MBD Simulation on a Piston Assembly

Aim: To model all parts of a piston assembly and assemble them and also run a motion analysis for the given three cases using SolidWorks.

Objective:

• To create a 3D model of different components.
• To run a motion study analysis for the following cases:

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

Theory:

Piston - A 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. In a pump, the function is reversed and force is transferred from the crankshaft to the piston for the purpose of compressing or ejecting the fluid in the cylinder. In some engines, the piston also acts as a valve by covering and uncovering ports in the cylinder.

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 aluminium 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 produced as a 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.

Crankshaft - Its main function is to transform the linear motion of the piston into rotational motion. The pistons are connected to the crankshaft through the connecting rods. The crankshaft is mounted within the engine block.

Gudgeon Pin - A crankpin, also known as a crank journal, is a mechanical device in an engine which connects the crankshaft to the connecting rod for each cylinder. It has a cylindrical surface, to allow the crankpin to rotate relative to the "big end" of the connecting rod.

Procedure:

1. Design of crank

2. Design of Piston Head

3. Design of Connecting Rod

4.Design of Wrist pin

5. Design of End Cap

Assembly of Components designed:

All the components are imported and assembled now while providing all the necessary constraints. The first component is made to float from being fixed. The temporary axes should be selected for ease of assembling. Desired motion is achieved due to the constraints applied.

First, the crankshaft is inserted whose axis is made coincident with the top and right plane then its front plane is provided with a certain distance from the front plane of the assembly. Then the connecting rod and end cap are inserted and both of their axes are made coincident and then their faces are made coincident so that they move together as one individual part.

Then the axis of the crank is made coincident to the axis of the connecting rod. One side of the connecting rod is selected and the corresponding side of the crank is selected and made coincident with a distance of 1mm which serves as clearance. Similarly, wrist pin and piston head are inserted and made coincident along their respective axes and planes so they move as one individual part.

Next, the wrist pin axis and connecting rod axis are made coincident and then the front plane of the piston head are made coincident. The right plane of the piston is made coincident with the assembly's right plane.

Motion Study and Analysis:

Case 1 : 0 mm Offset

A torque motor is provided for the axis of the crank with 2000 RPM in the clockwise direction. Solid-body contacts along with gravity were provided and the material selected was Steel(Dry). For 2000RPM, degrees per second almost translates to 12000 so that is provided as the frames per second for a smooth transition and to obtain more accurate results by running calculations.

Case 2: 10 mm Positive Offset

A torque motor is provided for the axis of the crank with 2000 RPM in the clockwise direction. Solid-body contacts along with gravity were provided and the material selected was Steel(Dry). For 2000RPM, degrees per second almost translates to 12000 so that is provided as the frames per second for a smooth transition and to obtain more accurate results by running calculations.

Case 3: 10 mm Negative Offset

A torque motor is provided for the axis of the crank with 2000 RPM in the clockwise direction. Solid-body contacts along with gravity were provided and the material selected was Steel(Dry). For 2000RPM, degrees per second almost translates to 12000 so that is provided as the frames per second for a smooth transition and to obtain more accurate results by running calculations.

Conclusions:

The amplitude for the plot with 0mm Offset is 302.499 mm and is maximum as compared to 10m Positive Offset(302.3094mm) and 10mm Negative Offset(302.3090mm) which is minimum, because there is reduction in the piston displacement which is due to the fact that the piston has to travel less distance from TDC to BDC because of the offsetting.

2.Offsetting the cylinder axis from the crankshaft axis minimizes rod angularity when cylinder pressure is at it's 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.