MBD Simulation on Piston Assembly

AIM: To study the motion of the piston assembly

OBJECTIVES:

1. Design all the components of piston assembly
2. Assemble all the components of the piston assembly
3. Perform motion analysis on the piston assembly
4. Obtain plots for linear displacement of the piston crown
5. Compare the plots of all three conditions and draw a conclusion

THEORY:

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.

The piston is a disc which reciprocates within a cylinder. It is either moved by the fluid or it

moves the fluid which enters the cylinder. The main function of the piston of an internal combustion

engine is to receive the impulse from the expanding gas and to transmit the energy to the crankshaft

through the connecting rod. The piston must also disperse a large amount of heat from the combustion

chamber to the cylinder walls.

The connecting rod is the intermediate member between the piston and the crankshaft. Its primary

function is to transmit the push and pull from the piston pin to the crankpin and thus convert the

reciprocating motion of the piston into the rotary motion of the crank. The usual form of the connecting

rod in internal combustion engines is shown in Fig. 32.9. It consists of a long shank, a small end and a

big end. The cross-section of the shank may be rectangular, circular, tubular, I-section or H-section.

Generally circular section is used for low speed engines while I-section is preferred for high speed engines.

A crankshaft (i.e. a shaft with a crank) is used to convert reciprocating motion of the piston into

rotatory motion or vice versa. The crankshaft consists of the shaft parts which revolve in the main

bearings, the crankpins to which the big ends of the connecting rod are connected, the crank arms or

webs (also called cheeks) which connect the crankpins and the shaft parts. The crankshaft, depending

upon the position of crank, may be divided into the following two types:

1. Side crankshaft or overhung crankshaft, and
2. Centre crankshaft.

Offsetting the cylinder axis from the crankshaft axis, minimizes 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 the beginning of the power stroke when pressure rises sharply about 12-15 degrees after TDC. Rather than revising a complete engine design, the easy way to do this is to offset the pin bore in the piston. This allows the same crank/rod pin axis geometry as an engine designed with offset cylinder axes, but within existing conventional engine architecture. The amount by which the cylinder axes need to be offset depends on many variables, but the main ones are the ratio of crank throw to con rod length and the angle after TDC at which maximum cylinder pressure occurs.

PROCEDURE:

Piston

Connecting rod

End cap

Wrist pin

Crank

Assembly

Insert all component and provide the following mates

Crank axis with assembly right plane

Crank axis with top plane

Front plane of Crank with assembly front plane with 36mm distance

Axis of end cap and axis of connecting rod

Face of end cap and face of connecting rod

Axis of Crank and axis of connecting rod

Face of Crank and face of connecting rod with 1mm distance

Axis of wrist pin and axis of piston

Front plane of wrist and front plane of piston

Axis of connecting rod and axis of wrist pin

Front plane of connecting rod and front plane of piston

Right plane of piston with right plane of assembly

For Case 2, with 10 mm positive offset and Case 3, with 10mm negative offset

Select piston->right-click on it->click open part option->select the sketch responsible for wrist pin hole and select edit option -> select the smart dimension-> click on circle center and origin and give 10mm distance->exit sketcher->save the file. An 10mm offset will be created in assembly.

Case 1 Assembly

Case 2 Assembly

Case 3 Assembly

Motion Analysis

First select Motion Analysis from SOLIDWORKS Add-Ins ribbon, then click on motion study. Click on drop down list on bottom-left side and select motion analysis.

Select Motion Study Properties, input 7200 fps in fps option box and tick the use precise contact.

Apply motor on crack axis in clockwise direction with 2000rpm.

Apply gravity from gravity option in Y direction.

Click on result option and select Displacement/Velocity/Acceleration->Linear Displacement->Magnitude->Select piston face.

Establish 4 contacts between crack and end cap, end cap and connecting rod, connecting rod and wrist pin, wrist pin and piston.

Click on calculate button to start motion analysis. Wait for some time, we can provide time but it takes too much time; so, we pause motion analysis after some time and get the plot. We export the result generated to spreadsheet by right-clicking result->export to spreadsheet.

RESULTS:

Plot below shows the superimposed plot of all the three cases. As we can observe we see very little difference. We can see difference if we decrease the unit scale of the plot.

Below we can see all the three plots of all the three cases one below the other. As we can observe there is difference in linear displacement of the piston head. The wrist pin offset does impact the linear displacement of the piston.

CONCLUSION:

As observed form the above graphs the linear displacement remains almost same in all the three cases.

1) When the wrist pin is not provided any offset, the piston reaches the top dead center (TDC) and bottom dead center (BDC) by making the connecting rod straight up and down which place a huge load on the connecting rod. The crank has to rotate past TDC or BDC in order to get the piston moving again which takes a lot of power and RPMs from the engine.

2) By introducing offset in the wrist pin of positive 10mm or negative 10mm we can observe that the motion of the connecting rod is not straight up and down when the piston is at TDC and BDC. This in turn allows the crank to rotate with much less resistance giving the engine more power and speed. This concept of offsetting the wrist pin is widely used in racing cars

REFERENCES:

https://x-engineer.org/automotive-engineering/internal-combustion-engines/ice-components-systems/internal-combustion-engine-piston/

https://auto.howstuffworks.com/engine2.htm

https://en.wikipedia.org/wiki/Piston

A Textbook of Machine Design by R.S. Khurmi

LINKS:

YouTube Video GDrive