Heat Transfer in Internal Combustion Engines

1 1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS345 E. 47 St., New York, 1001785-WA/HT-23 The society shall not be responsible for statements or opinions advanced in papers or indiscussion at meetings of the Society or of Its Divisions or Sections, or printed in its is printed only it the paper is published in an ASME Journal. Papers are availablefrom ASME for fifteen months after the meeting,Printed In USA, heat Transfer in Internal Combustion EnginesC. S. WANG and G. F. BERRYE nergy and Environmental Systems DivisionArgonne National LaboratoryArgonne, Illinois 60439 Presented at the Winter Annual MeetingMiami Beach, Florida November 17-21, 1985 ABSTRACTA heat Transfer model has been developed that usesquasi-steady heat flux relations to calculate the heat transferfrom Combustion gases through the cylinder wall to thecoolant in an Internal Combustion engine .

2 The treatment ofconvective heat Transfer accounts for the physical problemsof rotating and impinging axial flow inside the enginecylinder. The radiative heat Transfer includes gas radiation(CO2, H20, and CO) and soot-particle radiation. Cylinderwall temperatures can be accurately predicted from thismodel for both the gas and the coolant sides. The presentmodel's heat Transfer results for the motoring case are ingood agreement with results from empirical correlationsbased on instantaneous heat flux data. The calculatedradiative heat flux and gas emissivity show reasonableagreement with data in the , m2aparameter defined in Eq. 19 Cpheat capacity, J kg-1 K-1 Ddiameter, mGrGrashof numberHheat Transfer coefficient, W m-2. K-1 IPlanck function, W m-1kthermal conductivity, W m-1 K-1, or refractive indexKabsorption coefficient, m-1 Ldistance between cylinder head and piston top, mMmass flow rate, kg s-1nrefractive indexPrPrandl numberqheat Transfer rate, W m-2 Qtotal heat loss per cycle, W m-2 Rradius, mReReynolds numberT Temperature, KU overall heat Transfer coefficient, W m-2 K-1V axial velocity, m s-1 Greek Symbols gas absorptivity wall thickness, m emissivity density, kg m-3 , optical thickness Stefan-Boltzmann constant, W m-2 K-4 viscosity, Pa s scattering albedo or rotating speed crank angle, degrees equivalence ratioSubscriptsbbulkcconvective or coolant-sideeeffectiveggas stream or gas-sidehhydrauliciinside or indexooutside or wall surfacerradiative spectral variablevvalvewwall surfaceINTRODUCTIONIn Internal Combustion (IC)

3 Engines , heat loss fromcombustion gases through the cylinder wall to the coolantstrongly influences the thermodynamics of the engine heat loss is an important part of the energy balance,which influences gas temperature and pressure, piston work, engine performance, and , the heat Transfer from the cylinder gases to thewalls is calculated by estimating the engine -wall temperatureand basing the heat flux on the difference between the gasand wall temperatures. The heat Transfer coefficient used forthe calculation is usually adopted from empirical correlations(1-4). The weaknesses of this approach are as follows: The calculated results are sensitive to the walltemperature, which is treated as a given quantity. Theempirical correlations include no means of incorporatingsignificant changes in engine geometry or flow field.

4 These correlations do not correctly predict thecontribution of radiative heat of these weaknesses may be expected to have a strongeffect on the heat Transfer calculations. A more accuratemeans of calculating the heat flux is required for analysis ofthe IC engine 's thermodynamic cycle. Also, more detailedinformation on thermal conditions (such as walltemperatures) would be useful to engineers and objective of this paper is to provide a heat transfermodel that can be used to calculate the heat flux moreaccurately. This greater accuracy is to be achieved byestimating the coolant temperature, rather than the walltemperature, because coolant temperature can be correctlymeasured (or estimated) much more easily than walltemperature can.

5 A useful side benefit of this approach isthat, because the wall temperatures are computed as anintegral part of the calculations, their variation as a functionof the crank angle is provided. This side benefit isparticularly important for the adiabatic engine contrast to common practice, the actual heat transfermechanisms are not lumped together; instead, convection,radiation, and conduction are treated explicitly. Thetreatment of convective heat Transfer accounts for thephysical problems of rotating and impinging axial flowinside the Combustion chamber and cross flow outside thecylinder, using empirical correlations established for flowssimilar in nature to those occurring on the inside and outside(coolant side) of the engine cylinder . The radiative heattransfer includes gas radiation (CO2, H2O, and CO) and soot-particle radiation.

