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INTERCOOLING-SUPERCHAR GING PRINCIPLE: Part II-High-pressure High-Performance Internal CombustionEngines With Intercooling-Supercharging

L in -Shu W ang

ABSTRACTSince Brayton, Beau de Rochas, and Otto (1876) firstput forth the importance of compression before igni-tion/combustion,simple cycle internal-com bustionengineshave evolved into three successful types: gasoline engine,diesel engine, and gas turbine. Raising compression ratioleads to simultaneous increases in thermal efficiency andengine specific power output. A large part of the continu-ous improvement in performance of these three types ofengines has been achieved with increasing peak cyclepressure.Even designed at optimal peak cycle pressure, howeve r,significant heat loss remains in the exhaust of internalcombustion engines. In Part 1 of this two-part paper theintercooling-supercharging principle was introduce d as ameans that reduces exhaust heat loss. In Part 2, theproposed principle is reformulated by representing thethermal efficiency and the (mass) specific power in termsof their natural parameters: peak cycle pressure , peak cycletemperature (or, equivalent ratio), a newly-introducedintercooling-supercharging parameter, and a newly-introduced composite-engine parameter. A multi-variablesearch optimization procedure is then used to find theoptimum designs. The proposal shares with the seminalconcept of Brayton and Otto the unique characteristic, asan engineering solution, in producing simultaneousincreases in efficiency and engine specific power output.Intercooling-superc harging should raise the perform ance ofIC engines to new levels at peak cycle pressure higher thanthat required for the Otto cycle, the diesel cycle, and theBrayton cycle--me eting the challenge of the new

generation of industriallutility powerplants and the vehiclepropulsion engines.

1 . INTRODUCTIONIn Part 1 (Wang 1995) of this two-p art paper. theoriginal formulation of the intercooling-superchargingprinciple was introduced as a means for reducing exhaustenthalpy loss from "simple-cycle" internal con~bustionengines. The focal point was the thermal efficiencyimprovement in engine systems-without requiring hightemperature heat exchanger.In reviewing the simulation-study results, it was realizedthat the optimization procedures u se d were awkward orineffectual. In Part 2 the optimization procedure isreconsidered again by looking for the logical (natural)variables that characterize the intercooled-supercl~argedcycle engines as the direct extension of the "simplecycles." In terms of these variables, a multi-variablcunivariate search procedure is then formulated for ti d in gthe optimum design. The advan tage of the "simple cyclex"is their performance. The focal point of Pan 2 will be th eperformance of internal combustion engines. Thcintercooled-supercharged cycle engines will be presented a:,logical development of simple cycles, achieving a higherperformance level.

2. PERFORMANCEPerformance is used in this paper as the engine speci ti cpower (the reciprocal of engine specific weight or zngiuc

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specific volume) and the engine's fuel consumption (thespecific fuel consumption; alternatively, its reciprocaldivided by fuel heating value-the thermal effic iency ).Since the engine specific power depends on its massspecific power and the density of the working fluid, weshall consider performanc e as principally a fun ction of thethermal efficiency; the (mass) specific power: and theworking fluid density-at a particular (refe rence) point ofthe process, e.g., density at the bottom dead center beforethe compress ion stroke of the piston-cylinder component ofa composite engine.Performance of the " s i m ~ l e vcle" internalcombustion enqinesSince it was independently pointed out by Bray ton, Beaude Rochas, and Otto (and later by Diesel), the concept ofoptimal compression before ignition/combustion becamethe single most important concept after Carnot's idea ofachieving the highest combustion temperature in acombustion heat engine. As outlined by Beau de Rochas.his third and fourth conditions under which maximumefficiency could be achieved are (Heywood 1988:xx):

3 . The greatest possible expansion ratio4. The greatest possible pressure at the beginning ofexpansion

(Beau de Ro chas's first two conditions hold heat loss fromthe charge to a minimum.)We shall refer to the Brayton-cycle gas turbine, and theOtto-cycle and diesel-cycle naturally aspirated pistonengines as simple cycle engines. Performance-thenmalefficiency, mass specific power, and density-of simplecycle engines depends principally on two variables (threevariables, if one also consider the cylinder temperature andthe possibility of raising it). For the gas turbine,

where v,, p ,,, p,,, represent thermal efficiency, massspecific power, and c harge density at some referencr point.For piston engines,

where 4 is the equivalent ratio of charge. Standardizedignition and combustion processes for the gasoline engineand for the diesel engine are assumed in the abovrequations. It should be noted that 4 for both Otto anddiesel engines are at their respective maximum , and T,,,,,,is also at the maximum-unless high -tempera ture materialand lubrication advance make it possible for raising itsvalue.(Only principal design thermodynamic variables areconsidered in the above equations. Other design andoperating variables, which are also important to eneineperformance such as enginelcombustion-chambergeometry,ignition timing, valve timing, speed, flow fieldcharacteristics, ... , are assumed at their "optimal"standard design-values.)At the maximum 4, TqLinde,,r at the current gas turbineT (again depending on the material technology) thesingle remaining principal variable is peak cycle pressure.For each type of simple cycle ehgine there is acorresponding optimal P range. The optimal P valuesfor the thermal efficiency and for the specific power are ingeneral distinct, but not far apart; these two values define

