Multibody dynamics simulation of Piston assembly

Objective :

1. To study and perform motion analysis of the reciprocating motion of the piston for various positions of the wrist pin axis with respect to the axis of rotation of the crank

2. The offset between these axis is made positive or negative and linear displacement of the Piston head is studied for these configurations.

Parts

1. Piston

A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod.

Geometry:

2. Connecting Rod

A connecting rod is a rigid member which connects a piston to a crank or crankshaft in a reciprocating engine. Together with the crank, it forms a simple mechanism that converts reciprocating motion into rotating motion.

A connecting rod may also convert rotating motion into reciprocating motion, its original use. Earlier mechanisms, such as the chain, could only impart pulling motion. Being rigid, a connecting rod may transmit either push or pull, allowing the rod to rotate the crank through both halves of a revolution. In a few two-stroke engines the connecting rod is only required to push

Geometry:

3. Gudgeon pin

In internal combustion engines, the gudgeon pin (UK, wrist pin US) connects the piston to the connecting rod and provides a bearing for the connecting rod to pivot upon as the piston moves. In very early engine designs (including those driven by steam and also many very large stationary or marine engines), the gudgeon pin is located in a sliding crosshead that connects to the piston via a rod. A gudgeon is a pivot or journal.

4. Crank

Is a mechanical part able to convert the reciprocating motion from the piston into rotational motion.

Geometry:

5. End Cap

End cap is connected to the end of the connecting rod, which is assembled crank.

Assembly

Using assembly mate the piston, connecting rod , wrist pin , end cap and the crank are assembled as shown.First the crank is made coincident with the assembly planes so that it can rotate about its axis and then connecting rod and end cap assembly is made coincident with the crank axis. Then wrist pin is made coincident with the piston and with its front plane and it is then assembled to the connecting rod.

Analysis:

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 .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.

Results and Discussion:

Case 1: 0mm offset; [Clockwise]

• Crank speed or motor rpm =2000rpm.
• Number of frames per second=12000 FPS with precise contact.
• Wrist Pin Offset= 0mm.
• gravity in y direction.
• material selected for each contact is \'steel dry\'.

Plot - Linear Displacement vs Time

Case 2: 10mm offset (positive); [Clockwise]

• Crank speed or motor rpm =2000rpm.
• Number of frames per second=12000 FPS with precise contact.
• Wrist Pin Offset= 10mm positive offset.
• gravity in y direction.
• material selected for each contact is \'steel dry\'.

Plot - Linear Displacement vs Time

CASE 3 : AT 10 MM NEGATIVE OFFSET

• Crank speed or motor rpm=2000rpm.
• Number of frames per second=12000 FPS with precise contact.
• Wrist Pin Offset= 10mm negative offset.
• gravity in y direction.
• material selected for each contact is \'steel dry\'.

Superimposed graph

linear displacement graph from each case is exported to excel and then super imposed there.

Observations:

a) The linear displacement vs time graph is smooth for all the three wrist pin configurations.

b) When there isn\'t any offset the piston dispalcement will be maximum than the offset conditions as there is slight reduction in the piston displacement which is quite understood since the piston has to travel a bit less distance because of offsetting because the conneting rod will not be straight at TDC and BDC because offset. This can be seen in the superimposed graph.

c) The Linear Displacement increases as we move from negative wrist pin offset to positive wrist pin offset.