NUMERICAL ANALYSIS OF THE AIR AND EGR GAS MIXTURE IN THE INTAKE MANIFOLD

Transcription

1 Poceedings of COBEM 0 Copyight 0 by ABCM st Bailian Congess of Mechanical Engineeing Octobe 4-8, 0, Natal, RN, Bail NUMERICAL ANALYSIS OF THE AIR AND EGR GAS MIXTURE IN THE INTAKE MANIFOLD Heicson Faia de Moaes, heicson.moaes@naista.com.b Fabio Yuio Kuoawa, fabio.uoawa@naista.com.b MWM Intenational Motoes, Naista South Ameica Lui Felipe de Mendes Moua, felipe@fem.unicamp.b Uniesidade Estadual Campinas, Faculdade de Engenhaia, Depatamento de Engenhaia Témica e de Fluidos Abstact. The standads which dine the limits of the pollutant emissions poduced by the automotie ehicles hae become inceasingly esticted, specialty fo diesel engines. To meet these standads, the automotie companies hae deeloped seeal technologies to educe and contol the pollutant emissions esulting fom the combustion pocess in the engines. Among these technologies we can find the exhaust gas eciculation system, which is usually nown in the industy as EGR system. This system is ey functional; howee the success of its opeation is quite sensitie to the bounday conditions which suound its opeating. One of these bounday conditions is the homogeneity of the ai and EGR gas mixtue which is poided to the engine cylindes. The moe homogeneous wee this mixtue, moe ficient will be the EGR system opeation and lowe will be the pollutant emissions geneated by the engine. In this inestigation aimed to ealuate numeically the mixtue pocess. It was used the Computational Fluid Dynamics (CFD) techniques to deelop the analyses. The computational code used is the FLUENT TM 6.3.6, which is a code based on the finite olume method to sole the tanspot equations of the fluid mechanics. The bounday conditions of the poblems wee dined with basis on the test cycles which the engine is submitted to be ealuated by the egulatoy agencies. As it was adopted the pemanent egime to mae the system numeical analyses, it s pesented qualitatie esults of the flow and the mixtue pocess of the ai and EGR gas (in the eal phenomena the bounday conditions ay accoding to the engine opeation tansient egime). Keywods: diesel engine, CFD, EGR, emissions,. INTRODUCTION In 89 the enginee Rudolf Chistian Kal Diesel egisteed a patent fo a new intenal combustion engine conception. His concept was based on stat the combustion with the injection of liquid fuel into a chambe filled with heated ai, whee the ai was heated though a compession pocess. It allowed this new engine to each an ficiency twice times bigge than the engines of his time, Heywood (988). This themal machine has been called Diesel Engine, and due its obustness, duability and fuel economy, it has been used in geat pat of the wate and land tanspotation, and it s esponsible to geneate electical powe fo fams, constuctions and industial actiities, Lloyd and Cacette (00). Although the positie chaacteistics descibed aboe, the diesel engine has some negatie issues. One of those issues is common fo all intenal combustion engines: the pollutant emissions esultant fom the combustion pocess (Loueio, 005; Onusal and Gautam, 997). As the use of this ind of engine was gowing quicly, some counties stated to contol the pollutant emissions. Figue. Eolution of the standad emissions fo light duty ehicles in the USA

2 Poceedings of COBEM 0 Copyight 0 by ABCM st Bailian Congess of Mechanical Engineeing Octobe 4-8, 0, Natal, RN, Bail Figue show the eolution on the Ameican emission standads fo light duty ehicles. On this chat HC (unbuned hydo-cabon), CO (cabon monoxide) and NOx (nitogen oxides) ae combined only fo illustation. Among the all compounds esultant fom the diesel combustion which hae its limits contolled by the emission standads, the nitogen oxides ae the ones which had the biggest estiction. Figue show the eolution on the Ameican emission standads to contol the NOx and the soot poduced by heay duty ehicles. Figue. Eolution of the contol fo NOx and Soot poduced by heay duty ehicles in the USA To follow these standads, some technologies wee deeloped in ode to educe the NOx poduced by the engine. One of them is the Exhaust Gas Reciculation, usually called EGR. This system wos substituting a faction of the ai inducted into the engine with exhaust gases fom the combustion. The exhaust gas mass faction is intoduced into induction system though a connection between the exhaust manifold and the intae manifold, and then is mixed with the main ai flow. Afte that the mixtue flows to the cylindes, Bosh (004). Figue 3 show a schematic dawing of the EGR system. Figue 3. Schematic dawing of the EGR system The significant inceasing of thee EGR ate esults on geat NOx eductions. Unfotunately, big quantities of EGR mass incease the emissions of unbuned hydo-cabons, CO and the fomation of soot due the eduction of the oxygen in the chambe (She, 998; Siewet et al., 00; Zheng et al., 003). Othe poblem egading the use of EGR

