MBD Simulation on a Piston Assembly (SOLIDWORKS)
MBD Simulation on a Piston Assembly (SOLIDWORKS)
1. AIM- To model all the parts of the piston assembly and then assemble them to run a multibody dynamics simulation on that.
OBJECTIVE- The motion analysis is going to be carried out in three different conditions,
| Sl.No | Wrist Pin Offset | Crank Speed (Rpm) |
| 1 | "0"mm | 2000 |
| 2 | "10"mm positive | 2000 |
| 3 | "10"mm negative | 2000 |
2. INTRODUCTION-
A piston is a component of reciprocating engines, reciprocating pumps, gas compressor, pneumatic cylinder and among other similar mechanisms. It is a moving component that is contained by the cylinder and is made gas-tight with the help of piston rings. In an engine, its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via crankshaft, via 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.
Gif: Working Piston
If an engine has, larger bore than stroke, then its called over-square engine. Such engines will run faster and therefore not that reliable in the long term. Eg: Race car engines.
If an engine has, larger stroke than bore, then its called under-square engine.
These kinds of engines will be slower than the over-square engine, will be reliable as well as efficient due to higher compression.
Eg: Engine in Trucks & Locomotives.
If an engine has, equal bore and stroke, then its called square engine. Eg bike like Honda Shine.
3. PROCEDURE-
Step1) Create all the five models of piston assembly carefully and then assemble all the parts in such a way that piston can have only reciprocating motion and crank can have only rotating motion.
Step2) Go to SOLIDWORKS Add-Ins enable motion analysis there and perform motion analysis.
Step3) Set a rotating motor on crank having a speed of 2000RPM.
Step4) Set-up contacts between all the connecting pair where energy needs to transmit from one part to another.
Step5) Set gravity on Y-axis and then click on calculate.
Step6) Set the frame rate to be 2000*6=12000 to capture all the frames and enable precise contact.
Step 7) Stop the calculation after some time and view the desired results.
4. 3-D Model of-
1) 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.
2) Piston : Piston is a moving component that is contained by the 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 connecting rod.
- For 0mm positive wrist pin offset
- For 10mm positive wrist pin offset
- For 10mm negative wrist pin offset
3) Connecting rod : A connecting rod is a rigid member which connects a piston to a crank or crankshaft in a reciprocating engine.
4) End Plate : The end plate is used to fix the connecting rod with the crankshaft.
5) Gudgeon Pin : In an internal combustion engine, the gudgeon pin(wrist pin) connects the piston to the connecting rod and provides a bearing for the connecting rod to pivot upon as the piston moves.
5. Assembled model-
- For 0mm positive wrist pin offset
Animation:
- For 10mm positive wrist pin offset
- For 10mm negative wrist pin offset
6. RESULTS AND PLOTS-
- For 0mm positive wrist pin offset
- For 10mm positive wrist pin offset
- For 10mm positive wrist pin offset
- Comparison of all the linear displacement plots on a single graph:
Maximum Linear displacement produced in different cases are:
1. 0 mm offset - 302.708 mm
2. 10 mm positive offset - 302.522 mm
3. 10 mm negative offset - 302.521 mm
TDC in all three cases
1) When the wrist pin is in the center that is "0mm" 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 "10mm positive" or "10mm negative" we can clearly see that 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 oncept of offsetting the wrist pin is widely used in racing cars.
3) We see that Stroke length decreases as we introduce offset to any direction. Maximum of stroke length can be seen on the "omm" offset case.
4) Engine displacement is the multiplication of stroke length to the cross-sectional area of the cylinder as the stroke length decreases on introducing offset, the engine displacement also got decreased.
5) Generally, bigger-displacement engines develop more torque, but small engines can spin faster, which increases their horsepower output.
6) Having an offset pin relieves the stress that is caused in the rod when it reaches the top or bottom dead center.
7.Observation and conclusion-
From the plots obtained, we can see that Linear Displacement increases as we move from negative wrist pin offset to positive wrist pin offset i.e. the magnitude of the Linear displacement increases.
Also from the graph obtained after superimposing it we can see that for a given time frequency of rotation of piston with wrist pin offset negative is higher comparatively to one with zero offset and pin with zero offset has higher frequency than one with a positive offset.