MBD SIMUATION ON A PISTON ASSEMBLY
| MBD SIMULATION ON A PISTON ASSEMBLY |
CONTENTS
- AIM
- INTRODUCTION
- GEOMETRY / PART MODELLING
- ASSEMBLY MODELLING
- SIMULATION SETUP
- PLOTS AND RESULTS
- OBSERVATIONS AND CONCLUSIONS
AIM
1. To design various components of a piston assembly, assemble them and carry out its motion analysis for the following three conditions:
| Sr. No. | Wrist Pin Offset | Crank Speed (rpm) |
| 1 | 0 mm | 2000 |
| 2 | 10 mm Positive | 2000 |
| 3 | 10 mm Negative | 2000 |
2. To obtain the Linear Displacement of the Piston Head in all the three cases above, export the graphs into excel and finally, superimpose all the three graphs into a single graph.
INTRODUCTION
What is Motion Simulation / Motion Analysis / MBD?
Suppose an engineer is designing an elliptic trammel meant for tracing different ellipses. When he has defined mates in the CAD assembly, he can animate the model to review how the components of the mechanism move. Although assembly animation can show the relative motion of assembly components, the speed of motion is irrelevant and timing is arbitrary. To find velocities, accelerations, joint reactions, power requirements, etc., the designer needs a more powerful tool. This is where motion simulation comes in.
Motion simulation provides complete, quantitative information about the kinematics—including position, velocity, and acceleration, and the dynamics—including joint reactions, inertial forces, and power requirements, of all the components of a moving mechanism. Often of great additional importance, the results of motion simulation can be obtained virtually at no additional time expense, because everything needed to perform motion simulation has been defined in the CAD assembly model already, and just needs to be transferred to the motion simulation program.
In the case of the elliptic trammel described, the designer needs only to decide the speed of the motor, the points to be traced, and the motion results he wishes to see. The program does everything else automatically, without the user’s intervention. The motion simulation program uses material properties from the CAD parts to define inertial properties of mechanism components, and translates CAD assembly mating conditions into kinematic joints. It then automatically formulates equations that describe the mechanism motion.
Apart from this, motion simulation conducts interference checks in real time, and provides the exact spatial and time positions of all mechanism components as well as the exact interfering volumes.
Product developers can also use motion simulation for mechanism synthesis by converting trajectories of motion into CAD geometry. Designers can also use trajectories of motion to verify the motion of an industrial robot.
Thus, SOLIDWORKS CAD program together with SOLIDWORKS Simulation (FEA) and SOLIDWORKS Motion (i.e., the motion simulation) as add-ins represents the state of the art in integrated simulation tools.
The Piston Assembly
The assembly of the piston, connecting rod and crank shaft in an engine behaves like a mechanism during the working of the engine. It is generally an inversion of single slider crank mechanism. Thus having, 4 links (crank, connecting rod, piston and the cylinder), 3 turning pairs and one sliding pair (constrained). These are further explained during the 3D modelling of the various parts in the piston assembly.
The piston makes the crankshaft to turn by utilizing the energy supplied to it by the combustion of the fuel. It is cylindrical in shape and reciprocates inside the cylinder. Burning of the air-fuel mixture within the cylinder exerts a pressure on the piston, thus pushing the cylinder down. The action of the connecting rod and crankshaft converts this downward motion to a rotary motion.
Wrist Pin / Gudgeon Pin Offset
Wrist pin / gudgeon pin being a bit offset is the position of the wrist pin which is going to be varied as followed. To understand the reason for why it is being done, let us consider these following two cases:
(A) 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.
(B) 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.
1. 0 mm Offset
2. 10 mm Positive Offset for the crank rotation in the clockwise direction
3. 10 mm Negative Offset for the crank rotation in the clockwise direction
GEOMETRY / PART MODELLING
Part1: Crank:
A crank is generally a link in a mechanism which can have a complete rotation of 360 degrees. It is a link in pure rotation. It is sometimes also referred to as an arm attached at a right angle to a rotating shaft (crankshaft shown in red below) by which circular motion is imparted to or received from the shaft. When combined with a connecting rod, it can be used to convert circular motion into reciprocating motion, or vice versa.
|
Part2: Connecting Rod:
A connecting rod, also called a con rod, is the part of a piston engine which connects the piston to the crankshaft. It is the link in General Motion. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the crankshaft. The connecting rod is required to transmit the compressive and tensile forces from the piston, and rotate at both ends.
|
Part3: End Cap / Plate:
End cap is connected to the end of the connecting rod, which helps in its attachment with the crank.
|
Part4: Gudgeon Pin / Wrist Pin:
The Gudgeon Pin / Wrist Pin connects the piston to the connecting rod, and provides a bearing for the connecting rod to pivot upon as the piston moves.
|
Part5: Piston / Slider:
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. It is a link in Pure Translation. 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.
|
ASSEMBLY MODELLING
The piston, connecting rod, wrist pin, end cap and the crank are assembled as shown below using assembly mate options.
