Analysis of Recoverable Exhaust Energy from a Light-Duty Gasoline Engine

Transcription

1 Anlysis of Recoverble Exhust Energy from Light-Duty Gsoline Engine Tinyou Wng 1, Yjun Zhng 1, Zhng Jie 1, Gequn Shu 1, Zhijun Peng * 1) Stte Key Lbortory of Engines, Tinjin University, Chin ) Deprtment of Engineering nd Design, University of Sussex, UK * Corresponding uthor ABSTRACT While EER (Exhust Energy Recovery) hs been widely pursued for improving the totl efficiency nd reducing CO emissions of internl combustion engines, the mximum regenerted power from the exhust energy hs been proposed nd clculted in terms of endoreversible cycle. In this pper, bsed on the experimentl dt of n EER system instlled on light duty gsoline engine, the exhust energy nd mximum recoverble energy were nlysed, by defining new prmeter of the recoverble exhust energy efficiency (the frction of mximum recoverble exhust energy in the totl fuel energy) for reflecting the vilble energy of exhust energy in ll mentioned engine operting conditions. Combining those experimentl nd modelling dt, results show the engine exhust gses temperture increses with both of the speed nd lod in the overll operting conditions of vehicle, rnging mong 400ºC nd 850ºC. With wter s the working fluid for the EER system, the recoverble exhust energy efficiency rnges mong 5% nd 1% under different engine operting condition nd it could be up to 19% in rther wide lod rnge under generl engine operting speed. Keywords : internl combustion engine, exhust energy recovery (EER), recoverble exhust energy efficiency

2 1 INTRODUCTION In recognition of the need to further reduce vehicle exhust emissions nd the greenhouse gs CO s the oil price hs kept roring up, there hs been n incresing interest in the development of clener nd more efficient energy sving vehicle powertrin. It is thought future sustinble vehicle powertrin developments beyond the next decde re likely to be focused on four topics [1]: emission legisltion nd control, new fuels, improved combustion nd rnge of dvnced concepts for energy sving. And mong the vrious dvnced concepts, EER (Exhust Energy Recovery) for IC engine hs been proved to not just bring mesurble dvntges for improving fuel consumption but lso increse engine power output (power density) or downsizing, further reducing CO nd other hrmful exhust emissions correspondingly. It ws predicted by Vzquez et l. [] tht if 6% of the het contined in the exhust gses were converted to electric power, 10% reduction of fuel consumption cn be chieved. Erly reserches on EER hve investigted the bsic concepts, problems nd expected improvements for such system. An exmple could be found from the reserch conducted by Chmms nd Clodic [3], who presented the dvntges offered by Rnkine system designed for hybrid vehicles, up to 18% fuel economy improvement could be chieved when wter ws used to recover the exhust het. More recently reports [4] showed how further investigtion of the technology nd rchitectures re possible. For instnce, Teng et l. crried out series of experiments [5-7] on hevy-duty diesel engines to explore the potentil of EER, with hybrid energy systems combined the exhust system with the chrge ir cooler nd EGR cooler(s). Their results show tht up to 0% increse in the engine power nd 5% improvement in fuel svings over the ESC 13-mode test could be chieved by the EER system. Ringler et l. [8] selected two bsic EER configurtions (one just with exhust gs only nd nother with exhust gs plus coolnt) from numerous illustrted Rnkine cycle lyouts for detiled evlution of het recovery bsed on four-cylinder IC engine. Their experimentl

3 works demonstrted tht wste het recovery cn produce n dditionl power output of bout 10% t typicl highwy cruising speeds. Weersinghe et l. [9] identified the substntil potentil of EER for IC engines vi two most promising nd techniclly vible technologies: turbo-compounding nd exhust het secondry fluid power cycles. Their results reveled tht the two EER technologies would contribute more power output in the order of % nd fuel svings by -%. Vrious reserches hve underlined the interest in light to hevy duty vehicle pplictions nd suggest tht fuel economy improvements of up to 0% cn be expected from EER. In this pper, the study which hs been focused on the exhust energy from gsoline engine with the objective of exploring the vilble recoverble energy in exhust gs is presented. While the exhust temperture nd gs flow rte vry with engine operting conditions, the vilble exhust energy for EER nd its chrcteristic under different engine operting conditions would be understood. Then the optiml operting res for utilizing the exhust energy could be identified. In the current study, four-cylinder light-duty gsoline engine ws employed for experimentl recoverble exhust energy. PARAMETER DEFINITION AND MODEL DESCRIPTION The EER system which will be used in the present reserch is bsed on Rnkine cycle nd physiclly it comprises four min components: n evportor/het exchnger, n expnder, condenser nd pump. The lyout of the system is shown in Figure 1 nd its temperture-entropy digrm is presented in Figure.