3 Poceedings of COBEM 0 Copyight 0 by ABCM st Bailian Congess of Mechanical Engineeing Octobe 4-8, 0, Natal, RN, Bail technology is to ensue the ai and EGR mixtue will be distibuted to the cylindes in a homogeneity way. The low homogeneity mixtue will cause the incease of the NOx emissions and soot. These poblems ae caused due the chemical composition aiation and tempeatue aiation of ai and EGR mixtue that flows to the cylindes (Maiboon et al., 009). In ode to facilitate the mixtue pocess and aoid the poblems descibed aboe, it s nomally used a mixtue deice (called EGR mixe) on the engines. This wo aims to numeically ealuate the ai/egr mixtue into the intae manifold with pe-chambe mixe.. DESCRIPTION PROBLEM The pesent study focuses on the mixtue Ai/EGR in the intae manifold. This poblem is soled consideing compessible, tubulent, constant popeties fluid flow whee gaity and iscous dissipation fects wee neglected. Based on these assumptions, numeical esults of a 3D simulation ae obtained fo the domain showed in Fig. 4. Ai inlet EGR inlet Intae manifold outlet 3. MATHEMATICAL FORMULATION Figue 4 Imposed bounday conditions The steady-state Naie-Stoes equations fo an compessible tubulent flow with constant popeties fluid ae pesented in the thee-dimensional Cylindical coodinate system (,, ). It is assumed that the iscous heating and gaity fects ae negligigle. Continuity equation: ( ) ( ) ( ) 0 () Momentum equation in -diection:

4 Poceedings of COBEM 0 st Bailian Congess of Mechanical Engineeing Copyight 0 by ABCM Octobe 4-8, 0, Natal, RN, Bail p D () Momentum equation in -diection: p D (3) Momentum equation in -coodinate: p D (4) Enegy equation: T T T D p De (5) Whee and ae the fectie (molecula +tubulent) iscosity and themal conductiity. State equation: p RT (6) The standad - model is a semi-empiical model based on model tanspot equations fo the tubulent inetic enegy () and its dissipation ate (). The model tanspot equation fo is obtained modeling the exact equation, while the model tanspot equation fo was obtained using physical easoning and beas little esemblance to its mathematically exact countepat, Launde and Spalding (974). In the - model it was assumed that the flow is fully tubulent, and the fects of molecula iscosity ae negligible away fom solid walls. Tanspot equations fo the Standad - model ae (Naayama, 995): i T i G x x D (7) C G C x x D i t i ) ( (8) whee: G - epesents the geneation of tubulent inetic enegy due mean elocity gadients 3,, C C C - ae constants - tubulent Pandtl numbe fo - tubulent Pandtl numbe fo

5 Poceedings of COBEM 0 Copyight 0 by ABCM st Bailian Congess of Mechanical Engineeing Octobe 4-8, 0, Natal, RN, Bail The tubulent iscosity,, is computed by combining and : t c (9) t The bounday conditions pescibed at each domain suface ae summaied in Tab. : Table Imposed bounday conditions in the model Suface Inlet Ai Inlet EGR Wall Outlet Imposed bounday condition Pessue Mass Flow Adiabatic Pessue The alues used in the simulation wee obtained in the dynamomete test fo the thee opeation conditions, Tab.: Table Dynamomete test data Engine opeation point Engine speed (RPM) Engine Load (gf) Inlet EGR flow (g/s) Inlet ai pessue (Pa) 00 90,5 3, , ,3, , ,6, ,09 Outlet Atmospheic pessue With the numeical analysis concluded, it was calculated the EGR ate based on the EGR flow aailable fo each cylinde. The EGR ate is calculated though the following equation: m EGR EGR(%) x00 m m EGR AIR (0) whee m and m ae mass flow of the EGR and ai, espectiely. EGR AIR 4. COMPUTATIONAL METHODOLOGY The numeical simulations wee caied out using the commecial CFD softwae, FLUENT TM (007), esion The finite olume scheme (Fletche, 998), which inoles integating the goening equations about each contol olume, was applied. The goening equations wee discetied using the fist ode upwind scheme fo conectie tems and soled by a density-based fomulation, using the SIMPLE appoach fo pessue-elocity coupling. An analysis of gid independence was pomed using fie diffeent meshes. Table 3 summaies the numbe of elements fo each mesh. Table 3 Mesh Mesh Elements numbe Mesh 57,845 Mesh 6,984 Mesh 3 4,636 Mesh 4 33,09 Mesh 5 59,478 The simulations wee pomed using a mesh with hybid elements tetahedal, Fig. 5.

