MBD Simulation on a Piston Assembly

Aim

The objective of this task is to perform motion analysis on Piston assembly by varying the position of wrist pin offsets in the assembly and studying the linear displacements of each.

Introduction

"As the automobile goes, so goes the engine; and as the engine goes, so goes the piston"

Automobiles play a key role in today’s generations and is a prominent symbol of growth and improvement. The quest to get improved fuel economy, lower emissions and higher efficiency, automakers and engineers are constantly striving for lighter, lower-friction pistons with the stamina to endure tougher operating conditions.

Thus pistons play a key role in improving the efficiency of the vehicles. So to understand how such an important arrangement works in a sophisticated space, we need to understand how each component in the assembly actually works

What is a piston?

A piston is a moving disk enclosed in a cylinder. The disk moves inside the cylinder as a liquid or gas inside the cylinder expands and contracts. Piston helps in the transformation of heat energy into mechanical work and vice versa. There are various parts in the piston, namely piston skirt, Crown Valve Recess, piston rings ect.

What are piston rings and what are its uses?

Piston rings play a crucial in internal combustion engines. They must absorb the combustion pressure, cool the piston, keep the oil in the crankcase and provide a film of oil on the cylinder wall. Most modern 4-stroke engines are usually fitted with three piston rings per cylinder. The first ring holds back the combustion gases. The second ring helps to hold back combustion gases and also scrapes the oil downward. The third ring holds the oil where it needs to be.

What is wrist pin and what’s it application?

In IC engines, the wrist pin connects the piston to the connecting rod, and provides a bearing for the connecting rod to pivot upon as the piston moves.

What is a connecting rod and what’s it application?

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.

What is an end cap/rod cap?

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.

What is crank and crankshaft and where is it used?

A crank is an arm attached at a 90 degree angle to a rotating shaft. When combined with a connecting rod, it is used to convert circular motion into reciprocating motion, or vice versa.

A crankshaft is a rotating shaft which then converts reciprocating motion of the pistons into rotational motion. These are commonly used in internal combustion engines.

Advantages and application

The piston arrangement is often used and found in every place where there is an engine and the motion has to be transferred especially in Automobiles.

Modelling

Piston

Piston with Zero Offset

Piston with Positive Offset

Piston with Negative Offset

Connecting rod

Wrist Pin

End cap

Crank

Assembly

Working process

When the combustion process takes place, the pressure generated in the cylinder pushes the piston down. When the piston tends to move down, the wrist pin which is connected to the connecting rod is pushed down which creates a reciprocating mechanism. The connecting rod is assembled with the end cap having the crank in between. Now as the piston moves up and down the crank converts the reciprocating mechanism to rotatary motion.

Methodology of analysis

To perform the motion analysis and study how the mechanism works, a few situations are taken and studied.

• In the 1^st motion study the the wristpin offset is 0mm
• In the 2^nd motion study the the wristpin offset is 10mm posotive
• In the 3^rd motion study the the wristpin offset is 10mm Negative

Motion Analysis

Assembly and Mates

For constraining and setting the components in right places, Coincident and distance mates are used.

Motor, Solid Contacts, Gravity and FPS

Motor

Motor speed is considered to be 100 fps

Solid Contact

All the solid contacts are made considering the material to be steel dry.

Contact 1

Contact 2

Contact 3

Contact4

Gravity

FPS

Considering the speed to be 100, fps is ((100 x360)/60)=600.

Plots

Case-1 Zero wristpin offset

Linear displacement here is 310.5-210.5=100

Case-2:- Positive wristpin offset

Linear displacement here is 310.31-210.19=100.12 i.e. ~100

Case-3:- Negative wristpin offset

Linear displacement here is 319-222=97

Results

Based on the plots obtained its clear that linear dispalcemeny is almost same in 3 cases with slight variations.

When the wrist pin is provided with zero offset or positive offset the Top dead centre of the piston reached is 310 mm, but in case of negative offset it reached 319mm which is slightly higher than the rest.

Similarly, in case of return stroke in the case when the wristpin is given a positive offset or a zero offset the BDC reaches 210mm, whereas in negative offset it reaches 222mm.

Standard offset

Positive Offset

Negative Offset

Conclusion

The detailed study has been made for the piston arrangement ,considering the changes w.r.t to the position of offsets to the wristpin.