4 Figure 1 Lyout of the EER system Figure T-S digrm of the EER system With the evportor/het chnger, the working fluid is superheted by bsorbing therml energy provided from the exhust gs. Flowing out from the evportor s high temperture stem, the working fluid is driving the expnder to produce useful work. Then the wste stem from the expnder will be cooled down through the condenser to return to liquid phse. In the next step, the working fluid is run to mintin the circultion. For most internl combustion engines, there is pproximtely 0~40% of totl fuel energy which is dissipted through exhust gs, with the mjor prt s sensible enthlpy due to high exhust temperture nd minor prt s chemicl enthlpy due to incomplete combustion. To evlute energy mount in the exhust gs, it is necessry to obtin the thermo-physicl prmeters of exhust gses. Considering currently ll diesel engines nd most gsoline engines during dominnt operting period re driven with len combustion condition, n ssumption of complete in-cylinder combustion would be used for the following nlysis while the focus of this reserch work is on the therml energy recovery of exhust gs. Then smll mounts of incomplete combustion products such s CO nd unburnt hydrocrbon nd NOx emission components could be ignored nd the compositions of exhust gses could be considered s the mixture of CO, H O, N nd O.

5 Provided tht the stoichiometric ir-fuel rtio in the gsoline combustion is 0, nd the ctul one is α, the molr frctions for N nd O in the ir mixture is k N nd k O, respectively. While the tom numbers of crbon nd hydrogen in the hydrocrbon fuel moleculr re θ C nd θ H, respectively, the molr frctions of compositions in the exhust gses could be obtined by the following equtions, respectively. / 1 0 H O N N k k θ φ = / 1 ( 0 0) H O O O k k θ φ = / 1 0 H O C CO k θ θ φ = / 1 / 0 H O H O H k θ θ φ = (1) Considering the bove four compositions re ll idel gses, their constnt pressure het cpcity C p,i could be chieved by the empiricl formuls [10] : R T c T c T c c T c C i p ) ( , = () Given the exhust gses idel condition, the specific enthlpy could be clculted by: = i M i h i h ω (3) where, ω i M i nd h i re the molr frction molr mss nd specific enthlpy for ech composition. And the ltter could be expressed by: = T T i p i dt C h h 0, 0 (4) Combined the equtions (1) to (4), the specific enthlpy of exhust gses cn be chieved.

6 It should be noted the bove formule cn only be selected for clculting the specific enthlpy of exhust gses when ll therml recovery process did not involve stem condenstion of exhust gs nd there is only the sensible het of the exhust gses which is bsorbed by the therml recovery system. When the stem condensing het should be included if there is phse chnge of exhust gs vi the evportor, the exhust specific enthlpy ws obtined from NIST-Refprop dtbse. Then, the exhust het Q exh nd its frction in the totl fuel energy could be given by: Q = ( h h ) m exh exh out exh (5) η exh = Qexh h m f f (6) where, m exh nd m f re the mss flow rte of exhust gs nd fuel, respectively, h exh nd h f re the corresponding exhust gs enthlpies, nd the low heting vlue of fuel. For estimting the recoverble energy from exhust sensible het, the Chmbdl-Novikov efficiency or Curzon-Ahlborn efficiency [11] shown in the following formul is employed in the current clcultion. ηm, p = 1 T T L, in H, in (7) where T L,in nd T H,in represent the inlet tempertures of the cold nd hot het sinks with finite therml cpcitnce, i.e. the exhust temperture t the het exchnger exit T out nd the exhust temperture T exh,in for the clcultion of the mximum convertible energy of exhust. The Chmbdl-Novikov efficiency or Curzon-Ahlborn efficiency is for semi-idel engine operting t mximum power output in which het trnsfer is irreversible but other components re idel. It gives n upper bound on energy tht cn be derived from rel process tht is lower thn

