MBD Simulation on a Piston Assembly

Aim:

To run an MBD simulation on a Piston Assembly.

Objective :

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

Introduction :

Piston motion analysis is study of the reciprocating motion of the piston in the cyclinder and the motion of the assembly attached i.e. connecting rod, end cap, crank shaft & gudgeon pin.First of all for the assembly of connecting rod, crank shaft, end cap, gudgeon pin and piston itself we will prepare the geometric model of all the necessary components.

The parts required for the assembly is as follows.

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.

3D model of the piston created using Solid Works.

Gudgeon Pin (Wrist pin): In internal combustion engines, the gudgeon pin 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 the gudgeon pin is located in a sliding crosshead that connects to the piston via a rod. A gudgeon is a pivot or journal.

3D model of the Gudgeon pin

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.

3D model of the Connecting rod

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

3D model of the End cap

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

3D model of the Crank

Final assembly: The piston, connecting rod, wrist pin, end cap and the crank are assembled as shown below using assembly mate option. 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. This will result in the required assembly for the motion analysis.

The assembly is shown below.

The motion study :

In the Motion study the following steps are performed.

1. Select motion analysis in the dop down menu.
2. Motor is given to crank with the speed 2000 RPM.
3. Frames per second is given as 12000 with precise contact.
4. Contact is given between end pin and crank, crank and connecting rod, connecting rod and wrist pin and wrist pin and piston. Material selected for each case is steel (Dry).
5. Gravith in the Y direction.

Calculation is performed for the motion analysis for about 1 sec. Linear displacement of the piston head w.r.t time is obtained.

There are 3 cases

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. For the second and third case we need to offset the piston circle position to 10mm positive and negative respectively, and then in the assembly coincide the axis of that with the wrist pin.

Case 1: 0mm offset. Here the rotation to the motor is given in clockwise direction.

Motion analysis is performed on this and linear dispacement plot is obtained

Case 2: 10mm offset (positive). Here the rotation to the motor is given in clockwise direction.

Linear displacement graph

Case 3: 10mm offset (negative). Here the rotation to the motor is given in anticlockwise direction.

Linear displacement graph

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

Observation and conclusion:

• The linear displacement graph is smooth for all the 3 cases.
• 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 obvious 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 of 10mm offset.
• In the super imposition the plot of 10mm (negative) offset is opposite to the first two as the rotation given to the motor is in anticlockwise direction.
• 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. This results in piston slap.
• 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.
• This will result in reduction of stresses generated in the connecting rod and less piston slap can be reduced using this.

Animation link:

https://youtu.be/TUxe2gdu9_w