MBD Simulation on a Piston Assembly

INTRODUCTION TO PISTON KINEMATICS:

Automotive industry deals with reciprocating engines, in which the piston has a linear motion within the cylinder. The crank mechanism transforms the linear movement of the piston into rotational movement of the crankshaft. It consist of the following parts:

1. Piston Head
2. Connecting Rod
3. End Cap
4. Wrist Pin/Gudgeon Pin
5. Crank

OBJECTIVE:

To perform the motion analysis for the piston assembly for the following conditions:

Wrist Pin Offset	Crank Speed (RPM)
0 mm	2000
10 mm Positive	2000
10 mm Negative	2000

PROCEDURE:

3D PART MODEL:

• PISTON HEAD:

A piston is a component of reciprocating, reciprocating pumps, gas compressors and pneumatic cyclinders, among other similar mechanisms. Here the piston is drawn from front plane then it is revolved around 360 degree to get the part of piston head.

Zero Offset:

The circle is drawn for the Wrist pin is directly sketched in the origin.

Positive Offset:

The circle drawn for the Wrist pin is offset 10mm positively from the origin.

Negative Offset:

The circle drawn for the Wrist pin is offset 10mm negatively from the origin.

• CONNECTING ROD:

A connecting rod, also called a con rod, is the part of a piston engine which connects the piston to the crankshaft. 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. Here the connecting rod is been drawn in front plane and extruded to get the part of connecting rod.

• END CAP:

The big end of the connecting rod connects to the crank pin journal to provide a pivot point on the crankshaft. A rod cap is the removable section of a two piece connecting rod that provides a bearing surface for the crank pin journal. Here the end cap is been sketched in front plane and been extruded to get the end cap part.

• 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. Here the Gudgeon pin is sketched in Front plane and extruded up to 127mm from mid-plane.

• CRANK:

Almost all reciprocating engines use cranks to transform the back-and-forth motion of the pistons into rotary motion. Here the crank is been sketched and extruded and then it is mirrored to get the full part.

ASSEMBLY:

The assembly file contains the above components that are properly mated to get the motion of piston kinematics.

MOTION ANALYSIS:

In the motion analysis, the study is done in three different cases to find out the linear displacement.

Wrist Pin Offset - The position of the wrist pin is going to be varied as followed. But the centre 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.

Zero offset:

The circle is sketched from front plane and constrained with origin i.e. with Zero Offset. Then the Motion study is been performed and the following graph is obtained.

Here the Linear displacement of piston varies from min 240mm to max 303mm is obtained.

10 mm Positive Offset - If the crank rotation is in the Clockwise direction. Also the circle is sketched from front plane and offset 10mm positive from origin i.e. with positive 10 mm Offset. Then the Motion study is been performed and the following graph is obtained.

Here the Linear displacement of piston varies from min 240mm to max 303mm is obtained. The smooth constant curve indicates that the constant linear displacement of piston with respect to time.

10 mm Negative Offset - If the crank rotation is in the Clockwise direction. Also the circle is sketched from front plane and offset 10mm positive from origin i.e. with positive 10 mm Offset. Then the Motion study is been performed and the following graph is obtained.

Here the Linear displacement of piston varies from min 240mm to max 303mm is obtained. The smooth constant curve indicates that the constant linear displacement of piston with respect to time. Here the Linear displacement of the Piston Head in all the three cases is obtained and given above.

SUPERIMPOSED PLOTS:

From three studies we get nearly the same plot for all. For comparing the results of all the three plots of different analysis results, we are using a superimposed graph. With the help of a superimposed graph, we can plot all the three graph in a single graph which helps to understand the difference between plots. The superimposed graph is as follows.

RESULT:

Here in the piston kinematics, we are required to do the motion analysis of piston assembly by Designing different Parts of the Piston Assembly and assemble them in SolidWorks for the following three conditions:

Wrist Pin Offset	Crank Speed (RPM)
0 mm	2000
10 mm Positive	2000
10 mm Negative	2000

As a result of the motion analysis, the observations made on seeing the study and plots are listed below.

• When the piston pin is in the centre of the piston i.e. the hole in centre, the piston reaches TDC or BDC.
• As a result, the connecting rod is moving straight up & down that places huge load on the connecting rod. Then the crank has to rotate past back to TDC or BDC in order to get the piston moving again for motion. This results more power and rpm speed from the engine and provides less power as out.
• In case of offset, the piston wrist pin/gudgeon pin is slightly little offset to one side of the piston either positively or negatively hence that results when the piston is at TDC or BDC, the rod is not moving straight up and down. This in turn, it allows the crank to rotate with much less resistance that requires less power and RPMs from the engine and provides the engine more power and rpm speed as out.

As a whole, Positive offset or Negative offset pin relieves the stress that is caused in the connecting rod when it reaches TDC or BDC when compared to zero offset Piston Assembly.

Drive link for the Parts and Assembly with the excel of Piston Kinematics: Piston Kinematics