ABSTRACT-A piston with squish plate is designed and thermally analysed. Because of the piston squish the fuel entering the combustion chamber get mixed with air better than conventional piston. This increases the adiabatic flame temperature and due to this the surface temperature of the combustion chamber is raised. When the air is inducted into the combustion chamber it is exposed to high temperature. The fuel injected into the combustion chamber attains self-ignition temperature quickly and hence there is increase in the average pressure and temperature inside the combustion chamber and an obvious increase in the thermal efficiency can be observed. In this study theoretical and experimental analysis of heat transfer has been done. The results show there is an increase of heat transfer and performance in engines using squish piston.
KEYWORDS: Heat Transfer, Squish Piston, Adiabatic Flame Temperature.
1. INTRODUCTION
The heat transfer taking place in CI engines plays an important role in the performance and emission of the engine. The heat transfer should be in the optimum range it should not be very high or low. Diesel engine rejects about two third of the heat energy of the fuel, one third to the coolant and another one third to the exhaust leaving only about only one third as useful power output.
A realistic description of the gas-side wall heat transfer is an important prerequisite for analyzing and simulating the working process of IC engines as heat transfer highly influences brake thermal efficiency, component and cooling system design, exhaust gas emissions and warm-up. Heat transfer is mainly caused by forced convection, which is controlled by turbulent charge movement and the temperature gradient of the working gas to the combustion chamber walls.
The shape and geometry of the piston is an important parameter which has a greater impact on heat transfer of an engine. Squish type of pistons are mostly used in direct injection diesel engines. The fuel and air mixes very well in a squish type of engine than a conventional type of piston. The parameters that vary because of the intrusion of squish in a piston are mainly turbulence. Discussing the effect of squish plate piston without turbulence is a waste of
time as the main function of squish is to increase the turbulent intensity in the
combustion chamber. Without turbulence in the combustion chamber we
would burn the mixture at the laminar burning rate which is ten to twenty
times slower than the turbulent rate. This increase the gas to surface heat transfer rate.
The generator of turbulence is the piston movement. As the piston
displaces fluid during the up stoke it imparts kinetic energy to the fluid which
gets converted at a certain rate to turbulence. Obviously this increases with
rpm so higher rpm will have a higher turbulent intensity and thus faster burn
rate unless the dissipation rate is greater than the generation rate.
And then, the next generator of turbulence is the squish action. The squish
action causes gas to flow towards the center of the cylinder. The speed of this
flow is known as maximum squish velocity. This kinetic energy is converted to turbulence at a rate depending on the conditions inside the cylinder at that time.
2. MATHEMATICAL MODELLING
2.1. Newton’s Law of Cooling
Newton’s Law of Cooling is generally used to describe heat transfer. According to this law, heat flux is the product of a heat transfer coefficient ?? and the driving temperature difference between locally averaged gas temperature Tg and wall temperature Tw. For reasons of simplicity, zero-dimensional models are usually used for calculating the heat transfer coefficient in IC engines. These models are mostly based on the similarity theory developed by Nusselt and determine a spatially uniform heat transfer coefficient for the whole combustion chamber. In practical engine cycle simulation, the heat transfer correlation according to Woschni is widely used because of its easy usage. However, Woschni also introduced different constants for different engine types and operating conditions. Further correlations by Hohenberg are also used in practice. The latter approach is particularly suitable for direct injection diesel engines.
qw = ??(Tg ‘ TW )
Where ?? = Coefficient of heat transfer
Tg = Combustion chamber temperature.
Tw= Coolant temperature
2.2. Convective Heat Flux
The heat transfer coefficient is calculated according to the Reynolds-Colburn analogy, which is based on boundary layer theory and describes the correlation between fluid friction and heat transfer. In terms of density ??, specific heat cp, characteristic velocity vc and the friction coefficient cf, the heat transfer coefficient ?? is defined by the following equation:
?? = 1/2 ?? Cp??cCfPr2/3
where
?? – Coefficient of heat transfer ?? – density
Cp – specific heat
??c – characteristic velocity
Cf – coefficient of friction
Pr – Prandtl number
2.2.1 Characteristic Velocity
The characteristic velocity is the one which represents the overall effect of the various velocities acting on the combustion chamber. The characteristic velocity can be calculated from the axial and squish velocities of the piston.
