Engineering Research Paper

TOPIC: DESIGN AND PREDICTION OF TEMPERATURE DISTRIBUTION OF PISTON OF RECIPROCATING AIR COMPRESSOR
AUTHORS:  Bhaumik Patel, Ashwin Bhabhor
ABSTRACT
In this study, the work is carried out to design of Piston of reciprocating air compressor with the help of MATLAB software. It can be seen that MATLAB gives the accurate result of design of piston and also reduce the time to design. Also the work is done to predict the temperature distribution on the surface of the piston. Because distribution of temperature will predict that actual condition of the piston during the operating conditions, so damage of the piston can be reduced before taking into operations. Also it is seen that in the flat topped piston the center of the crown is hottest and the outer edge cooler by 20 to 50 0C and if the heat flux is higher so temperature at this point is also higher and it is quite difficult to reduce the temperature.
INTRODUCTION
The main requirement of piston design is to measure the prediction of temperature distribution on the surface of piston which enables us to optimize the thermal aspects for design of piston at lower cost. Most of the pistons are made of an aluminum alloy which has thermal expansion coefficient, 80% higher than the cylinder bore material made of cast iron. This leads to some differences between running and the  design  clearances.  Therefore,  analysis  of  the piston thermal behavior is extremely crucial in designing more efficient compressor.Good sealing of the piston with the cylinder is the basic criteria to design of the piston. Also to improve the mechanical efficiency and reduce the inertia force in high speed machines the weight of the piston also plays major role. To allow for thermal expansion, the diameter of the piston must be smaller than that of the cylinder. The necessary clearance is calculated by estimating the temperature difference between piston and  cylinder  and  considering  the  coefficient  of thermal expansion of piston.

PISTON FUNCTION AND ASSOCIATED PROBLEM AREAS
The function of the piston is to absorb the energy by the gas / air which enters in the cylinder and then accelerates to produce useful mechanical energy. So to prevent the leakage & proper compressor of air / gas  the  piston  must  be  sealed  with  the  cylinder surface. This is accomplished by the piston rings which also help to prevent oil from entering in the cylinder   from   underneath   the   piston.   Another function of the rings is to keep the piston from contacting the cylinder wall. Less contact area between the cylinder and piston reduces friction, thereby increasing efficiency. As a result of this process,  heat  is  transferred  from  the  combustion gases into the piston and other components that make up  the  cylinder  walls.  This  reduces  the thermodynamic efficiency of the process, and therefore also diminishes power.

PISTON COOLING
The reduction of Temperature in the piston can be done  by  heat  pipe  cooling  method.  This  system allows for a channel inside the piston skirt that directs heat away from the piston itself. This will increases the heat transfer through the piston, which does not help the efficiency of the compressor, but we can use the special light alloys to form the piston. Magnesium and its alloys have much larger creep rates than other metals and therefore can usually not sustain the same load and temperatures as steel or aluminum. A heat- pipe system can drastically reduce the temperature of the piston crown from about 700ºC to only 350ºC. Therefore, using heat-pipe technology makes it easier to employ magnesium alloys in pistons.

PISTON TEMPERATURE DISTRIBUTION
Generally the heat flux is highest in the center of the cylinder head, in the exhaust valve seat region, and to the center of the piston. Cast-iron pistons run about 40 to 800 C hotter than aluminum pistons. In the flat topped piston the center of the crown is hottest and the outer edge cooler by 20 to 500 C. The maximum temperatures occur where the heat flux is high and access for cooling is difficult. Such locations are the bridge between the valves and the region between the exhaust   valves   of   adjacent   cylinders.   The   heat generated by friction between the piston and the liner is a significant fraction of the liner thermal loading. Temperature distributions in the piston can be calculated from the knowledge of the heat fluxes across the  component surface using  finite  element analysis  techniques.  For  steady-state  engine operation, the depth within a component to which the unsteady temperature fluctuations (caused by the variations in heat flux during the cycle) penetrate is small, so a quasi-steady solution is satisfactory.

DESIGN DATA OF COMPRESSOR:

• Power Capacity : 5 H.P

• Speed : 1440 R.P.M

• Piston Displacement : 500 LPM

• Atmospheric Pressure : 1.01325 bar

• Working Pressure : 10 bar

• Temperature on top surface of Piston: 158.530 C

The above data is taken for the design of piston through which various geometries of the piston can

be found out.The material of the piston is Aluminum alloy 6061.

RESULTS AND DISCUSSIONS

The prescribed operating temperature inside the cylinder penetrates the piston crown through nearly 45 % of its thickness before piston ring dissipates some of heat. The average piston temperature beneath the piston ring is about 1300C, from the top surface of the piston 0.57148 to 0.71435 W/mm2 amount of heat transfer. The material of the piston will work well under the prescribed operating conditions and

the design is safe to resist the pressure of 10 bar or 10 Mpa.

CONCLUSION

It can be seen from the thermal analysis of piston of reciprocating air compressor that the stresses

produced during the operations are less as compared to the design stress. The distribution of the

temperature can be determined by this study. Average piston temperature beneath piston ring is

about 1300C.

This study is also useful to reduce the time to design of piston.

	Presented By Choudhary Ritesh Roll No : 24 PGDIE 42