MBD Simulation of the Piston Assembly

Introduction:

An internal combustion engine is a heat engine where the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine, the expansion of the high-temperature and high-pressure gases produced by combustion applies direct force to some component of the engine. The force is applied typically to piston , turbine blades, rotor or a nozzel This force moves the component over a distance, transforming Chemical energy into useful mechanical energy.

Having the cylinder axis intersect the crank axis is a time-honored tradition among engine designers. It has been the normal practice in production engines for well over a century, but time has a way of adjusting our thinking to newer and better ways.

As engines evolved, engineers determined that offsetting the piston (wrist) pin yields two major benefits. First, it improves the noise characteristics of the engine due to piston slap at top dead center (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.

Offsetting the cylinder axis from the crankshaft axis, minimizes rod angularity when cylinder pressure is at its highest. Reduced angularity 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. Rather than revising a complete engine design, the easy way to do this is to offset the pin bore in the piston. This allows the same crank/rod pin axis geometry as an engine designed with offset cylinder axes, but within existing conventional engine architecture.

In this project we model the pston assembly and perform the motion analysis on it to determine the linear displacement of the piston head at different conditions.

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

The different parts involved in our assembly are as follows:

Piston

Connecting Rod:

Gudgeon Pin:

Crank:

The above Parts are Assembled:

Simulation Setup:

we perform all the three cases at 2000RPM which leads to 33.33 Revolutions per second, inorder to obtain the values of linear displacement at each degree of the crank rotation we adjust the frames to capture at 12000 frames per second and we use the precise contact.

We provide contacts for all the mates like

crank with end cap

crank with connnecting rod

connecting rod with gudgeon pin

gudgeon pin with piston head

we use the material as steel dry for all the parts of the assembly

and also we provide gravity in terms of y direction downwards

we follow same procedures for all the three cases to obtain the linear displacement graph

The graph obtained on performing the Motion analysis of this assembly:

Conclusion:

As observed form the above graphs the linear displacement remains almost same in all the three cases.

The linear displacement at 0 offset : 302.432825609819

The linear displacement at 10mm positive offset: 302.308213379358

The linear displacement at 10mm negative offset: 302.304495861784

as we can see the difference in the linear displacement we can say that by offsetting the piston pin we can avoid the over traveling of the piston at TDC and avoid the most of the sound and vibration. It also provides time for the flame propagation