7 tht predicted by Crnot for Crnot cycle, nd ccommodtes the exergy destruction occurring s het is trnsferred irreversibly [1]. Although sometime the Crnot efficiency is used to clculte the mximum recoverble energy, the Chmbdl-Novikov efficiency or Curzon-Ahlborn efficiency ws proposed on the bsis of endoreversible cycle with the trget of gining the mximum output power which is the optiml purpose for EER system, becsure the ltter one considered the temperture difference of het (cold) source nd the working fluid which would cuse irreversible loss. It is lso thought the Crnot efficiency would be equl to the Chmbdl-Novikov efficiency or Curzon-Ahlborn when the power outputs of Crnot het engine rech the mximum [1]. Klein checked this expression nd suggested tht it gives more relistic estimte of the efficiencies seen in rel het-power cycles thn the Crnot Approch [13]. By using the Chmbdl-Novikov efficiency or Curzon-Ahlborn efficiency, the mximum energy recovery from exhust gses cn be estimted s follows. Q exh, r = Qexhη m, p (8) Then, the exhust recoverble energy efficiency bsed on the totl fuel energy cn be expressed s : η = (9) exh, r ηexhη m, p The frction of recoverble exhust sensible het in the totl fuel energy is ctully working s the recoverble energy dptive coefficient. The coefficient is proposed in the current study for reflecting the fluctutions of exhust temperture nd recoverble exhust energy with the engine operting conditions, nd lso for estimting the vilble energy in the exhust gs under ll mentioned engine operting conditions.

8 After those bove prmeters were defined, simultions for estimting recoverble exhust energy with the bove EER system nd for finding the optiml operting conditions of the gsoline engine to chieve the mximum power output were crried out on MATLAB/SIMULINK pltform. In the simultion model, performnce prmeters for those criticl components re considered s follow. The Energy blnce in the evportor could be given s: Q η = Q (10) e h w Where, Q e nd Q w represent the het provided by the exhust gs nd the het bsorbed by the working liquid, respectively. η h is the efficiency of evportor. This formul could be further expressed by: m exh ( hexh out ) h w w out w in h ) η = m ( h h ) (11) Where, m exh h exh-in h exh-out stnd for the mss flow rte, inlet nd outlet specific enthlpy for exhust gses respectively, nd m w h w-in h w-out re the corresponding prmeters for working fluid. The function of evportor model is to simulte the het trnsfer process in the evportor by clculting the outlet temperture of the working fluid t given pressure nd flow rte. With the ssuming inlet prmeters for the exhust gses nd working fluid nd the sizes of the evportor, the outlet temperture could be yielded by itertive progrm. Bsing on the design prmeters of the evportor, the het trnsferred in it could be lso clculted s Q w = U A T (1) m m Where, U m represents the verge het trnsfer coefficient of the het trnsfer process, A is the overll het trnsfer re of evportor, nd T m is the logrithmic men temperture difference in

9 the het trnsfer process. For the expnder, the specific enthlpy h w-out nd entropy s exp in the inlet of expnder could be obtined with the results from the evportor model. And the pressure p exp-out in the outlet of expnder would be determined by condensing pressure. Assuming the working fluid s expnsion process is isentropic, the specific enthlpy in the outlet of expnder could be identified by p exp-out nd s exp. Therefore, the power output produced by the expnder could be clculted s W exp mw hexp out hw out ) = ( η (13) exp Where, η exp represents the expnder efficiency. In the pump, the process cn be expressed with the following formul when the process is ssumed s isentropic. W pump = m ) η (14) w ( hpump out hpump in pump 3 TEST DESCRIPTION As close loop is still under construction, the present experiment is conducted on n open Rnkine cycle system which is connected with 1.3-liter gsoline engine nd the system structure is shown in Figure 3. The min specifictions of the engine is listed in Tble 1. In the present test, wter ws chosen s the working fluid.

