MBD Simulation on a Piston Assembly

Aim:

The aim is to perform a MBD simulation on a Piston Assembly using Solidworks.

Introduction & Basic Theory:

Multi-Body Dynamics or MBD is the study of the movement of various parts in the assembly with each other. By performing MBD one can see the behaviour or path trached by the different parts present in the assembly. Thus using this method an engineer can visualize the motion and the behaviour of the complete assembly and take the steps further steps for manufacturing thus reducing the time nad cost.

A Piston assembly is the heart of any ICE. This assembly consists of a Crank, Connecting Rod, Gudgeon Pin and the Piston.The explosion that happens on the combustion chamber provides tha linear motion to the piston which in turn leads to the rotaion of the crank from which the output can be extracted.

In our case we are going to model the above mentioned parts in solidworks and we will provide the rotary motion to the crank to study the linear displacemnent of the piston.

Procedure:

Using the dimmensions shown in the cad models of each parts has been created the screen shots of the same can be seen below:

GEOMETRY CREATION:

1. All the parts are designed in Solidworks individually which have been shown in the video.
2. Some options should be carefully selected while designing, for e.g.- the curved slot in the piston head and in crank as well, it should be done via the extruded cut option in which the flip side cut option must be chosen. (This is done because the sketch was open, I personally prefer to close the sketch so we don't have to worry about it.)
3. Another important thing is that the fillets should be given to every sharp corner mostly if we take into account stress concentration at sharp corners and the simulation to be more precise.

Solidworks motion analysis-

1. After complete assembly, the SolidWorks motion is enabled.
2. Rotary motion is provided to the crank (2000 rpm).
3. Solid-Body contact is done between the bodies that are in contact

• connecting rod and crank.
• connecting rod and piston head
• Piston and Wrist pin

There are a total of 3 contact pairs -

1. Gravity is also enabled in a downward direction w.r.to the assembly.
2. Now the most important point is that we add precise contact and frames per second is increased to 7200 for the purpose that we don't want to miss any peak values of the desired parameters and also so that the simulation is smooth and more accurate.
3. Tried results for more than 5 revolutions of the crank.

Sl. No	Wrist Pin Offset `	Crank Speed (Rpm)
1	"0" mm	2000
2	10 mm Positive	2000
3	10 mm Negative	2000

10 MM POSITIVE OFFSET:

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.

Gudgeon 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. Gudgeon is a pivot or journal.

Connecting Rod:

A connecting rod is a rigid member that 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

End Cap:

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

Crank:

The assembly:

The piston, connecting rod, wrist pin, end cap, and the crank are assembled as shown below using the 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 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 motion analysis.

MOTION STUDY AND ANALYSIS :

Case 1: Wrist/gudgeon pin with 0 mm offset with acrank roation of 2000rpm

• A torque/motor was provided for the axis of crank with 2000 rpm in clockwise dirction. Solid body contacts were provided for different components with the material selected as Steel(Dry), Gravity was provided in the downward direction, precise contact and 12000 fps were provided as input for smooth transition and to obtain results more accurately and were solved by running calculations.
• Plot obtained for linear displacement of piston from results -

Plot obtained for angular velocity of crank from results -

Case 2 (10 mm Positive Offset) -

• A torque/motor was provided for the axis of crank with 2000 rpm in clockwise dirction. Solid body contacts were provided for different components with the material selected as Steel(Dry), Gravity was provided in the downward direction, precise contact and 12000 fps were provided as input for smooth transition and to obtain results more accurately and were solved by running calculations.
• Plot obtained for linear displacement of piston from results -

• Plot obtained for angular velocity of crank from results -

• Case 3 (10 mm Negative Offset) -
• A torque/motor was provided for the axis of crank with 2000 rpm in clockwise dirction. Solid body contacts were provided for different components with the material selected as Steel(Dry), Gravity was provided in the downward direction, precise contact and 12000 fps were provided as input for smooth transition and to obtain results more accurately and were solved by running calculations.
• Plot obtained for linear displacement of piston from results -

• Plot obtained for angular velocity of crank from results -

• Superimposing all these data in 1 chart:

CONCLUSION

1) When the wrist pin is in the center that is "0mm" offset, the piston reaches the top dead center (TDC) and bottom dead center (BDC) by making the connecting rod straight up and down which place a huge load on the connecting rod. The crank has to rotate past TDC or BDC in order to get the piston moving again which takes a lot of power and RPMs from the engine.

2) By introducing offset in the wrist pin of "10mm positive" or "10mm negative" we can clearly see that the connecting rod is not straight up and down when the piston is at TDC and BDC. This in turn allows the crank to rotate with much less resistance giving the engine more power and speed. This concept oncept of offsetting the wrist pin is widely used in racing cars. As engines evolved, engineers determined that offsetting the piston (wrist) pin yields two major benefits.

3) First, it improves the noise characteristics of the engine due to piston slap at top dead center (TDC). This is a major NVH (noise vibration and harshness) concern to production engineers who want to eliminate alarming noises anywhere they can. The second reason is to improve engine output by reducing internal friction. 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, especially at the beginning of the power stroke when pressure rises sharply about 12-15 degrees after TDC. Rather than revising a complete engine design, the easy way to do this is to offset the pin bore in the piston. This allows the same crank/rod pin axis geometry as an engine designed with offset cylinder axes, but within existing conventional engine architecture.

4) We see that Stroke length decreases as we introduce offset to any direction. Maximum of stroke length can be seen on the "omm" offset case. Giving offset to the wrist pin decreases the distance travelled by the piston more in positive offset than negative offset. This reduces the piston slapping and also reduces the friction loss thus increasing power output of the engine.

5) Engine displacement is the multiplication of stroke length to the cross-sectional area of the cylinder as the stroke length decreases on introducing offset, the engine displacement also got decreased. From the plots obtained, we can see that Linear Displacement increases as we move from negative wrist pin offset to positive wrist pin offset i.e. the magnitude of the Linear displacement increases.

6) Generally, bigger-displacement engines develop more torque, but small engines can spin faster, which increases their horsepower output.

7) Having an offset pin relieves the stress that is caused in the rod when it reaches the top or bottom dead center.

8) 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 little higher comparatively to one with zero offset and pin with zero offset has little higher frequency than one with a positive offset.

LINK FOR VIDEOS :

CASE 1 : https://drive.google.com/file/d/1sPjm-C9ojn20WPTbHW004i9twFyNwl1c/view?usp=sharing

CASE 2 :https://drive.google.com/file/d/1pQU4gGTQrxirSN2o1cHs4TFA3AU9j4WI/view?usp=sharing

CASE 3 :https://drive.google.com/file/d/1msnY0RZJ0l6Hb7CKbXn8EzJqJUcjgHdh/view?usp=sharing