MBD Simulation on a Piston Assembly

Aim: To analyze the motion of the Piston Assembly and plot Linear Displacement of the piston for different cases of wrist pin offset with time.

Theory: The Piston – Crankshaft assembly is based on the Slider-Crank Mechanism. The piston acts as a slider whereas crankshaft as a crank. These are extensively used in Internal Combustion Engines to convert the linear motion of the piston in rotary motion via the crankshaft. This rotary motion is then transferred to wheels with help of gearbox, differential, and some other parts.

Wrist Pin: It is used to connect the connecting rod to the piston. The main function of the wrist pin is to transfer motion between the two. It is also called as Gudgeon Pin.

Wrist Pin Offset: The location of the wrist pin may be deflected from the geometrical center of the piston in the same plane. This is done in order to avoid piston slap and also to reduce connecting rod angularity at the time of maximum pressure condition.

Geometry Modelling: The parts are designed as per given instructions and dimensions. The crank is constrained for all translation motions and is allowed to rotate only along its axis. It is further connected to Connecting Rod-End Cap assembly which acts as a link between crank and piston (via Gudgeon pin). Connecting Rod-End Cap is centered on Crank (with help of Width Mate). Connecting Rod is attached to the Gudgeon Pin-Piston assembly in a similar fashion.

Motion Study Setup: The material used is Steel (Dry) which provides a static friction coefficient of 0.3 and a kinematic friction coefficient of 0.25.

A motor is attached to the crank to induce motion. Further contacts are made connecting all solid bodies.

Contacts:

1. Piston with Gudgeon Pin
2. Gudgeon Pin with Connecting Rod
3. Connecting Rod with Crank
4. End Cap with Crank

Setup Settings:

Precise Contact	ON
Gravity	Negative Y-direction
Motor RPM	2000
Frames per second	12000

Cases Run:

1. Zero Offset
2. 10mm Positive Offset
3. 10mm Negative Offset

Results: The graphs obtained for all the three simulations are smooth indicating smooth functioning of the mechanism.

Combined Graphs:

The Stroke length found in all the three cases differ as follows:

Case	Stroke Length (mm)
Zero Offset	99
10 mm Positive Offset	100
10 mm Negative Offset	100

Conclusion: The stroke length varies with wrist pin offset.

Animations, Part and Assembly files, Excel Sheet for combined graph