carried out for all three cases and the results were exported to excel and the plots were superimposed.
Result obtained:
The linear displacement plot of the piston head displayed a sinusoidal curve. From the data, it was found that the amplitude, frequency and wavelength for all three cases is same. The only difference observed was, when an offset is introduced in the model, the waves shift from its original position.