MBD Simulation on a Piston Assembly
AIM: Multi-Body Dynamics Simulation on Engine piston assembly.
Objectives of the project:
- To study and perform the motion analysis of the piston assembly for various position of wrist pin axis with respect to axis of rotation of the crank.
- To obtain the plot for linear displacement of the piston head when the wrist pin axis can be offset as positive or negative with respect to axis of crank rotation in three different cases as stated.
Introduction:
A reciprocating engine, also often known as a piston engine, is typically a heat engine that uses one or more reciprocating pistons to convert pressure into a rotating motion.One of the main type is the internal combustion engine, used extensively in motor vehicles.
Reciprocating engines operate by converting the heat and pressure released during combustion of fuel mixed with air into mechanical energy. Fuel is injected into the cylinder where it reacts with air entering from the intake manifold. The resulting exothermic reaction creates gases under high temperature and pressure, which expand by driving the piston downward to produce rotational energy.
3D Models of Parts and Assembly of Engine Piston:
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. 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.
Crank:
A crank is an arm attached at a right angle to a rotating shaft by which circular motion is imparted to or received from the shaft. When combined with a connecting rod, it can be used to convert circular motion into reciprocating motion, or vice versa. The arm may be a bent portion of the shaft, or a separate arm or disk attached to it. Attached to the end of the crank by a pivot is a rod, usually called a connecting rod.
Connecting Rod:
A connecting rod, also called a con rod, is the part of a piston engine which connects the piston to the crankshaft. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the crankshaft. The connecting rod is required to transmit the compressive and tensile forces from the piston, and rotate at both ends.
End Cap:
End Cap is connected to the end of the connecting rod to make assemble as a crank in engine piston assembly.
Gudgeon or 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, including those driven by steam, and many very large stationary or marine engines, the gudgeon pin is located in a sliding crosshead that connects to the piston via a rod. A gudgeon is a pivot or a journal.
Assembly of Engine Piston:
In Engine piston assembly, we assemble all the part such as crank, connecting rod, end cap, wrist pin and piston using the mate command. Firstly, the crank part coincides with the right and top plane of the assembly and then the connecting rod is made coincident with the end cap to make as a defined crank. Then, the connecting rod coincides with the axis of crank rotation and on the other end of it mate with the wrist pin which is in coincidence with the piston part and finally the assembly is completed by making right plane of assembly coincident with the plane of piston.
Motion Analysis of Engine Piston Assembly:
Crank Speed=2000Rpm
No. of frames per second (fps) =8000 with precise contact
Material selected for each contact=Dry steel
Gravity given =y direction
Given Conditions:
Sr. No. Wrist Pin Offset Crank Speed (Rpm)
- “0” mm 2000
- “10”mm Positive 2000
- “10”mm Negative 2000
Results obtained:
Case 1: When the position of wrist pin is offset by 0mm and speed given to crank is 2000rpm (Clockwise).
Plot of Linear Displacement Vs Time:
Animation link:https://youtu.be/vZXjlBJuqz8
Case 2: When the position of wrist pin is offset by 10mm (positive) and speed given to crank is 2000rpm (Clockwise).
Plot of Linear Displacement Vs Time:
Animation link:https://youtu.be/BwqIfrbbMHU
Case 3: When the position of wrist pin is offset by 10mm (negative) and speed given to crank is 2000rpm (Clockwise).
Plot of Linear Displacement Vs Time:
Animation link:https://youtu.be/ym8Lrolq8UU
Super impose Graph of above all 3 cases:
The data for linear displacement for above all three case are exported to excel and then super imposed there to make a combine graph.
Observation and conclusion:
- The plot for linear displacement vs time is smooth for all the 3 cases.
- From all the above conditions, it is observed that in case 1 the piston displacement will be maximum than the other two offset conditions. This happens because there is a slight reduction in piston displacement since the piston has to travel little less distance for 0mm case. While offsetting, the connecting rod will not be straight at TDC and BDC and hence it has to travel more as in case 2 or 3 for 10mm.
- When the wrist pin is in centre of the piston, and piston reaches from top to bottom dead centre then the rod is straight up and down placing more load on the rod. The crank has to rotate TDC or BDC in order to get the movement of piston again which results in more resistance to obtain both power and speed from the engine.
- Now with an offset condition, the wrist pin is little offset to one of side of piston therefore the rod is not straight up and down when it is at TDC BDC. This allow the crank to rotate with less resistance which result in more power and speed of the engine.
- It is also observed that the linear displacement is more in positive offset (case2) than that of the negative offset (case3).