MBD Simulation on a Piston Assembly Using SolidWorks

Multi-Body Analysis On Piston Assembly

AIM :-  To Model Piston-Assembly  and then assemble them in SolidWorks. The motion analysis Of the Piston-Assembly is carried out in three different Offset cases..

OBJECTIVE :-

• To create 3D model of different components of piston assembly,
• To run motion study analysis for the following cases -

•  To Plot a Linear displacement of the Piston Head in all the three cases. Then export the graphs into Excel. Then Superimpose all the three graphs into a single graph

Theory :-

A schematic of the basic one-cylinder slider-crank mechanism and the terminology for its principal parts

The explosion of the combustible mixture in the cylinder drives the piston to the left as in above Figure, turning the crank. The crank torque that results is ultimately delivered to the drive wheels of the vehicle through a transmission to propel the car, motorcycle, or other device. The same slider-crank mechanism can also be used "forward-driven," by motor-driving the crank and taking the output energy from the piston end. It is then called a piston pump and is used to compress air, pump gasoline and well water, etc.

In the internal combustion engine of above Figure, it should be fairly obvious that at most we can only expect energy to be delivered from the exploding gases to the crank during the power stroke of the cycle. The piston must return from bottom dead center (BDC) to top dead center (TDC) on its own momentum before it can receive another push from the next explosion. In fact, some rotational kinetic energy must be stored in the crankshaft merely to carry it through the TDC and BDC points as the moment arm for the gas force at those points is zero. This is why an internal combustion engine must be "spun-up" with a hand crank, pull rope, or starter motor to get it running.

Increasing the length of the connecting rod reduces the amount of friction between the piston and the cylinder wall, but it also makes the engine bigger (left). A shorter connecting rod produces more friction loss (center). The offset cylinder (right) enables both reduced loss and a more compact design.

In general, the piston in an engine is pushed down by the explosive force of combustion in the combustion chamber and this is the origin of the drive force created by an engine. Strictly speaking, however, the piston is not actually pushed straight downward. Although it is small, there is in fact a diagonal component to the downward force that causes the piston to be pushed against the wall of the cylinder. This results in friction and a corresponding amount of horsepower loss. This friction-induced power loss can be reduced by lengthening the connecting rod, but lengthening the rod makes the engine heavier and larger overall.

The offset cylinder is a design that solves both of these inherently conflicting problems. This is accomplished by “offsetting” the cylinder from the crankshaft so that the axis of the piston and connecting rod are aligned vertically at the moment of combustion—when the pressure in the cylinder is at its peak. Doing this greatly reduces the friction from the diagonal force pushing the piston against the cylinder wall. At the same time, this enables more effective use of the energy produced by combustion.

3D CAD MODEL (VARIOUS COMPONENTS INVOLVED) 

Step 1 : - Design of Piston 

The piston is a reciprocating part of IC engine that performs a number of functions. The main functions of the piston are as follows:

(i) It transmits the force due to gas pressure inside the cylinder to the crankshaft through the connecting rod.

(ii) It compresses the gas during the compression stroke.

(iii) It seals the inside portion of the cylinder from the crankcase by means of piston rings.

(iv) It takes the side thrust resulting from obliquity of the connecting rod.

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STEP 2 :- DESIGN OF CONNECTING ROD 

The connecting rod consists of an eye at the small end to accommodate the piston pin, a long shank and a big end opening split into two parts to accommodate the crank pin.The basic function of the connecting rod is to transmit the push and pull forces from the piston pin to the crank pin. The connecting rod transmits the reciprocating motion of the piston to the rotary motion of the crankshaft. It also transfers lubricating oil from the crank pin to the piston pin and provides a splash or jet of oil to the piston assembly.

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STEP 3 : - DESIGN OF WRIST PIN AND END CAP 

Wrist Pin

A Wrist pin is a pin through the skirt of a piston in an internal-combustion engine, to which the little end of the connecting rod is attached.The bearing at the other end of the connecting rod is known as the little end and  olds the wrist pin that is mounted in the piston.The wrist pin connects the piston head to the connecting rod.

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END CAP

The maximum force acting on the cap and two bolts  consists only of inertia force at the top dead centre on the exhaust stroke. The maximum force acting on the cap and two bolts consists only of inertia force at the top dead centre on the exhaust stroke.It is treated as a beam freely supported at the bolt centres and loaded in a manner intermediate between uniformly distributed and centrally concentrated load.

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STEP 4 :- DESIGN OF CRANK

The crankshaft is an important part of IC engine that converts the reciprocating motion of the piston into rotary motion through the connecting rod. The crankshaft consists of three portions—crank pin, crank web and shaft. The big end of the connecting rod is attached to the crank pin. The crank web connects the crank pin to the shaft portion. The shaft portion rotates in the main bearings and transmits power to the outside source through the belt drive, gear drive or chain drive.

