MBD Simulation on a Piston Assembly
AIM
To perform MBD simulation of a piston assembly in SolidWorks
PARTS OF PISTON ASEMBLY
Piston: It is the moving component that is contained by a cylinder and transfers the force from the expanding gas in the cylinder to the crankshaft via connecting rod. Grooves are made for the piston rings in order to make it air tight.
Connecting rod: It is also referred to as the con rod. It is the part of piston engine which connects the piston to the crankshaft. Together with the crank, it converts the reciprocating motion of the piston to the rotary motion of the crankshaft.
Gudgeon pin: The connecting rod is connected to the piston by a swivelling gudgeon pin (wrist pin). The pin is mounted within the piston.
Crankshaft: It is a rotating shaft which converts the reciprocating motion of the piston into rotary motion, with the help of connecting rod. It consists of a series of cranks and crank pins to which connecting rods are attatched, for multiple cylinder engines.
ASSEMBLY
1) In order to create the piston assembly, first all the parts were created separately using suitable dimensions
2) For the connecting rod, the rod and the end cap were created separately and later assembled to form the connecting rod.
3) The assembly was done by applying the necessary mates. Once the assembly was created, the motion analysis of the assembly was performed.
4) The crankshaft was given a rotaion of 2000 RPM in clockwise direction and the contacts between the bodies were defined. The material for each component was selected as dry steel.
5) After that, the frame rate was set to 12000 frames per second to capture each degree of rotation of the crankshaft ( 2000 RPM = `2000*``360/60` = 12000 deg/sec). Precise contact was selected.
6) The analysis ws done and the plot for linear displacement of the piston was obtained against time.
7) The above steps were repeated for a wrist pin offset of 10 mm on either side (positive and negative).
CASE 1 : No offset
Linear displacement of piston
CASE 2: Negative offset (10 mm)
Linear displacement of piston
CASE 3: Positive offset (10 mm)
Linear displacement of piston
Superimposed plots
PISTON ASSEMBLY ANIMATION
Component design in SolidWorks
https://drive.google.com/drive/folders/1qUJ1Om1Fll5czmAoc90hNj5H2moS2r8l?usp=sharing
CONCLUSION
1) The point of maximum displacement represents Top Dead Centre (TDC) whereas the point of minimum displacement represents Bottom Dead Centre (BDC).
2) From the superimposed graphs, we can see that thw piston displacement is maximum at 0 mm offset because the connecting rod is straight at BDC and TDC, which is not the case when the offset is given.
3) One reason for using an offset pin is to reduce piston side loading. By slightly offsetting the piston pin, the rod can be in a more vertical position during the power stroke. By having the rod in a more vertical position when the load on it is at its highest, the piston will ‘scrub’ up against the cylinder wall less. This reduces friction, power loss, and wear. In this way, offsetting the piston pin has a similar effect to using a longer rod, but without the need to raise the cylinder deck.
4) Of course, an offset pin decreases rod angularity during the power stroke at the expense of increased rod angularity at other times. But at these other times, the loading of the rod is lower. So side loading here is less of a concern.
5) Another reason for offsetting the pin is that it reduces cold piston slap which improves the noise characterstics of the engine.
REFERENCES