10 Figure 3 The open Rnkine cycle system for exhust energy recovery with gsoline engine Tble 1 Specifictions of CA4GA1 Engine Engine type CA4GA1 Number of cylinders 4 Bore Stroke (mm) Displcement (L) Compression rtio 10 Number of vlves 16 Cmshft type DOHC Rted power/speed (kw/rpm) 67/6000 Mximum torque/speed (Nm/rpm) 10/400 4 RESULTS AND DISCUSSION 4.1 Distribution of Exhust Gs Temperture For studying EER, severl prmeters of ir mss flow rte nd exhust gses, such s temperture, mss flow rte, would be required for the clcultion of exhust energy. In those prmeters, the exhust temperture plys so importnt role, such s for the design of recovery system, the choice of working fluids nd the optimiztion of system [14]. Therefore, in order to mke full dvntges

11 of exhust energy, it is necessry to hve dequte informtion of the distribution of exhust gses temperture under different engine operting conditions. As shown in Figure 4, it is the mesured exhust gs temperture s function of engine speed nd corrected engine lod (torque). It could be found tht the exhust gses temperture depends strongly on the engine speeds nd lods. Here it should be noted the ctul vehicle opertion normlly needs the engine speed rnges from 000 to 4000 rpm. During the mid-rnge of the engine lod (40-80 Nm), the corresponding exhust gses temperture of the test engine cn be 500ºC to 700ºC, while it will be up to 850ºC t the full lod. It should be pointed out the dt in Figure 1 were mesured in the outlet of 3-wy ctlyst, which is followed downstrem by the het exchnger of EER system. Therefore, bsed on the exhust temperte distribution, n pproprite evporting temperture nd mss flow rte would be determined for the working fluids in the thermodynmic cycle of exhust energy recovery. Figure 4 Distribution of exhust gses temperture of the test engine s function of engine speed nd torque 4. Model Vlidtion nd Process Prmeters The role of evportor is so significnt in the EER system becuse it directly domintes the potentil to recover the exhust energy. Using the bove model, the performnce of evportor ws

12 simulted under vrious operting conditions. With the recovered efficiency s the efficiency of the evportor/het exchnger, the performnce of the evportor cn be found in Figure 5. Figure 5 Efficiency of evportor s function of mss flow rte of working fluid under different evporting pressure. From Figure 5, it cn be found the efficiency of evportor depends strongly on the flow rte of working fluid, while it presents wek dependence on the evporting pressure. Bsed on the present open loop, the clculting model of the EER system ws vlidted. Showed in Figure 6, the experimentl nd modeling results of the reltionship between the working fluid flow rte nd pressure re in very good greement. 4.3 Frction of Mximum Recoverble Energy After the vlidtion to the modeling method, three kinds of energy efficiencies s defined in Eqution 6, Eqution 7 nd Eqution 9 were clculted under different exhust tempertures nd results cn be found in Figure 7.

13 Figure 6 Comprison of experimentl nd modeling results regrding working fluid mss flow rte under different evporting pressure (δt - over-het degree) Figure 7 Vrition of η m,p, η exh, nd η exh,r s function of the exhust gses temperture It could be seen tht ll three efficiencies present nerly linerity with the exhust gses temperture. And the frction of mximum recoverble energy in the totl fuel energy η exh,r could exceed 0% when the temperture is over 800. The temperture chrcteristics, combined with the distribution

14 of exhust gses temperture, could infer further the optiml conditions tht would benefit most to the EER system. In the rel drive cycle of vehicle, the engine speed cn rnge round 000 rpm to 4000 rpm for most driving conditions. Here, three typicl speeds were chosen to demonstrted the vritions of the exhust recoverble efficiency η exh,r with different engine speeds, s presented in Figure 8. Figure 8 Vritions of recoverble exhust energy efficiency η exh,r with different operting conditions of the test engine In Figure 8, it could be seen tht, given certin speed, η exh,r increses firstly s the lod increses, nd reches up to mximum when the torques increse mong 90 Nm nd 100 Nm. Then it will decrese if the torques continues to increse further. There exists n optiml operting condition for ech engine speed to produce the mximum recoverble exhust energy. Comprison mong the cses in different speeds could mke cler tht the higher the engine speed, the lrger η exh,r will be possible. This could be explined by the fct tht the increse of speed would give rise to the increse of the exhust gses temperture. Bsing on the distribution of the exhust gses temperture with the engine speeds nd lods s shown in Figure 4, the distribution of η exh,r s function of operting conditions could be chieved in

