MBD Simulation on a Piston Assembly

INTRODUCTION:

Piston is a cylindrical engine component that slides back and forth in the cylinder bore by force produced during the combustion process. The piston acts as a movable end of the combustion chamber. The stationary end of the combustion chamber is the cylinder head. Pistons are commonly made of a cast aluminum alloy for excellent and lightweight thermal conductivity

Aluminum expands when heated, and proper clearance must be provided to maintain free piston movement in the cylinder bore. Insufficient clearance can cause the piston to seize in the cylinder. Excessive clearance can cause a loss of power, blowby, etc.

Piston pin offset refers to the piston wrist pin being a bit offset to one side of the piston i.e. for it to not be in the centre of the piston.

To understand the reason for which it is done, let us consider these following two cases:

1. When the piston pin is in the centre of the piston, now when the piston reaches top or bottom dead centre then, in this kind of setup the rod is straight up & down placing huge load on the rod. The crank has to rotate past TDC or BDC in order to get the piston moving again. This robs a lot of both power and RPMs from the engine.
2. Now with an offset pin, the piston wrist pin is a little offset to one side of the piston therefore 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.

Having offset pin relieves the stress that is caused in the rod when it reaches top or bottom dead centre. For the rod does not have to slam the piston up and then back down. The rod travels more in an arc. They have been used in real race engines for long.

AIM:

To obtain the linear displacement of the Piston Head in the following three cases:

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

PROCEDURE:

• After finishing the piston assembly, go to solidworks motion.
• Assemble the different components and make sure that the required rotational freedom for the crankshaft as well as linear motion for the piston is present.
• Select the motor and center axis of the crank and give the input as 2000rpm.
• Select the contact forces between the parts and set the material as steel dry.
• Now set it to 7200 frames per second and select precise control.
• Run the motion simulation for the with 0mm offset and obtain the linear displacement vs time graph.
• Adjust the mates of the right plane of the piston with that of the assembly for 10mm forward or backward and run the simulation as before.

PISTON ASSEMBLY:

GRAPHS:

The results are plotted in the form of excel with columns showing linear displacement vs time.

With no offset:

With negative and positive offsets:

OBSERVATION:

• For 0mm offset, the linear displacement achieved is maximum and time taken to complete a cycle is more when compared to the other two conditions.
• The time taken to complete a cycle is reduced due to the offset irrespective of the direction of offset.
• The linear diaplacment is same for both negative and positive offsets.