MOTION ANALYSIS OF IC ENGINE PISTON ASSEMBLY MECHANISM

AIM: MOTION ANALYSIS OF IC ENGINE PISTON ASSEMBLY MECHANISM.

THEORY:

In IC Engine, the combustion is driven by Slider Crank Mechanism. Inside the cylinder, the piston performs reciprocating motion which results in compression and expansion of gas inside the cylinder. This motion of the piston is guided by the Slider Crank Mechanism. In this mechanism, the crank is rotated by the crankshaft which set the motion for connecting rod and makes piston (slider) to slide linearly inside the cylinder.

Slider-crank mechanism, the arrangement of mechanical parts designed to convert straight-line motion to rotary motion, as in a reciprocating piston engine, or to convert rotary motion to straight-line motion, as in a reciprocating piston pump.

OBJECTIVE:

• The objective of the project is to determine the linear displacement of the piston with the following three cases:                                                       

3-D MODELLING:

• First, the 2-D sketch was created according to the given dimensions of all the components. Then, relevant 3-D model creation steps were performed to form the 3-D model.
• After creating models of all components, piston assembly was created by combining all components with proper mates and constraint.
• Three different piston assembly was created each for 0, 10 mm (positive), 10 (mm) negative offset of wrist pin inside the piston.

 PARTS:

                 piston (offset=10)                          piston (offset=10 mm(positive))       piston(offset=10 mm(negative))

                                              Connecting Rod                                                            End Cap

                                                          Wrist Pin                                                     Crank

 ASSEMBLY:        

                             offset=0                           offset=10 mm(positive)               offset=10 mm(negative)

CAD MODEL FILES: https://drive.google.com/drive/folders/1OPqiq1Qa7mfK4tdRmGurTEYg_RmBKsi3?usp=sharing

MOTION ANALYSIS:

• Multi-body Dynamics analysis of piston assembly is carried out for three cases of wrist pin offset.
• The same RPM was provided to the crank (2000 RPM) in all three cases.
• Linear Displacement of the piston was recorded for all the cases
• The plot of Linear Displacement for all three cases was superimposed to get a useful result.

CASE1: offset=0

CASE2: offset= 10 mm (positive)

CASE3: offset= 10 mm (negative)

Combined Result:

EXPLANATION OF PLOTS:

From the results obtained, we can see that there is a very slight difference in the linear displacement of the piston in all three cases. we can derive from the simulation that by providing the wrist pin offset gives us a better performance of the engine as it reduced the loads which comes during the operation of engine mechanism.

Basically, when the offset is zero, the connecting rod is in a straight position when the piston is at TDC while with offset given to the wrist pin, the connecting rod is slightly tilted which gives smooth working of the mechanism and there is no high loads which come on the assembly. This reduces the stress levels in the parts. This leads to lower piston thrust forces, hence lower frictional losses during the period of maximum cylinder pressure, especially at the beginning of the power stroke when pressure rises sharply about 12-15 degrees after TDC.

CONCLUSION:

Offsetting the piston (wrist) pin yields two major benefits. First, it improves the noise characteristics of the engine due to piston slap at the top dead centre (TDC). This is a major NVH (noise vibration and harshness) concern to production engineers who want to eliminate alarming noises anywhere they can. The second reason is to improve engine output by reducing internal friction.