MBD Simulation on a Piston Assembly

AIM : To model all the parts of piston assembly and then assemble them in solidworks and run motion analysis.

OBJECTIVE :

• To create 3D model of different components of piston assembly.
• To run motion study analysis for the following cases -

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

"0" mm offset - if the crank rotation is in the clockwise direction.

"10" mm positive offset - If the crank rotation is in the clockwise direction.

"10" mm negative offset - If the crank rotation is in the clockwise direction.

• To plot the linear displacement of the piston head in all three cases and then superimposing graphs by exporting them to microsoft excel.

THEORY -

Piston : The assembly of the piston, connecting rod and crank shaft in an engine behaves like a mechanism during the working of the engine. The piston makes the crankshaft to turn by utilizing the energy supplied to it by the combustion of the fuel. It is cylindrical in shape and reciprocates inside the cylinder. Pistons are provided with groves near the top and provide and air tight fit. The pistons do not allow the high pressure mixture from the combustion chambers into the crankcase. The piston has four strokes in total two upside and two downside.

Connecting rod: The connecting rod links the piston and the crankshaft. It has a hole at the upperend and is connected to the piston by the wrist pin. The lower end also called as big end is attached to the crank shaft. The small end is press fit and can swivel in the pston. The connecting rods are usally made of alloy steel , titanium and sometimes with aluminium. They are not rigidly fixed at either end so the angle between connecting rod and piston can change as the rod moves up and down and rotates around the crankshaft. Higher factor of safety is provided to connecting rod as the failure is very likely under heavy stresses.

Crankshaft : The turning motion to the wheels is provided by the crank shaft which converts the reciprocating of the pistons into a rotary motion. The crank shaft is usally either alloy steel or cast iron.

Gudgeon Pin - In internal combustion engine 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.

PROCEDURE :

• 3D modelling of different components :
• At first different components are designed according to dimensions provided.
• design of crank

• Design of piston

• Design of connecting rod

• Design of pin

• Design of end cap

• Assembly

• Assembly of all the components :

1. The components modelled are now assembled by importing into assembly. The first component inserted is made to float from being fixed. The temporary axes should be selected for the ease in the assembling. All the components are assembled with necessary constraints, so the desired motion can be achieved
2. At first the crank shaft is inserted whose axis is made co-incident with the top and right plane and then its front plane is placed at some distance from the assembly's front plane . Then the connecting ros and the end cap are inserted and both of their axis are made coincident and then their faces are made coincident so that they move together.
3. The axis of the crank is made coincident with the axis of the connecting rod. Then one side of the connecting rod is selected and the corresponding side of the crank is selected and made coincident with a distance mate of 1 mm to provide some clearance similarly piston head and wrist pin are inserted and made coincide along their respective axis and planes.
4. The wrist pin axis and connecting rod axis rae made coincident and then the front plane of the connecting rod and the front plane of the piston head are made coincident. The right plane of the piston is made co incident with assembly right plane.

• MOTION ANALYSIS:

After complete and proper assembly, motion study using solidworks add-ins were carried out for the cases provided in the challenge statments.

• CASE 1 ('0' mm offset)-

1. A torque/motor was provided for the axis of crank with 2000 rpm in clockwise direction. Solid body contacts were provided for different components with the material selected as steel(Dry) , Gravity was provided in the downward direction(Y-axis) precise contact and 12000 fps were provided as input for smooth calculations.
2. Plot obtained for linear displacement of piston from results -

• CASE - 2 ('10' mm Positive offset ) :

1. A torque/motor was provided for the ais of crank with 2000 rpm in clockwise direction. solidbody contacts were provided for different components with material selected as steel(Dry) , Gravity was provided in the downward direction(Y-axis),10 mm offset for hole position in piston head. Precise contact and 12000 fps were provided as input for smooth calculations.
2. Plot obtaines for linear displacement of piston from results -

• CASE - 3 ('10' mm negative offset ) :

1. A torque/motor was provided for the ais of crank with 2000 rpm in clockwise direction. solidbody contacts were provided for different components with material selected as steel(Dry) , Gravity was provided downward direction(Y-axis),10 mm offset for hole position in piston head. Precise contact and 12000 fps were provided as input for smooth calculations.
2. Plot obtaines for linear displacement of piston from results -

LINKS FOR ANIMATION

• CASE 1 ('0' mm offset) - https://youtu.be/ZT0aqAbgyOc
• CASE - 2 ('10' mm Positive offset ) - https://youtu.be/vf5Jkc-Sc6c
• CASE - 3 ('10' mm negative offset ) - https://youtu.be/Jnn9u2E48YE

CONCLUSIONS:

1. Offsetting the cylinder axis from the crankshaft axis minimizes the 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 th ebegining of the power stroke.
2. Because of offsetting there is reduction in piston displacement piston has to travel less distance from TDC to BDC.