MBD Simulation on a Piston Assembly using Solidworks

AIM- To model all the parts of the piston assembly and then assemble them to run a multibody dynamics simulation on that with different cases.

THEORY- An internal combustion engine is a heat engine where the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine, the expansion of the high-temperature and high-pressure gases produced by combustion applies direct force to some component of the engine. The force is applied typically to piston , turbine blades, rotor or a nozzel This force moves the component over a distance, transforming Chemical energy into useful mechanical energy.

Fig.1 Internal Combustion Engine

Piston:-

A piston is a cylindrical engine component that slides back and forth in the cylinder bore by forces produced during the combustion process. The piston acts as a movable end of the combustion chamber. The stationary end of the combustion chamber is the cylinder head. Piston is a moving component that is contained by the cylinder and is made gas-tight with the help of piston rings. Pistons are commonly made of a cast aluminum alloy for excellent and lightweight thermal conductivity. Thermal conductivity is the ability of a material to conduct and transfer heat. Aluminum expands when heated, and proper clearance must be provided to maintain free piston movement in the cylinder bore. Insufficient clearance can cause the piston to seize in the cylinder. Excessive clearance can cause a loss of compression and an increase in piston noise.

Fig.2 Piston Assembly

The parts are as follows :

• Piston Head
• Gudgeon Pin (Wrist pin)
• Connecting rod
• End Cap
• Crank

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.

Fig.3 Piston

Gudgeon Pin (Wrist pin)- In an internal combustion engine, 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.

Fig.4 Gudgeon Pin

Connecting rod- A connecting rod is a rigid member which connects a piston to a crank or crankshaft in a reciprocating engine.

Fig.5 Connecting rod

End Cap- The End Cap is used to fix the connecting rod with the crankshaft.

Fig.6 End Cap

Crank- A Crank or crankshaft is a mechanical part able to perform the conversion between reciprocating motion and rotation motion. It translates reciprocating motion of the piston into rotation motion. Whereas in the reciprocating compressor, it converts rotational motion into reciprocating motion.

Fig.7 Crank

Creating model of all the parts of Piston Assembly

1) Piston Head- The piston is shown below and the steps taken to draw it are shown in part tree.

Fig.8 2D diagram of Piston Head

Fig.9 Steps to draw Piston

2) Gudgeon Pin (Wrist pin)- The Gudgeon Pin is shown below and the steps taken to draw it are shown in part tree.

Fig.10 2D diagram of Wrist pin

Fig.11 Steps to draw Wrist pin

3) Connecting rod- The Connecting rod is shown below and the steps taken to draw it are shown in part tree.

Fig.12 2D diagram of Connecting rod

Fig.13 Steps to draw Connecting rod

4) End Cap- The End Cap is shown below and the steps taken to draw it are shown in part tree.

Fig.14 2D diagram of End Cap

Fig.15 Steps to draw End Cap

5) Crank- The Crank is shown below and the steps taken to draw it are shown in part tree.

Fig.16 2D diagram of Crank

Fig.17 Steps to draw Crank

Piston Assembly- Now opening the new file as assembly and inserting all the parts one by one and providing them contraints/mates to make th complete assembly. Mates being used were shown in figure-

Fig.18 Mates used for Piston Assembly

Fig.19 Various 2D views of Piston Assembly

Fig.20 Various 3D views of Piston Assembly

Piston motion analysis:

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

In the Motion study the following steps are performed.

1. Select motion analysis in the drop-down menu.
2. Motor is given to crank with the speed 2000 RPM clockwise. Fig.21 Motor established at Crank
3. Frames per second are given as 12000 with precise contact. Fig.22 Setup of Motion Study Properties
4. There are total 4 contact pairs -
   • connecting rod and crank
   • end cap and crank
   • connecting rod and piston head
   • Piston and Wrist pin
5. Gravity in the Y direction.
6. And now for all the motion analysis done by clicking on calculation and setting the time key 0.5sec to get the results.
7. Now after the calculation obtain the plot for the linear displacement of the piston w.r.t time.

The motion analysis is going to be carried out in three different conditions

S.No	Wrist Pin Offset (mm)	Crank speed (RPM)
1.	0 mm	2000
2.	10mm Positive	2000
3.	10mm Negative	2000

Negative 10mm Offset Zero Offset Positive 10mm Offset

Fig.23 Various positions of Piston

Following steps (changes to be done in Piston and mate) as shown in figure to be taken for modelling in different cases

1. For Zero Offset Fig.24 Steps to draw Zero Offset Fig.25 Zero Offset
2. Positive 10mm Offset Fig.26 Steps to draw Positive 10mm Offset Fig.27 Positive 10mm Offset
3. Negative 10mm Offset Fig.28 Steps to draw Negative 10mm Offset Fig.29 Negative 10mm Offset

RESUlTS & PLOTS:

1) Zero Offset

Plot 1: Linear Displacement of Piston Head with zero offset w.r.t time

2) Positive 10mm Offset

Plot 2: Linear Displacement of Piston Head with Positive 10mm Offset w.r.t time

3) Negative 10mm Offset

Plot 3: Linear Displacement of Piston Head with Negative 10mm Offset w.r.t time

4) Superimposed Graph

Plot 4: Linear Displacement of Piston Head with all cases w.r.t time (Superimposed plot)

As observed form the above graphs the linear displacement remains almost same in all the three cases.

1) The linear displacement at 0 offset : 302.432825609819

2) The linear displacement at 10mm positive offset: 302.308213379358

3) The linear displacement at 10mm negative offset: 302.304495861784

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.