MBD Simulation on a Piston Assembly

MBD Simulation on a Piston Assembly

1. Aim

Model all the parts of a piston assembly and perform the motion analysis in different conditions.

2. Objectives

1. Performing Motion Analysis of the Piston Assembly in given below condition of the piston.

Note: Crank rotation is in the Clockwise direction.

a) Case 1: No Offset - Centre of wrist pin and centre of crank should conincide in a single plane.

b) Case 2: Positive Offset

c) Case 3: Negative Offset

2. Obtaining Plots for Linear Displacement of the Piston Head in these three cases.

3. Superimposing the three graphs in excel to compare the results.

3. Introduction

The Piston Assembly Consist of mainly five parts such as Piston itself, Gudgeon pin, Connecting Rod, Endcap, and Crank. In an engine the piston assembly's 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.

A slider crank linkage is a four-link mechanism with three revolute joints and one prismatic, or sliding, joint. The rotation of the crank drives the linear movement the slider, or the expansion of gases against a sliding piston in a cylinder can drive the rotation of the crank.

There are two types of slider-cranks: in-line and offset.

1. In-line: An in-line slider-crank has its slider positioned so the line of travel of the hinged joint of the slider passes through the base joint of the crank. This creates a symmetric slider movement back and forth as the crank rotates.
2. Offset: If the line of travel of the hinged joint of the slider does not pass through the base pivot of the crank, the slider movement is not symmetric. It moves faster in one direction than the other. This is called a quick-return mechanism.

4. CAD Geometry

The various parts for the Piston assembly are modelled in solidworks according to dimensions. i.e Piston Head , Connecting Rod, Wrist Pin , Crank and End Cap and the following models are given below. Major tools used for modelling the geometry is Sketch, Extrude Boss/base, Extrude cut, Revolve Boss/base, Fillet and Chamfer.

4.1 Piston Assembly

4.2 Part 1 - Piston head

4.3 Part 2 - Connecting rod

4.4 Part 3 and 4 - Wrist pin and End cap

To connect the Piston and the Connecting Rod, a Wrist Pin is designed. The End Cap is designed with respect to the Connecting Rod End in order to have a proper fit.

4.5 Part 5 -Crank

A crankshaft is a mechanical part able to perform the conversion between reciprocating motion and rotation motion. It translates reciprocating motion of the piston into rotation motion. Whereas in the reciprocating compressor, it converts rotational motion into reciprocating motion. To transmit the motion from Connecting Rod, a Crank is designed with shaft size in accordance with that of the End Cap and the Connecting Rod.

5. Motion study setup

The motion features of the software can assist with a wide range of motion study depending on how complicated your requirements are, and they can be divided into three categories:

• Animation: If you simply wish to create some nice visuals for presentation or marketing without consideration of mass and gravity effects, then animation is for you.
• Basic Motion: For an extra layer of complexity that takes into consideration the effects of mass, springs, gravity and physical collision detection, then a Basic Motion study is more suited for your requirements.
• Motion Analysis: This is the top tier of motion study and takes into account a wider range of physical interactions such as impact effects, damping, force, momentum, etc.

5.1 Steps for motion analysis

1. Activate the motiton analysis plugin of solidwokrs in add-ins tab.
2. Select the motion analysis from the left drop down menu where there are other option including animation and basic motion. Animation will be selected as default so we need to select motiton analysis from the list.
3. In all cases add the motor to the crank as input. The motor is set to 2000 RPM in all cases.
4. As the piston should move during the motion analysis in relative to connecting road we need to give information to solidwworks that these are solid contact bodies. For this we use 'Contact' option from motion analysis options. Solid contact bodies to be set is given below in diagram
5. Motiton study properties can be selected to input necessary accuracy details and also to enable precision contact. As the motor is rotating in 2000rpm = 33.333 RPS. To get every degree change as a frame we need to set 33.333*360 = 12000 frames per second in the motion analysis settings. Also on the 'precision contact' setting to get good results.
6. Add gravity in Y direction down using the gravity icon next to contact button.
7. After including all settings to analyse the motion click the calculate button.
8. Using 'Results and plots' option we can plot different graphs and get results. Linear displacement can be selected from dropdown menu Displacement/Velocity/Acceleration > Linear displacement > Magnitude and select the top surface of the piston.

Solid contact bodies to be set is given below

Gravity and motion precision settings

5.2 Animation

6. Results, Observation and Conclusions

The linear displacement v/s time graph obtained is a smooth sinusoidal curve since the frames per second(fps) used in motion analysis is high (12000).

Case 1: No Offset

The linear displacement plot of the piston head displayed a sinusoidal curve. Maximum value being 302.49mm and min being 202.44mm.

Case 2: Positive Offset

The linear displacement plot of the piston head displayed a sinusoidal curve. Maximum value being 302.47mm and min being 202.50mm.

Case 3: Negative Offset

The linear displacement plot of the piston head displayed a sinusoidal curve. Maximum value being 302.47mm and min being 202.50mm. which is similar to positive offset case.

Superimpose all the three graphs

Note: This graph is made with more data from the above same cases.

1. The linear displacement plot of the piston head displayed a sinusoidal curve.
2. From the superimposed data study it is found that the maximum displacement of the piston without offset is slightly more than that with other two offset cases. It is negligible so that it is not very noticable in the graph. Maximum value being 302.49mm and min being 202.44mm in case of zero offset conditions and maximum value being 302.47mm and min being 202.50mm in case of positive and neagtive offset conditions. Stroke lenght of a zero offset piston assembly is given by 2 x radius of crank = 2x50 = 100mm. Calculating the strokelenght from graph we get stroke lenght = 302.49 - 202.44 = 100.05mm which is very close to the theoretical value. So the graph is correct. For offset cases the stroke lenght = 302.47 - 202.50 = 99.97mm. Thus we can say that by introducing an offset the linear displacement or stroke length of the piston decreases. As the offset is very small and hence the change in stoke length.
3. The other only difference observed was, when an offset is introduced in the model, the waves shift from its original position. But the horizontal lag has no significance since the time period is still the same.
4. In the zero offset case, when the piston reaches the top dead center (TDC) and bottom dead center (BDC) the connecting rod and psiton will be totally alligned and the connect rod will be straight vertical. The crank has to takes a lot of power from the engine to rotate from these dead centre positions in zero offset case.
5. By giving an offset in the wrist pin the connecting rod will not be straight up vertical 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.

CAD FILE

The 3D model files : Link

https://drive.google.com/drive/folders/1plz-An4Kono-f9Lmy2JOduPfUqkR3MAr?usp=sharing