Nomenclaturep,,, = engine specific powerpms = mass specific powerP = pressure ratior = intercooling superchargingparameter defined by equation

( 1 2 4s = composite engine parameter,defined by equation (15a)T = temperature

cr = intercooling superchargingparameter, defined by equation(12)

fl = (non-integer) exponent inequations (15), (15a)T~~= thermal efficiencyp = density$ = equivalent ratioSubscript

back = piston back pressure. ortemperaturecy l inde r = cyl inder wal ltemperaturepeak = peak cycle valuesref = reference point at which thecharge density is evaluated asthe most relevant to p,,super = supercharging pressureratio

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the optimal range. Ineach case, raising Ptoward its optimal valuerange increases thermale f f i c i e n c y . s p e c i f i cpower. and chargedensi ty-leading tosimple cycle engines ofgood perfomlance. It isimportant to note that themain reason for theincrease in thermalefficiency by raisin g Pis the lowering ofe x h a u s t c h a r g etemperature.

3. INTERCOOLINGSUPERCHARGINGPARAMETEREven at the optimalP significant enthalpyloss remains in theexhau st of a sim ple cycleengine. Intercooling-s u p e r c h a r g i n g w a sproposed in Part 1 as ameans to further reducethe exhaust enthalpy loss.It is noted that equation( 3 ) i n P a r t 1-representing the oldoptimizationprocedure-is not anexplicit function of P

middle diagram represents the "optimal" placement of intercoolers.

Figure 8 Temperature versus entropy for increasingP with "optimal" placementofintercoolers.,

We shall begin by expressing performance of theintercooled-supercharged gas turbine in terms of P thusestablishing a direct link with the simple-cycle gas turbine.A second variable, the intercooling superchargingparameter. a or r , is defined as

Ilr = 2{[lnP,/lnP,,,] - 1)Either a! or r can be used for characterizing the placementof the two intercoolers. They are simply related as

Instead of equation (3). we have now

r is defined between 0 and 1: a! defined between 0 and 713.With r = 0, equations (13),(14) reduce to equation:,(6),(7), the simple cycle case. With r = 1, equations(13),(14) become the thermal efficiency function and thespecific power function for the conveutional intercooledcycle turbine, where the two intercoolers are placedbetween compressors of equal p ressure ratios.

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4. THE TWO-VARIABLE UNlVARlATE SEARCHO P T I M I Z A T I O N F O R I N T E R C O O L E D -SUPERCHARGED GAS TURBINEWith a constant T for the gas turbine, equations(13),(14) suggest a two-variable univariate searchmethod. Instead of the procedure represented by Fig.3 inPart 1, the two-variable univariate search is representedby the T-S diagrams in Fig.7 and Fig.8. Fig.7 showsschematically the T-S iagrams of increasing r (from leftto right) under constant Pm . Fig.8 shows schematicallythe T-S diagrams of increasing P (from left to right)under constant r .An example of the computed two -step results (first steprepresented in Fig.7, second step represented in Fig. 8)for the thermal efficiency and the mass specific powerusing GA TEICYC LE software (see Wang and Pan 1994for details) is shown in Fig.9. A complete performancecurve map is given in Wang and Pan (1994), where therespective performances of the simple cycle, theintercooled-supe rcharged cycle, and the conventionalintercooled cycle are shown.The first step of the univariate search shows theexistence of optimal r value for the maximum thermalefficiency. Increasing r thus simultaneously increasesspecific power, thermal efficiency, and charge density. Thesecond step shows largely increase in thermalefficiency-associated with the lowering of exhaustenthalpy-as well as increase in charge density, of course.With the additional variable r in equations (13),(14),performance of an optimum designed intercooled-supercharged cycle gas turbine is capable of reaching alevel significan tly higher than that o f the simple cycle case.The optimal peak cycle pressure for maximum performanceis at a value much higher than that of simple cycle gasturbine.

