MBD Simulation on a Piston Assembly

AIM

To Simulate of the Piston Assembly and running motion analysis for different cases.

THEORY

PISTON

A piston is a moving disk enclosed in a cylinder which is made of gas-tight by piston rings. The reciprocates inside the cylinder as fluid inside the cylinder expands and contracts. Piston helps in the transformation of heat energy into mechanical work and vice versa. Piston transfers the force output of an expanding gas in the cylinder to a crankshft, which provides rotational momentum to a flywheel thereby transfer of energy takes place.

CRANKSHAFT

Crankshaft is a part in a reciprocating engine that rotates due to the transfer motion of the pistons. However in a pump, the crankshaft's motion causes the piston to move back and forth. The crankshaft is connected to the pistons by 'gudgeon pin' which have an offset axis allowing this conversion of motion to be accomplished. The crankshaft is connected to a flywheel which is used to store rotational energy in order to reduce the pulsation or vibrations of the pistons.

CONNECTING ROD

A connecting rod transfers motion from the piston to the crankshaft and functions as a lever arm. Connecting rods are commonly casted aluminum alloy and are designed to withstand dynamic stresses from combustion and piston movement. The small end of the connecting rod connects to the piston with a piston pin. The piston pin provides a pivot point between the piston and connecting rod. The big end of the connecting rod connects to the crankpin journal to provide a pivot point on the crankshaft. A rod cap is the removable section of a two-piece connecting rod that provides a bearing surface for the crankpin journal.

GUDGEON PIN

Gudgeon pin connects the piston to the connecting rod. It is located in a sliding crosshead that connects to the piston via rod. It is a pivot or journal.

PISTON ASSEMBLY

OBJECTIVE

Model all the parts and then assemble them in SolidWorks. The motion analysis is going to be carried out in three different conditions,

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

Wrist Pin Offset - The position of the wrist pin is going to be varied as followed. But the center 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.

"0" mm Offset - Run a simulation

10 mm Positive Offset - If the crank rotation is in the Clockwise direction.

10 mm Negative Offset - If the crank rotation is in the Clockwise direction.

Obtain the Linear displacement of the Piston Head in all the three cases. Then export the graphs into Excel. Then Superimpose all the three graphs into a single graph.

3D MODELS OF DIFFERENT PARTS OF PISTON ASSEMBLY

1. DESIGN OF PISTON

2. CONNECTING ROD

3. CRANK

4. GUDGEON PIN

END CAP

NOW restricting the different degrees of freedom of different parts and mating them in order to get the required motion of while assembly

ASSEMBLY

LINK TO VIEW PARTS AND PLOTS OF PISTON ASSEMBLY

https://drive.google.com/drive/folders/19DhILeuLoRhK09wgT4VNbzpJoQ-eV1lY?usp=sharing

MOTION STUDY AND ANALYSIS

Steps followed for all 3 conditions

• Motor is installed on the axis of crank with 2000 rpm.
• Solid body contacts were provided for different mating parts with STEEL (dry) material.
• 7200 fps is provided @ input for smooth simulation run and to avoid sudden jerks were we more appropriate results.
• Gravity is provided in downward direction.

CASE 1- @0mm offset

CASE 2- @10mm +ve offset

CASE 3- @10mm -ve offset

4. Superimposition of all above 3 plots

OBSERVATIONS AND CONCLUSIONS

1. The position of piston varies with crank angles, @0 degree crank angle the piston position is @ TDC were as @ 80 degree crank angle, the piston is positioned at BDC.

2. The linear displacement plot of piston vs time follows sinusoidal path.

3. The angular velocity of crank almost remains constant throughout the motion.

4. The amplitude for the plot with "0" mm Offset is 302.499 mm and is maximum as compared to 10 mm Positive Offset (302.3094 mm) and 10 mm Negative Offset (302.3090 mm) which is minimum,and is because there is reduction in the piston displacement which is due to the fact that the piston has to travel a less distance from TDC to BDC because of offsetting,

3. Providing Wrist pin offset i.e wrist pin is offset with piston axis. When the piston pin or the wrist pin is offset to the piston axis, the to and fro travel distance of piston is less when compared to to and fro travel when there is no offset provided. With an offset pin, the rod is not straight up and down when the piston is at TDC or BDC. This in turn allows the crank to rotate with much less resistance giving the engine more power and speed. Providing offset pin relieves the stress that is caused in the rod when it reaches TDC or BDC.

4. When there is no offset provided, the system consumes lot power and RPMs from the engine. But providing offset, the engine runs quieter and produces more horsepower due to lower friction during the combustion event.