MBD Simulation on a Piston Assembly

MBD SIMULATION ON A PISTON ASSEMBLY

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 smooth 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. In a pump, the function is reversed and force is transferred from the crankshaft to the piston for the purpose of compressing or ejecting the fluid in the cylinder. In some engines, the piston also acts as a valve by covering and uncovering ports in the cylinder.

GUDGEAON PIN: The piston pin -also known as the gudgeon pin- transfers the entire generated power out of the combustion process from the piston, via the connecting rod down to the crankshaft. Due to the fact that the piston pin does not rotate, he is the engine component that have to withstand the highest stresses in engine operation.

CONNECTING ROD: A connecting rod, also called a con rod, is the part of a piston engine which connects the piston to the crankshaft. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the crankshaft.

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

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

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 MOTION STUDY:

1. In the Motion study the following steps are performed.
2. Select motion analysis in the dop down menu.
3. Motor is given to crank with the speed 2000 RPM
4. Frames per second is given as 7200 with precise contact
5. 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)
6. Gravity given 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 THREE CASES TO BE CALCUATED

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.

CASE 1: "0" mm Offset

CASE 2: 10 mm Positive Offset

CASE 3: 10 mm Negative Offset

SUPER IMPOSED GRAPH : The graphs of all the three cases are super imposed into one graph to understand the linear displacement of the piston in all the three cases by comparing them.

OBSERVATION AND CONCLUSION:

1. The linear displacement graph is smooth for all the 3 cases.
2. 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.
3. 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.
4. 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.
5. 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
6. This will result in reduction of stresses generated in the connecting rod and less piston slap can be reduced using this.
   

SIMULATION LINK: THE OUTPUT OF THE SIMULATION OF MOTION ANALYSIS