MBD Simulation on a Piston Assembly

Aim : Multibody Dynamics Simulation Using Solidsworks MBD on a Crank-Piston Assembly.

Introduction : Piston Crank mechanism is a demonstration of a simple mechanical system of linkage and motion chains. The objective is to display the working of a piston inside a engine cylinder and how reciprocating motion of the piston gets converted to rotational motion of the crank.

When the piston is manually pushed and pulled in vertical direction, the connecting rod which connects the piston to the crank transmits the power to the crank wheel which rotates about its crank shaft axis.

The power/torque of the crank shaft rotation is transmitted to the wheels which drives the vertical.

The parts required for the assembly is as follows.

• Piston : The power in an engine all comes from the force pushing on the top of the piston. That force is determined as the area of the piston multiplied by the gas pressure. Bigger piston and higher gas pressure will provide more power. The top surface of the piston is called the crown or head. The piston rings fit around the piston bridging the small clearance between the piston and cylinder wall. There are typically three piston rings on a piston. The top two rings are called compression rings or pressure rings. And their main role is to prevent gases from getting through the small clearance between the piston and cylinder wall. The lower ring on the piston is an Oil control ring. For minimal friction , an thin oil firm is needed and the function of the oil control ring is to remove excess oil and leave an ideal oil firm for the compression rings and piston skirt to glide over.

• Wrist Pin : The piston is attached to the connecting rod through a hollow, hardened steel tube known as wrist pin or gudgeon pin. This wrist pin carries the full force of combustion. The wrist pin may be slightly offset laterally rather than exactly centered on the piston. This is known as offset wrist pin.

• Connecting Rod : The connecting rod carries the force from the piston to the crankshaft, it is constantly subjected to stretching, squashing and bending forces as it acts as the intermediary in this push-pull relationship.

• End Cap : End cap is connected to the end of the connecting rod which is assembled with crank.

• Crank : The crankshaft is the moving part of an engine. The piston is connected to the crankshaft through the connecting rod. The main function of the cranks is to transform the liner motion of the piston into rotational motion.

• Piston Assembly : Piston, wrist pin, connecting rod and crank are assembled as shown below.

Procedure :

1. All the parts are designed individually in Solidsworks.
2. Now go for the assembly of the following parts.
3. Then start the motion analysis .

Motion analysis –

• After complete the motion, Solidsworks is enabled.
• A motor is applied to the crank and give a rotation of 2000rpm in the clockwise direction.
• The position of the wrist pin is going to varied as followed.
• Apply solid body connect on all parts.

There are total 4 connect pairs –

• Crank and end cap
• Crank and connecting rod
• Connecting rod and wrist pin &
• Wrist pin and piston.

And then-

1. Gravity is enabled in a downward direction w.r.t the assembly.
2. Add precise contact and frames per second is increased to 7200 for the purpose that we don’t want to miss any peak value of the desired parameter and simulation becomes more smooth and accurate.
3. Finally calculate and go for results and plots.

Motion study :

Case 1 – 0mm offset and 2000rpm in clockwise direction.

Case 2 – 10mm positive offset and 2000rpm in clockwise direction.

Case 3 – 10mm negative offset and 2000rpm in clockwise direction.

Results :

Case 1 - 0mm offset and 2000rpm in clockwise direction.

Case 2 – 10mm positive offset and 2000rpm in clockwise direction.

Case 3 – 10mm negative offset and 2000rpm in clockwise direction.

Superimposed graph of all three :

Observation :

• The linear displacement graph is smooth for all the 3 cases.
• An offset pin is to reduce piston side loading. It reduces the knocking noise of the engine.
• In superimposed graph we see that linear displacement of piston head is slightly same for every cases.