15 the sme wy nd the result is presented in Figure 9. The dt in Figure 9 indictes tht η exh,r rnges mong 0.05 nd 0.1, with generl tendency of proportionl increse with speeds nd lods. For the speeds mong 000 rpm nd 3000 rpm, the frction of mximum recoverble exhust energy could rech up to 19% of the totl fuel energy t the high lods, while the frction will become lrger in more extensive lod scope when the speed is beyond 3000 rpm. As the mximum recoverble exhust energy represents the mximum recoverble energy of the recovery system, its frction lso gives how much improvements the EER system will contribute to the engine efficiency. Therefore, it could be predicted tht the recovery system would operte with high efficiency in n extensive lod scope when the engine speed increses over 3000 rpm. Figure 9 Recoverble exhust energy efficiency for the test engine s function of the engine speed nd torque 4.4 Influence of the Instlltion of EER System on Engine Performnce From those results shown in bove section, it hs indicted the recoverble exhust energy efficiency cn be round 0% under some engine operting conditions. This mens, big

16 improvement on the engine efficiency cn be chieved with EER system. While the bove results were produced with the ssumption tht the EER system would not give ny side effect of the exhust gs flow, it is necessry to find if this is true or not. () (b) Figure 10 Influence of EER system on engine: () Exhust bck pressure; (b) Brke specific fuel consumption Shown in Figure 6, the exhust bck pressure nd the engine fuel consumption with nd without the EER system re presented. It cn be found tht the exhust bckpressure ws incresed no more thn 0.14 kp while the specific fuel consumption vried slightly except t 0.3Mp (33.6 vs , by 5.3%). This suggests the present EER system cn only increse the exhust bck pressure nd the engine fuel consumption with very limited mount. Compred the improvement on the totl engine efficiently due to the instlltion of EER system, the increses (on the exhust bck pressure nd the engine fuel consumption) cn be ignored. Following the present test, new evportor is under design with id of CFD for reducing the

17 exhust gs flow resistnce further. It is expected the new design will be possible to mke the increse of exhust bck pressure very close zero. 5 CONCLUSIONS In the current study, experiments were performed on light duty gsoline engine for obtining thermodynmic prmeters of exhust gses under different operting conditions. Bsed on the experimentl dt, the exhust het nd mximum recoverble exhust energy for n EER (Exhust Energy Recovery) system bsed on Rnkine Cycle were clculted nd nlyzed, including the chrcteristics in terms of temperture nd other operting conditions. From those results, the following conclusions cn be derived: A new prmeter, the recoverble exhust energy efficiency which is ctully the frction of mximum recoverble exhust energy in the totl fuel energy is recommended for reflecting the effects of exhust temperture nd possible exhust het on EER efficiency under different engine operting conditions. The exhust gses temperture increses with both of the speed nd lod in the overll operting conditions of vehicle, rnging mong 400ºC nd 850ºC for the tested light duty gsoline engine. Experimentl results of relevnt prmeters would be beneficil to determining the pproprite evporting temperture nd mss flow rte for the working fluids in the thermodynmic cycle of EER. For the EER system instlled on the light duty gsoline engine with wter s the working fluid, the recoverble exhust energy efficiency rnges mong 5% nd 1% under different engine operting condition. It tends to increse with engine speed nd lod. When the speed is beyond 3000 rpm, the coefficient could be up to 19% in rther wide lod rnge.

18 ACKNOWLEDGEMENTS Finncil supports from the Ntionl Bsic Reserch Progrm of Chin (973 Progrm) through the project of 011CB70701 nd the Ntionl Nturl Science Found of Chin (NSFC) through the project of re grtefully cknowledged. REFERENCES 1. Tylor A.M.K.P, Science review of internl combustion engines, Energy Policy. 36 (008) Vzquez J, Znz-BobiMA, Plcios R, Arens A. Stte of the rt of thermoelectric genertors bsed on het recovered from the exhust gses of utomobiles. Proceedings of 7th Europen workshop on thermoelectrics, El Chmms, R.G. nd D. Clodic, Combined Cycle for Hybrid Vehicles. SAE 005 World Congress & Exhibition, 005. SP-1973 ( ). 4. Tinyou Wng, Yjun Zhng, Zhijun Peng, Gequn Shu. A review of reserches on therml exhust het recovery with Rnkine cycle. Renewble nd Sustinble Energy Reviews (011) Teng H, Regner G, Cowlnd C. Achieving high engine efficiency for hevy-duty diesel engines by wste het recovery using supercriticl orgnic-fluid Rnkine cycle. SAE pper , Teng H, Regner G, Cowlnd C. Wste het recovery of hevy-duty diesel engines by orgnic Rnkine cycle Prt I: hybrid energy system of diesel nd Rnkine engines. SAE pper 007-

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