5 . COMPOSITE ENGINES"The piston engine is em inently suitable to deal withrelatively small volumes at high pressure andtemperature and the turbine, by virtue of its highmechanical efficiency and large flow areas, to dealwith large volumes at low pressures. Clearly thelogical developm ent is to com bine the two in series toform a compound unit. "

H. R. Ricardo[quoted in Sm ith (1955:279-280)]With the exception of the utility and large industrialpowerplants, there is a compelling logic in consider the

'igure 9 The two-step optimization procedure.composite internal combustion engines that combine thcpiston engine and the turbine/compressor. Their potentials.as envisioned by R icardo, have not been realized. We willmake the case that intercooling is the key in realizing th efull advantage of the composite engines, and will outlinethe reformulated methods in applying intercooling-supercharg ing principle to "combine " the piston engine andthe turbine.Without intercooling, the concept of optimal r and thebenefit of higher optimal P,, do not exist. (The optimalP,, of the simple cycle applies.) Supetcharging increasescharge density in the cylinder, thus increases enginespecific power. However. there will be no increase in massspecific power and no increase in thermal efficiency-nosimultaneous increases in power and efficiency.With intercooling, sim ultaneous increases in power andefficiency may lead to extraordinary gain.Three types of arrangements are possible according toshaft power characteristics. The case of both crank shaftand turbine shaft producing power is known as th eturbocompounding engine. The remaining two:

intercooled-supercharged gas generator engine-composite engine with single (turbine) power-shaftintercooled-turbocharged piston engine-compositcengine with single (piston crank) power-shaft

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will be considered in this paper, due to their advantageover the turbocoapounding engine by avoiding thecomplexity of dual power shafts.Due to their single power-shaft nature, importantadvantage can be gained by "shift ing" charge expansionenthalpy into the device producing power, away from thenon-net-power-producing component. A third principalvariable for the performance functions is to be introducedin the form of the ratio of piston back pressure (ratio),P,,,,, to the supe rcharging pressu re (ra tio ),

A best selection of PP may be made according tomaximum thermal efficiency at a given r and P@. Thisvalue does not remain constant, however, for variable rorlan d variable P From the limited results obtained, the?'best ratio is approximately proportional to P,,;""l"l. Thenew variable, the composite engine parameter, s, isdefined accordingly

where R is a constant exponent, to be selected for the gasgenerator engine, and for the turbocharged enginerespectively. It should be repeated that this definition of sis not unique; its definition is chosen for facilitating theoptimization procedu re without excessive iteration required.B is expected to be a constant for the best ratio only in afinite range of r's and P,,s. For the gas generator engineR may also be selected according to thermal load (insteadof maximum thermal efficiency), represented by constantT,,,,. In this case, R is found to be '/2 in our previousstudy.Performance of the composite engines is nowrepresented by functions of three pressure-related variables.

6. THREE VARIABLE UNlVARlATE SEARCHMETHOD FOR INTERCOOLED-SUPERCHARGEDGAS GENERATOR ENGINEFor gas generator engines the performance functionsare:

For a constant T,,,,,,, the optimum design of the a sgenerator engine is determined by an iterative three stepunivariate search. The engine schematic diagrani is shownin Fig.5 in Part 1. P,,, (pressure ma intained in the mainExlin Manifold) is typically greater than P,,,. A newadditional feature, variable (late) inlet valve closing timing,is introduced in the present (Part 2) consideration. Theiterative steps are

At preset r an d P v alu es, co nsid er the matching ofdifferent two-stage turbines (see Fig.5) with thepiston component to vary the back pressure, P,,,,. Thesupercharging pressure ratio and the compression ratioof the piston compon ent may have to be adjusted withinnarrow ranges to maintain constant r and Pw.. ChangeP until maximum thermal efficiency is found; thisdefines the optimal s. (If the correspondin g T,,, exceedsthe thermal load limit, a constant T,,,, limit will be usedinstead of maximum thermal efficiency to define theoptimal s.)At the same preset P,,-defining the mechanicalload-and the sam e s as determined, change r value byvarying supercharging pressure ratio and inlet valveclosing timing incombination maintaining the constancP Diffe rent matching two-s tage turbines may beneeded to maintain constant s. The index constant B isto be determined in this step by repeating step 1 so thatconstant s defines approximately the best matchingturbine for maximum thermal efficiency at eachindividual r case. Change r until maximum thermalefficiency is found. Thermal efficiency and specificpower results are recorded for each r.

i6 At the same s and r as determined, consider increasing

piston compression ratios for higher and higher Pvalues. Other system parameters (turbine-pistonmatching, supercharging pressure, inlet valve closingtiming, . ) again may need to be adjusted to niaintiiinconstant s and r. Change P until thermal efficiencyand specific power beginning to decline.In the region of near optimal r and P after the firstthree steps, consider R and s values again. Change Avalue if necessary, and revise s value if necessary.Repeat step 2 and step 3 for a finite range of r and Pto construct perfonnance curves of thermal efficiencyvs. specific power (or thermal efficiency vs. enginespecific power) for a chosen best s value.