1. The crank is made coincident with the assembly planes so that it can rotate about its axis.
2. The connecting rod and end cap assembly is made coincident with the crank axis.
3. The wrist pin / the gudgeon pin is made coincident with the piston and with its front plane.
4. The wrist pin / the gudgeon pin is assembled to the connecting rod.
5. The front plane of the piston is made coincident or given an distance mate of 10mm in either of the directions as per the above stated condition for the wrist pin offset.
Here a distance mate for 10 mm offset negative is shown when the crank is rotating in clockwise direction. Note that this is to be adjusted as per the different cases of the motion analysis in this challenge.
For a better understanding of the mates and how the various components of this piston assembly are brought together, a short clip in exploded view is created as shown below.
SIMULATION SETUP
1. 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.
2. Enable SOLIDWORKS Motion, in the Add-Ins tab of the SOLIDWORKS. If the Add-Ins tab is not visible on the screen, It can be made visible by right-clicking any of the tabs in the SOLIDWORKS. The same goes with hiding and making any of the other tabs visible as per the convenience.
3. Set the 'Motor'. Select the Center Axis of the Crank as the reference and give the input as '2000rpm'.
4. Set the 'Solid Body Contact' between the various parts with the material as 'Steel (Dry)'. There are four Solid Body Contacts made as shown below.
5. Now go to the 'Motion Study Properties' and set the frames per second to '12000' and select 'Use Precise Contact'. Also, the gravity in the proper direction is added, in this case the y-direction.
12000 frames per second is selected because,
2000 rpm = 2000 rotations / minute
2000 rpm = (2000 x 360) degrees / 60 seconds
2000 rpm = 12000 degrees / seconds.
thus, for 2000 rpm selecting 12000 fps will give us the results for each degree rotation of the crank. Thereby, resulting in smooth curvatures of our plots.
6. Run the motion simulation with 0mm offset and obtain the linear displacement vs time graph.
7. Adjust the mates of the right plane of the piston with that of the assembly for 10mm forward or backward (i.e., giving the negative or the positive offsets one after the another) and run the simulations as before. We have created the three motion studies for the same and prefered not to update once the offsets are made and the particular study has been run.
Animation for the Motion Analysis: (speed .3 seconds extended to 10 seconds)
PLOTS AND THE RESULTS
After the successful completion of each simulation, the plots were exported into the spreadsheet / excel files as attached below. We have also made used of some excel formulas (MAX MIN & AVERAGE) to show the maximum, minimum and the mean value for the linear piston displacement in each case.
Here,
Plot1 refers to 00 mm wrist pin offset,
Plot2 refers to 10 mm wrist pin offset positive for clockwise rotation of the crank and,
Plot3 refers to 10 mm wrist pin offset negative for clockwise rotation of the crank.
OBSERVATIONS AND CONCLUSIONS
In all the three cases, crank has been rotated in clockwise direction with 2000 rpm but with different positions of wrist / gudgeon pin. The simulation ran smoothly but took a while to complete. Based on the plots obtained following values of the results could be observed.
The maximum linear displacement of piston occurs when there is 0 mm wrist pin offset. When an offset is made, the liner displacement is less. This is obvious due to the fact that the connecting rod does not need to be exactly straight to reach TDC and BDC.
Finally, a comparison is made between the plots by superimposing them on one another as shown below.
It could be observed here that the time frequency for the linear displacement of piston in case of 10 mm negative wrist pin offset is less as compared to the other two cases.
Thus, we can say that offsetting results in reduction of piston slaps, at the same time, reduces the impact energy and the piston skirt friction loss. It also reduces the stresses generated in the connecting rod and lead to the easy rotation of the crankshaft. Also, offsetting influence the piston secondary motion, which needs a further complex analysis.