6 Conduction is treated in a wall temperatures, heat flux rates, and total heatloss are calculated. The results of the present analysis arecompared with the existing predictions based on empiricalcorrelations (1-3). The predicted radiative heat fluxes andgas emissivities from the present model will also becompared with experimental measurements (5). heat Transfer MODELThe physical system under consideration is quasi-steady, one-dimensional heat flow through the solid mediumseparating the cylinder gas and coolant. The basicassumptions used with the present model are as follows: The coolant temperature is known. The cylinder wall comprises seven heat Transfer areas(intake valve, exhaust valve, cylinder head, liner, pistontop, cup wall, and cup bottom), each of which is at itsuniform temperature and has its uniform heat flux atevery instant of the cycle.

7 (See Fig. 1.) The convective heat Transfer coefficients on both sidesof the walls are uniform over each heat Transfer area. The equivalent wall thickness at each heat Transfer areais the same. The possible effects of surface scales or deposits oneither side of the cylinder wall are not considered, andthe heat losses through leaks at the valve seats or at thegap between the liner and the piston are neglected. The heat flow is quasi-steady, and the wall temperaturesmay be determined using simple network process by which heat is transferred from thecylinder gas through the wall to the coolant consists of threeparts: convective and radiative heat Transfer from cylindergas to Combustion -chamber surface, conductive heat transferthrough the cylinder wall, and convective heat Transfer fromthe cylinder wall to the coolant.

8 Under the quasi-steady flowassumption, the heat flux is considered to be the same acrosseach element:Fig. 1 Schematic of engine Cylinder3where hg, the heat Transfer coefficient at gas-side, includesboth convection and radiation. The overall heat transfercoefficient, U, may be written in terms of thermal resistancescorresponding to gas-side (1/hc and coolant-side (1/hg ) heattransfer processes:The thermal resistance of the wall, /k, should be replaced byRi ln(RO/Ri)/k for the heat Transfer at the liner, where Ri andRO are the inner radius and outer radius of the liner,respectively; and L is a distance between the cylinder headand piston top. The gas-side and coolant-side walltemperatures (Twg and Twc, respectively) can be obtainediteratively from Eqs. 1-4 by satisfying Eq. 5. Theinstantaneous heat flux can be obtained by summing the heatfluxes through all heat Transfer areas at each crank angle.)

9 Where the index i denotes the number of heat Transfer total heat loss per cycle from the Combustion -chamberwalls can be integrated according to the following equation:CONVECTIVE heat TRANSFERHeat Transfer from cylinder Gas to WallsA very fundamental rotating-disk flow and forcedrotating-tube flow are considered for the calculation of heattransfer between the cylinder gas and the wall in swirlengines. For piston top and cylinder head, the turbulent flowover a disk rotating at constant speed about its axis isassumed. The analysis, based on Hartnett (6), is performedby integrating the continuity, momentum, and energyequations for the fully turbulent flow. The heat transfercoefficient, hgc , over the disk plate is written as follows:where Re (= gR2 / g), Pr, kg and R are Reynolds number,Prandtl number, thermal conductivity of gas, and equivalentradius, respectively.

10 For the liner and the piston-cup wall, theturbulent fluid flowing inside a pipe rotating about itslongitudinal axis is assumed. The heat Transfer coefficient isobtained from the results of Cannon and Kays (7): heat Transfer from cylinder Wall to CoolantThe heat Transfer from the cylinder head and liner to thecoolant is modeled by turbulent cross-flow forcedconvection. An average heat Transfer over the cylindricalsurface was correlated by McAdams for water andhydrocarbon oils (8).where Prc, kc, Re (= cVcDO/ c ), and DO are the Prandtlnumber of the coolant, thermal conductivity of the coolant,Reynolds number, and outside diameter of the enginecylinder, respectively. Vc is a mean velocity of the heat Transfer from the back surface of the valve tothe inlet gas is treated differently during the periods when thevalves are open and closed.