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 STEP 5:- ASSEMBLY OF ALL THE ABOVE COMPONENTS-

The Developed Components are now assembled by importing into assembly. the primary component inserted is formed to float from being fixed. The temporary axes should be selected for the convenience in assembling. All the components are assembled with necessary constraints, in order that desired motion are often achieved, i.e., piston should reciprocate about it's axis while crank can only rotate about it's axis.
At first, the crankshaft is inserted whose axis is formed co-incident with the highest and right plane then its front plane is placed at a long way from the assembly's front plane. Then the rod and therefore the end cap are inserted and both of their axes are made coincident then their faces are made coincident in order that they move together together individual part.
Then the axis of the crank is formed coincident with the axis of the rod. Then one side of the rod is chosen and therefore the corresponding side of the crank is chosen and made coincident with a distance mate of 1 mm to supply some clearance. Similarly piston head and gudgeon pin are inserted and made coincident along their respective axes and planes, in order that they move together together individual part.
Then gudgeon pin axis and rod axis are made coincident then the front plane of the rod and therefore the front plane of the piston head are made coincident.The proper plane of the piston is formed co incident with the assembly's right plane.

 CASE 1 :- (0mm OFFSET)

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Result in CASE 1 ; -

A torque/motor was provided for the axis of crank with 2000 rpm in clockwise dirction. Solid body contacts were provided for different components with the material selected as Steel(Dry), Gravity was provided in the downward direction, precise contact and 12000 fps were provided as input for smooth transition and to obtain results more accurately and were solved by running calculations.

 CASE 2 :- (10mm(Positive while crank rotate Anti-clockwise) OFFSET BETWEEN WRIST PIN AND PISTON AXIS)

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RESULT IN CASE 2 (10mm Negative Offset) :- 

A torque/motor was provided for the axis of crank with 2000 rpm in clockwise direction. Solid body contacts were provided for various components with the fabric selected as Steel(Dry), Gravity was provided within the downward direction, precise contact and 12000 fps were provided as input for smooth transition and to get results more accurately and were solved by running calculations. Plot obtained for linear displacement of piston from results -

 CASE 3 :- (10mm(Negative while crank rotate Anti-clockwise) OFFSET BETWEEN WRIST PIN AND PISTON AXIS) 

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RESULT IN CASE 3 (10mm PositiveOffset) :- 

A torque/motor was provided for the axis of crank with 2000 rpm in clockwise dirction. Solid body contacts were provided for different components with the material selected as Steel(Dry), Gravity was provided in the downward direction, precise contact and 12000 fps were provided as input for smooth transition and to obtain results more accurately and were solved by running calculations.
Plot obtained for linear displacement of piston from results

 SUPERPOSITION OF ALL THE CASES INTO A SINGLE CHART : -

ANOTHER REPRESENTATION - 

CONCLUSION : - 

1. The plot for linear displacement when the crank angle is at 0° or at time as 0 second, the piston is positioned at TDC and therefore the displacement reaches the utmost value. After half rotation (180°), the piston is at BDC and therefore the displacement reaches minimum value and follows sinusoidal path. The angular velocity of crank almost remains constant throughout the motion.
2. The amplitude for the plot with "0" mm Offset is 310.9506 mm and is maximum as compared to 10 mm Positive Offset (310.765 mm) and 10 mm Negative Offset (310.7651mm) which is minimum, and is because there's reduction within the piston displacement which is thanks to the very fact that the piston has got to travel a less distance from TDC to BDC due to offsetting,
3. Wrist pin offset refers to the piston gudgeon pin being a touch offset to at least one side of the piston i.e. for it to not be within the centre of the piston. When the piston pin is within the centre of the piston, now when the piston reaches top or bottom dead center then, during this quite setup the rod is straight up & down placing huge load on the rod. The crank has got to rotate past TDC or BDC so as to urge the piston moving again. This robs tons of both power and RPMs from the engine. With an offset pin, the piston gudgeon pin may be a little offset to at least one side of the piston therefore the rod isn't straight up and down when the piston is at TDC or BDC. This successively allows the crank to rotate with much less resistance giving the engine more power and speed. Having offset pin relieves the strain that's caused within the rod when it reaches top or bottom dead center .
   
   
4. Offsetting the cylinder axis from the crankshaft axis, minimizes rod angularity when cylinder pressure is at its highest. Reduced angularity results in lower piston thrust forces, hence lower frictional losses during the amount of maximum cylinder pressure, especially at the start of the facility stroke when pressure rises sharply about 12-15 degrees after TDC.