MBD SIMULATION ON A PISTON ASSEMBLY

AIM:  To run a MBD simulation on a Piston Assembly with zero, 10 mm positive offset and 10 mm negative offset of gudgeon pin at a crank speed of 2000 rpm.

PISTON:  A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders 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 connecting rod. In a pump, the function is reversed and the force is transferred from the crankshaft to the piston for the purpose of compressing or ejecting the fluid in the cylinder.

CRANKSHAFT: A crankshaft is a shaft driven by a crank mechanism, consisting of a series of cranks and crankpins to which the connecting rods of an engine are attached. It is a mechanical part able to perform a conversion between reciprocating motion and rotational motion. In reciprocating engine, it translates reciprocating motion of the piston into rotational motion, whereas in a reciprocating compressor, it converts the rotational motion into reciprocating motion.

CONNECTING ROD: A connecting 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.

GUDGEON PIN: In IC 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.

GUDGEON PIN OFFSET: As engines evolved, engineers determined that offsetting the gudgeon pin yields two major benefits. First, it improves the noise characteristics of the engine due to piston slap (rocking of the piston in the cylinder) 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 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.

PROCEDURE:-

• Make 3D models of piston head, gudgeon pin, connecting rod, end cap, and crank from the given drawings.

                                                                                                                PISTON HEAD

                                                                                                                      GUDGEON PIN

                                                                                                                    CONNECTING ROD

                                                                                                                            END CAP

                                                                                                                                     CRANK

• Open assembly to assemble the parts.
• Make the axis of crank and right plane of assembly coincident. Then make the axis of crank coincident with the top plane.
• Using distance constraint, enter a distance of 36 mm between front plane of assembly and front plane of crank.
• Make the axis of connecting rod and end cap coincident.
• Make flat bottom face of connecting rod and upper face of end cap coincident.
• Make the axis of connecting rod and crankshaft coincident.
• Enter a distance of 1 mm between the faces of crankshaft and connecting rod.

• Make the axis of gudgeon pin and axis of bore of piston head coincident.
• Make the front plane of gudgeon pin and front plane of piston head coincident.
• Make the axis of gudgeon pin and axis of small end of connecting rod coincident.
• Make the front plane of piston head and front plane connecting rod coincident.
• Make the right plane of piston head and right plane of assembly coincident.

                                                                                        PISTON ASSEMBLY WITH ZERO OFFSET OF GUDGEON PIN

• Go to motion analysis and add a motor by selecting the axis of crank. Enter value of speed as 2000 rpm.

• Select crankshaft, end cap, connecting rod, gudgeon pin, and piston head as a pair in contact and select material as steel (dry).
• In gravity select Y axis.
• Go to motion study properties and enter value of 12000 frames per second with precise contact option.
• Drag the end time to set the time to 1 second and click on calculate study to run the simulation.
• Select top flat face of piston head in results and plots to show the graph of linear displacement vs. time of piston head.

                                        SIMULATION OF PISTON ASSEMBLY WITH ZERO OFFSET OF GUDGEON PIN AT 0.1x PLAYBACK SPEED

• Similarly make the 3D models of piston head by offsetting the gudgeon pin bore 10 mm (positive and negative), assemble the parts and run the simulations.

                                                                         PISTON HEAD WITH 10 mm POSITIVE OFFSET OF GUDGEON PIN BORE

                                                                       PISTON HEAD WITH 10 mm NEGATIVE OFFSET OF GUDGEON PIN BORE

                                                                      PISTON ASSEMBLY WITH 10 mm POSITIVE OFFSET OF GUDGEON PIN

                                           SIMULATION OF PISTON ASSEMBLY WITH 10 mm POSITIVE OFFSET OF GUDGEON PIN AT 0.1x PLAYBACK SPEED

                                                                        PISTON ASSEMBLY WITH 10 mm NEGATIVE OFFSET OF GUDGEON PIN

                                        SIMULATION OF PISTON ASSEMBLY WITH 10 mm NEGATIVE OFFSET OF GUDGEON PIN AT 0.1x PLAYBACK SPEED

OBSERVATIONS: The above graph and data shows that when the offset of gudgeon pin is zero mm the maximum linear displacement of piston head is 310.95 mm at 0.007 s and minimum linear displacement is 214.92 mm at 0.022 s. In case of 10 mm negative offset of gudgeon pin the maximum displacement of piston head is 310.77 mm at 0.038 s and minimum linear displacement is 214.63 mm at 0.023 s. When there is 10 mm positive offset of gudgeon pin the maximum linear displacement of piston head is 310.76 mm at 0.007 s and minimum linear displacement is 214.63 mm at 0.022 s.

CONCLUSION: The maximum linear displacement of piston head decreases (310.76-310.77 mm) when there is a 10 mm positive as well as 10 mm negative offset of gudgeon pin  as compared to maximum linear displacement of piston head (310.95 mm) when there is  zero offset of gudgeon pin. The minimum linear displacement of piston head in both cases (10 mm positive and negative offset of gudgeon pin) increases to 214.63 mm from 214.92 mm (zero offset) at the same time i.e. 0.022 s to 0.023 s. However, the piston head takes more time i.e. 0.038 s to reach maximum linear displacement of 310.77 mm when there 10 mm negative offset of gudgeon pin as compared to zero offset and 10 mm positive offset of gudgeon pin which have difference in maximum linear displacement at the same time equal to 0.007 s.

REFERENCES:-

• WIKIPEDIA
• http://blog.wiseco.com/what-is-wrist-pin-offset