MBD Simulation on a Piston Assembly

AIM:

To carry out motion analysis study on piston assembly and it's various stroke length arrangements

INTRODUCTION:

the piston assembly under consideration is made of the following parts:

• piston head
• connecting rod
• end cap
• gudgeon pin
• crank

PART FILE (link): https://drive.google.com/drive/folders/1xR1eQ2jykauIAzXmLGYrUrIMtXqosexB?usp=sharing

Assembly link: https://drive.google.com/drive/folders/1YDKIS7CU3LerOrI2Wz3BoGMOKqJNVsPi?usp=sharing

video link:https://youtu.be/81A77Ipw4kc

GEOMETRY:

we are studying the kinematics of piston under three different stroke positions. the following figure represents important aspects to gain more insight into the relation between reciprocatory motion of piston head and rotary motion of the crank.

by examining the representation, we can derive the following details:

• bore = 127 mm
• offset distance of piston stroke = 10 mm (each side)
• center to center distance of con. rod = 212.5 mm
• center to center distance of crankshaft axis to joint of con rod (crank radius)= 50 mm

STROKE LENGTH:

for case-1 with no offset or neutral position, the stroke length of piston is obvious and that of the diameter of crank i.e. crank diameter = 100 mm

hence S1 = 100 mm

for second and similarly third case, the stroke length is found by following figure:

for a line of 10 mm offset, we can see that the stroke length is the same as the previous case, hence stroke length is independent of the offset of the axis on which piston reciprocates.

S2 = 100 mm and,

S3 = 100 mm

for finding stroke length in offset cases, we have to align crank radius with connecting rod in order to visualize gudgeon pin position for both the extreme positions. i.e top dead center & bottom dead center.

LINEAR DISPLACEMENT OF PISTON:

in all three cases, the displacement is taken as the difference between positions of any point on the piston which travels through TDC and BDC. now, considering the part of the piston head as a rigid body, all the points of piston head are going to have the same displacement because the piston simply does reciprocation without any rotary motion. however, the software here takes the origin of piston head in part file ( and not the center of mass of piston head) as a point that represents all the points of piston head which can be seen by the following figure.

in all 3 cases, the plot obtained for linear displacement vs time shows initial values around 302 mm and lowest values of 202 mm obtained at 14400 fps with precise contact.

however, the MBD solver of the software plots displacement of all three cases on different time steps, which makes the superposition of the plots difficult. this problem is illustrated in following section.

MOTION ANALYSIS:

the study setup for simulation is as follows:

• motor rpm: 1200 c.w
• fps: 14400 (solver takes readings at each 0.5 degrees rotation of crank)
• timespan: 0.1 second
• gravity: activated in the downward direction
• precise contacts are activated
• material: all parts are of steel(dry)

speaking about the difficulty, the time range for all 3 studies is the same as 0.1 seconds, yet the solver takes different time steps to formulate the results. this is illustrated by the following figure:

as we can see, there is an incongruity between time step results which makes it impossible to impose all three displacement plots on a single time scale. however, the data for positive offset and negative offset is the same because of the same displacement characteristics of the piston in both cases.

for solution, we approximate the time scale is the average of the first two columns and we'll then plot neutral and offset position displacements on a single scale.

by observing the plots, we can deduce that after the first half cycle, the displacement graph of neutral position lags behind that of the offset positions. from the given data file, we can get the value of time lag to achieve same position or the displacement lag at the same time.