Vc= (‘Va +Vs)/2
Where Va ‘ axial velocity
Vs ‘squish velocity
2.2.2. Axial velocity
The axial velocity component is caused by the piston moving up and down and thus is the main movement of charge. the axial velocity can be calculated by using half of the instantaneous piston velocity
2.2.3. Radial velocity
In the squish area, a high velocity inwardly-directed flow develops in bowl-in piston engines when the piston approaches TDC. A theoretical squish velocity Vsq can be determined by means of the continuity equation. In equation Vpis the instantaneous piston velocity, Asq is the squish area and Ag the gap area
Vsq=Va??(Asq/Ap)??(Vbowl/Vbowl??(??/4)B2??h)
Where Vsq – squish velocity
Va – axial velocity
Asq- squish area
Vbowl- bowl volume
3. FABRICATION OF SQUISH PISTON
The major part of this work is to fabricate a squish piston with optimum dimensions to operate in the engine by replacing the conventional type piston.The conventional piston is modified in this work into a squish piston.
Figure 1 Fabricated Squish Piston
4. RESULTS AND DISCUSSION
The brake thermal efficiency has been improved marginally over the lower loads an it has shown an improvement in higher loads for squish piston over conventional piston. The increase in brake thermal efficiency is the result of increase in the pressure and temperature of the combustion chamber.
Figure 2 Brake Thermal Efficiency vs Load
Figure 3 Total Fuel Consumption vs Load
Total fuel consumption has been decreased for squish piston than that of the conventional piston. As the combustion chamber temperature for a squish piston is higher than that of the conventional piston there will be a rise in the combustion rate which results in the lower total fuel consumption.
Figure 4 Specific Fuel Consumption vs Load
The specific fuel consumption has been decreased for squish piston than the conventional piston because of the better combustion parameters and higher temperature prevailling in the combustion chamber.
Figure 5 Hydrocarbon vs Load
The emission levels clearly shows that there is an impact of the squish in the emissions. Because of the higher temperature better combustion there is an increase in the temperature of the combustion chamber. Hence there will be an increase in the NOx and decrease in HC.
Figure 6 Oxides of Nitrogen vs Load
Figure 7 Pressure vs Crank Angle
The pressure vs crank angle has been measured using the combustion analyser and it shows the pressure generated in the combustion chamber during combustion for both conventional and squish pistons. From the analysis it has been shown that at the time of peak pressure there has been an increase of 6 bar for squish piston when compared with that of it counterpart conventional piston. The increse in pressure has been result of the swirl motion of air developed in the combustion chamber and incresed heat transfer.
Figure 8 Pressure vs Volume
This graph shows the variation of pressure in accordance with the volume which changes for every crank angle. It is higher for conventional piston as the volume has been slightly reduced in the squish piston. The variation in volume is also an important parameter in increasing the temperature. As the volume decreases there will be increase in pressure which raise the temperature.
Figure 9 Gas Temperature vs Crank Angle
This graph shows the relation between crank angle and temperature. There is an improvement of around 40K for squish piston over the conventional piston. Because of the improvement there will be a increase in the heat transfer which will result in increase of thermal efficiency of the engine.
Figure 10 Heat Flux vs Crank Angle
This graph clearly shows the increase in convective heat transfer during peak temperature conditions and lower heat transfer at the time of negative heat transfer. The convective heat transfer is responsible for the transfer of energy from the gas to the side walls so there will be a increase in surface temperature. This increases the thermal efficiency of the engine.
5. CONCLUSION
From the above piston model analysis and experimental validation, it is concluded that by adding a squish plate over a piston crown, the thermal efficiency can be improved without compensating the strength of the piston.
‘ The squish piston has been fabricated and performance emission and combustion analysis has been performed experimentally.
‘ The convective heat transfer has been increased for squish piston than the conventional piston.
‘ There is an increase of about 40K temperature in the engine using squish piston.
‘ The thermal efficiency of the engine also increases than the conventional piston.