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This map of performanc e curves together with thermal loadand mechanical load consideration will serve as the guidefor the design of the intercooled supercharged gasgenerator engine.

7. INTERCOOLED TURBOCHARGED PISTONENGINEThe starting point af designing the intercooledturbocharged piston engine (ITurbo" engine) is therecognition that the piston crank shaft is the power shaft.Charge exp ansion energy should therefore be sh ifted awayfrom the turbocharger turbine into the piston-cylinder bykeeping P lower than P,,r--the piston intakepressure-and reducing blowdow n loss. Performa ncefunctions are:

For spark-ignition homog eneous charged engine, equivalentratio. 9. may be chosen at a constant 0.9.The optimization procedure including a three-stepunivariate search is as follows:Choose preset r and P, and a particular intake volumeand effective compression stroke. Consider differentcombinations of (longer) piston stroke and (later) inletvalve closing (IVC) timing such that the intake volumeremaining constant, and then match the piston with thebest turbocharger turbine unit. Repeat until thermalefficiency becomes maximum for a particularcombination of piston stroke, IVC timing, andturbocharger turbine. Note the corresponding P valueand s value. (For non-constant-pressure turbochargerturbin e, P,,, is determined as the effec tive value of anequivalent constant-pressure turbocharger turbineacc ord ing to equ al h,,, value. )Using the same borelstroke design and at the preset Pand the s value as determined, vary the combination ofsupercharging (turbocharging) pressure and IVC timingsuch that P remain ing constant and se lectturbocharger turbine unit suc h that s remaining constant.The index constant D is to be determined in this step byrepeating step 1 so that constant s defines approximatelythe best matching turbine for maximum thermalefficiency at each individual r case. Continue until

thermal efficiency reaching maximum. Record qih andpms as function of r.Consider increasing effective compression stroke ratioand the corresponding P under constant s and r. untilthermal efficiency and mass specific power beginning todecline.In the region of near optima l r and P,, after the firstthree steps, consider D and s values again. Change I3value if necessary, and revise s value if necessary.Repeat step 2 and step 3 for a finite range of r and Pto construct performance curves of thermal efficiencyvs. specific power (or thermal efficiency vs. enginespecific power) for a chosen best s value.

Again this map of performance curves together withthermal load and mechanical load considera tion will serveas the guide for the design of the ITurbo" engine.

8. CONCLUSIONSince it was independen tly pointed ou t by B rayton, Beaude Rochas, and Otto, the concept of optimal compressionbefore ignition/combustion became the single mostimportan t concept-after the Ca rno t's idea of achieving thehighest combustion temperature in a combustion heatengine. The single-variable optimization led to "simplecycle" internal combustion engines of good performance.Simple cycle IC engines remain the dominant powerplantof choice because of their performance, even with criticalconcern of fuel economy and emission which are not thestrong points of simple cycles.The application of the intercodling-superchargingprinciple unfolds the possibilities of multi-variableoptimization that raise perfor mance of internal conlbustionengines to new leve ls. These high-perform ance engines arecharacterized by high peak cycle pressure.While detailed studies for the gas generator engine andthe turbocharged engine remain to be carried ou t, a drasticincrease in engine specific power, pWhe, is expected fo rthe two composite engines, resulting in a quantum leap i nperformance. As a market-driven solution, which favor5high-performance technology, is the preferred solutio~~osocietal well-being, the three proposed internal combustioncycles may represent the most effective solution to furleconomy and emission.To paraphrase Carnot: The application of intercoolinp-supercharging for the development of the high-pressurrintercooled engines presents in practice great challenges. I f

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we should succeed in overconling them, it would doubtlessoffer a notable advantage over simple-cycle engines.

REFERENCESHeywood, J. B . , 1988. Internal Combustion EngineFundamentals, McGraw-Hill, New York.Smith, G .G ., 1955. Gas Turbines and Jet Propulsion,Philosophical Library, New York.Wang, L. S. , 1995, "Intercooling-SuperchargingPrinciple: Part 1-Reduction of Exhaust Heat Loss fromInternal Combustion Engines by Intercooling-Supercharging, " xxxxxxxxWang. L.S., and Pan, L. , 1994, "Optimum Peak CyclePressure for the Intercooled-Supercharged Gas TurbineEngine," Therm odynam ics and the Design, Analysis, andImprovement of Energy Systems, ASME 1994.