MBD Simulation on a Piston Assembly

AIM:

To analyse the motion of Piston-Crank assembly in SolidWorks using Motion Analysis feature.

INTRODUCTION AND THEORY:

Piston-Crank assembly is a part of IC engine where the chemical energy of fuel is converted into mechanical energy. The chemical energy is used to drive the piston in linear direction and this linear motion is converted into rotary motion by slider-crank mechanism to crank. The rotary motion available at the crank is transmitted to the driving wheels by the crankshaft.

DATA GIVEN:

Crank is to be rotated clockwise.

For the above three cases, motion analysis is to be done.

DESIGN MODELS:

The 3D models are made in SolidWorks using the dimensions given in the CAD Drawing Sheet.

PISTON HEAD:

GUDGEON PIN:

CONNECTING ROD:

END CAP:

CONNECTING ROD ASSEMBLY:

CRANK:

PISTON-CRANK ASSEMBLY:

ZERO GUDGEON PIN OFFSET:

GUDGEON PIN OFFSET:

As you can see from the above picture, the crank axis and the gudgeon pin axis are on the same plane. But there are problems associated with this orientation. Once the piston head reaches the TDC (Top Dead Centre) in thermodynamic cyclic process, the piston head will tranverse towards the BDC (Bottom Dead Centre). While it is doing so, the piston head and connecting rod are normal to each other at the moment when piston head reaches TDC. So, there is a stress induced in the connecting rod due to compression in the axial direction which reduces the life of connecting rod. It is observed that the noise from the engine is loud due to friction in this case. In order to prevent these from occuring, an offset is given to the gudgeon pin so that the connecting rod is not axially compressed when the piston head is at TDC. The engine also runs quieter due to lesser friction. There are two types of offset given to the gudgeon pin: Positive and Negative Offset. Assume the rotation of the crank in clockwise direction. The pictures shown below depicts the types of offset.

POSITIVE GUDGEON PIN OFFSET:

NEGATIVE GUDGEON PIN OFFSET:

EXPLANATION:

1. The SolidWorks model of the components are made and assembled.

2. The Motion Analysis is carried out with the necessary input data.

3. The frames per second is set to 7200 fps and Precise Contact is used to get better results.

4. The contact between the components are defined and the material is applied as 'Steel (Dry)'.

5. The study is carried out and the output result is plotted.

RESULT:

ANGULAR VELOCITY - CRANK:

CASE 1:

CASE 2:

CASE 3:

SUPERIMPOSED PLOT:

As you can see from the superimposed plot, there is a time difference between the zero, positive and negative offset cases to reach the TDC and BDC. So, positive offset plot is slightly ahead of zero offset plot and negative offset is ahead of the other two.

P.S. There is a non-uniformity in the negative offset plot.

CONCLUSION:

Hence the motion analysis of piston assembly is carried out and the result is obtained.