MBD Simulation on a Piston Assembly
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AIM
To model all the parts of the engine piston and crank assembly and to perform motion analysis in the solidworks.
Introduction
Piston motion analysis is the analysis of a piston and crank assembly which consists of a piston head, wrist pin, connecting rod, end cap, and a crank.
Objective
Design the parts of a piston and crank assembly, assemble them and simulate their motion by restricting the degrees of freedom of every component so as to avail their motion in the required direction. A certain torque or angular velocity is provided to the crank and the motion of the piston is analyzed.
The parts required for the assembly is as follows.
Piston: 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.
Gudgeon Pin (Wrist pin): In internal combustion engines, the gudgeon pin connects the piston to the connecting rod and provides a bearing for the connecting rod to pivot upon as the piston moves. In very early engine designs the gudgeon pin is located in a sliding crosshead that connects to the piston via a rod. A gudgeon is a pivot or journal.
Connecting rod: A connecting rod is a rigid member which connects a piston to a crank or crankshaft in a reciprocating engine. Together with the crank, it forms a simple mechanism that converts reciprocating motion into rotating motion.
End cap: End cap is connected to the end of the connecting rod, which is assembled crank
Crank: Is a mechanical part able to convert the reciprocating motion from the piston into rotational motion.
Final assembly: The piston, connecting rod, wrist pin, end cap and the crank are assembled as shown below using assembly mate option. First the crank is made coincident with the assembly planes so that it can rotate about its axis and then connecting rod and end cap assembly is made coincident with the crank axis. Then wrist pin is made coincident with the piston and with its front plane and it is then assembled to the connecting rod. This will result in the required assembly for the motion analysis.
Motion Specification
1)Speed of rotation of the crank = 2000rpm
2)The wrist pin must be positioned as follows with respect to the plane of the axis of rotation of the crank.
| Sl.No |
Wrist Pin Offset ` |
Crank Speed (Rpm) |
| 1 | "0" mm | 2000 |
| 2 | 10 mm Positive | 2000 |
| 3 | 10 mm Negative | 2000 |
3)Precise contact is enabled.
4)The crank is rotated for 1minute in the clockwise direction.
5)The frames per second are 12000.
6)"Contact" is enabled with Steel(dry) contact to every pair of contacts.
7)Gravity is enabled in the Y direction.
Motion Study
Case 1: Offset is 0mm.
Output: A plot of linear displacement with time
Case 2: Offset is 10mm.
Output: A plot of linear displacement with time
Case 3: Offset is -10mm.
Output: A plot of linear displacement with time
Observation and conclusion:
- The linear displacement graph is smooth for all the 3 cases.
- When there isn't any offset the piston dispalcement will be maximum than the offset conditions as there is slight reduction in the piston displacement which is obvious since the piston has to travel a bit less distance because of offsetting because the conneting rod will not be straight at TDC and BDC because of 10mm offset.
- In the super imposition the plot of 10mm (negative) offset is opposite to the first two as the rotation given to the motor is in anticlockwise direction.
- When the piston pin is in the centre of the piston, now when the piston reaches top or bottom dead centre then, in this kind of setup the rod is straight up & down placing huge load on the rod. The crank has to rotate past TDC or BDC in order to get the piston moving again. This robs a lot of both power and RPMs from the engine. This results in piston slap.
- Now with an offset pin, the piston wrist pin is a little offset to one side of the piston therefore the rod is not straight up and down when the piston is at TDC or BDC. This in turn allows the crank to rotate with much less resistance giving the engine more power and speed.
Animated Link