MBD Simulation on a Piston Assembly

MBD Simulation on a Piston Assembly

Aim of the Project - To model piston assembly and to do the motion analysis to obtain the linear displacement of the piston head in three different cases.

Objective - In this project, the main objective is to model a piston and its associated parts i.e. piston head, wrist pin, connecting rod, end cap, and crank then run an MBD simulation on that. Motion analysis is done at 2000 rpm and frame rate 7200fps with precision contact for three different parameters then plots of the linear displacement of the piston is considered.

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.

Piston assembly consists of the piston head, connecting rod, gudgeon pin, crank and crankpin
The piston is the part of an engine that reciprocates to produce power to rotate the crankshaft. The way the piston moves, the amount of swept volume and the wrist pin offset plays an important role in the power produced inside an engine. In this Project, our concern will be limited to only the wrist pin offset.
Wrist pin offset refers to the piston wrist pin being a bit offset to one side of the piston i.e. not exactly in the centre of the piston. Offsetting the cylinder axis from the crankshaft axis minimizes rod angularity when cylinder pressure is at its highest.
Reduced angularity leads to lower piston thrust forces, hence lower frictional losses during the period of maximum cylinder pressure and hence more power
It also improves the noise characteristics of the engine due to piston slap at the top dead center (TDC).

Geometry of the parts

• Piston Head: 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.

Gudgeon Pin (Wrist pin): Drawing a circle and then using extrude option and then giving chamfer to each end.

Connecting rod: Drawing two sets of two concentric circles at some distance and then joining them with a line than giving fillet to it then use the extrude option and use extrude cut to remove unwanted part of connecting rod then using extrude and extrude cut we will prepare the bolting area.

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. The 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 :

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 7200 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.

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

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

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.

Also from the graph obtained after superimposing it we can see that for a given time frequency of rotation of piston with wrist pin offset negative is higher comparatively to one with zero offset and pin with zero offset has higher frequency than one with a positive offset.

All the model and assembly files are attached for verification.