MBD Simulation on a Piston Assembly

AIM : Multibody Dynamics Simulation on a Piston Assembly.

INTRODUCTION :

PISTON : It is a moving disk enclosed in a cylinder which is made gas-tight by piston rings. The disk moves inside the cylinder as a liquid or gas inside the cylinder expands and contracts. A piston aids in the transformation of heat energy into mechanical work and vice versa. Because of this, pistons are a key component of heat engines. Pistons work by transferring the force output of an expanding gas in the cylinder to a crankshaft, which provides rotational momentum to a flywheel. Such a system is known as a reciprocatig engine.

A piston must follow a cyclical process in order for it to continuously convert heat energy to work, and there are many ways to complete this cycle. For example:

1. By inputting heat to the gas inside the cylinder, the gas will expand increasing the volume in the cylinder and provide useful work.
2. By removing heat from the cylinder, the gas's pressure will decrease, allowing it to be compressed more easily.
3. By inputting work to the piston, the piston will compress back to its initial state, ready to perform the cycle once again.

CONNECTING ROD :

A connecting rod is an engine component that transfers motion from the piston to the crankshaft and functions as a lever arm. Connecting rods are commonly made from cast aluminum alloy and are designed to withstand dynamic stresses from combustion and piston movement. The small end of the connecting rod connects to the piston with a piston pin. The piston pin, or wrist pin, provides a pivot point between the piston and connecting rod. Spring clips, or piston pin locks, are used to hold the piston pin in place.

The big end of the connecting rod connects to the crankpin journal to provide a pivot point on the crankshaft. Connecting rods are produces as one piece or two-piece components. A rod cap is the removable section of a two-piece connecting rod that provides a bearing surface for the crankpin journal. The rod cap is attached to the connecting rod with two cap screws for installation and removal from the crankshaft.

CRANK : The crankshaft is essentially the backbone of the internal combustion engine. The crankshaft is responsible for the proper operation of the engine and converting a linear motion to a rotational motion. Crankshafts should have very high fatigue strength and wear resistance to ensure long service life. The crankshaft experiences high levels of cyclical loading. A high value of the coefficient of thermal expansion can be a problem when choosing materials for crankshafts and camshafts.Crankshafts are one of the most crucial components of internal combustion engines and are subjected to very high dynamic loads during engine use. The selection of materials and manufacturing methods depend on the type of engine and the geometry and design of the crankshaft. It is necessary to use high strength materials to ensure a long operational life.

GUDGEON PIN : The piston pin -also known as the gudgeon pin- transfers the entire generated power out of the combustion process from the piston, via the connecting rod down to the crankshaft. Due to the fact that the piston pin does not rotate, he is the engine component that have to withstand the highest stresses in engine operation.The piston pin connects the piston and the connecting rod and must withstand extremely high loads in changing directions. It is commonly made out of case-hardening or nitraded steel with a hardened, smooth ground and polished surface.

In order to withstand the occuring loads out of the combustion process as well as the continuous increasing firing pressures, it is essential to permanently optimize the load-bearing capacity of this component..

Piston pins are characterized by precision, application diversity and the highest material quality.Due to the oscillating forces of the ignition pressure and mass inertia forces in the opposite direction, the piston pin is bent through, oval-shaped and subjected to shear. The piston pin boss can be damaged by excessively large deformations of the piston pin, which can lead to the complete destruction of the piston.Only in the case of small bolt deformations, the stresses remain below the permissible values.

In this project, we will model all the parts and then assemble them in SolidWorks. The motion analysis is going to be carried out in three different conditions,

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

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

We will obtain the Linear displacement of the Piston Head in all the three cases. Then export the graphs into Excel. Then Superimpose all the three graphs into a single graph.

PROCEDURE :

• Create 3D drawings of the crank, gudgeon pin, connecting rod, end cap and piston head by providing the correct parameter values.

crank

connecting rod

end cap

gudgeon pin

piston head

• The final assembly after applying the proper mates between different components is obtained as

• Now we will perform motion analysis on the assembly for the 3 cases.

Case 1 : 0 mm offset

Case 2 : 10 mm Positive Offset - If the crank rotation is in the Clockwise direction.

Case 3 : 10 mm Negative Offset - If the crank rotation is in the Clockwise direction.

• Superimposition of all the 3 graphs on a single graph :

Conclusions : By introducing the offset in the wrist pin by 10 mm positive and negative, the connecting rod moves in an inclined way when the piston travels from top dead centre to bottom dead centre and vice versa. This reduces the friction for the crank and allows the crank to rotate more easily thereby improving the power and speed output of the engine.

Also by introducing the offset, we are also reducing the stroke length which in turn increases the number of strokes per unit time.