6 Poceedings of COBEM 0 Copyight 0 by ABCM st Bailian Congess of Mechanical Engineeing Octobe 4-8, 0, Natal, RN, Bail Figue 5 Example of mesh used in the simulations. 5. RESULTS Figue 6 shows the esults of the gid independence analysis. Based on that, we can conclude thee ae no significant aiations on EGR ate eceied by each cylinde fo meshes with moe than 4,000 cells. Figue 6. Gid independence analysis esults Accoding to the methodology descibed and the mesh sie dined by the gid independence analysis it was analyed the EGR system configuation intae manifold with pe-chambe. Figue 7 shows the esults of the intae manifold with pe-chambe mixe analysis. As show the esults, thee ae big discepancies on the EGR ates alues among the cylindes. As mentioned, this condition can incease the NOx emissions and soot fomation.

7 Poceedings of COBEM 0 Copyight 0 by ABCM st Bailian Congess of Mechanical Engineeing Octobe 4-8, 0, Natal, RN, Bail Figue 7. EGR ate pe cylinde Pe-chambe mixe Figue 8 shows the EGR mass faction contous. It s possible to chec the mixtue poided to the cylindes 3 and 4 is much iche in EGR than the mixtue poided to the fist two cylindes. Figue 8. EGR mass faction contous

8 Poceedings of COBEM 0 Copyight 0 by ABCM st Bailian Congess of Mechanical Engineeing Octobe 4-8, 0, Natal, RN, Bail The pesented esults don t conside the opening and closing of the ales due to complexity of the phenomena. Fo the qualitatie analysis, the esults show that the mal-distibution of the mixtue is mainly caused by intae manifold geomety. On the eal phenomena, with the opening and closing of the ales, the distibution of the mixtue would be moe homogeneous on the intae manifold outlet than on consideed situation. 6. FINAL COMMENTS Een with the simplifications consideed on the analysis and nowing that on the eal phenomena the opening and closing of the ales facilitate the mixtue pocess, it s possible to conclude that intae manifold geomety is not the optimal design fo the engine. Due the intenal wall, which diides the intae manifold in two olumes, the dop pessue imposed to the flow gets to the cylindes and is much bigge than the dop pessue imposed to the flow gets to the cylindes 3 and 4. As the mixtue doesn t get homogenous enough on the pe-chambe (boe the outlet pots), and with the dop pessue discepancies among the cylindes, the cylindes and eceie a poo EGR mixtue and the cylindes 3 and 4 eceie a ich EGR mixe. Fo a qualitatie analysis and to guide the designing pocess of the intae manifold, the esults obtained ae ey satisfactoy. If quantitatie esults wee equied, it ll be necessay to couple the CFD code with a D code of the engine in ode to tansfom the stationey bounday condition in tansient. With this the opening and closing of the ales will be consideed on the flow, esulting in a situation close to the eal phenomena. The D code must be deeloped and calibated based on dynamomete bench test data. 7. ACKNOWLEDGEMENTS Fo all people who helped us on the deelopment of this wo. 8. REFERENCES Bosch, 004, Automotie Handboo, 6 ed., Stuttgat, 3 p. CETESB, 009, Pocone, 0 Jun.009 < Fletche, C. A. J., 998, Computational Techniques fo Fluid Dynamics, Fundamental and Geneal Techniques, Sping-Velag, Belin,.. Fluent Inc., 007, Fluent Documentation:, esion Heywood, J. B., 988, Intenal combustion Engine Fundamentals, Ed. McGaw Hill, New Yo, USA, 930 p. Launde, B. E. and Spalding, D. B., 974, The Numeical Computation of Tubulent Flows, Compute Methods in Applied Mechanics and Engineeing, n. 3, pp Lloyd, A. C. and Cacette, T. A., 00, Diesel Engines: Enionmental, Impact and Contol, J. Ai & Waste Management Association,. 5, pp Loueio, L. N., 005, Panoâmica sobe Emissões Atmosféicas Estudo de Caso: Aaliação do Inentáio de Emissões Atmosféicas da Região Metopolitana do Rio de Janeio paa Fontes Móeis, Dissetation (Maste), Uniesidade Fedeal do Rio de Janeio, 53 p. Maiboon, A., Tauia, X. and Hetet, J. F., 009, Influence of EGR Unequal Distibution fom Cylinde to Cylinde on NO x -PM tade-off of a HSDI automotie Diesel engine, Applied Themal Engineeing,. 9, n. 0, pp Naayama, A., 995, PC-Aided Numeical Heat Tansfe and Conectie Flow, CRC Pess Inc. Onusal, B. and Gautam, S. P., 997, Vehicula Ai Pollution: Expeiences fom Seen Latin Ameican, Wold Ban Technical Pape, n She, E., 998, Handboo of Ai Pollution fom Intenal Combustion Engines: Pollutant Fomation and Contol, ed., San Diego, Academic Pess, 663 p. Siewet, R. M., Kiege, R. B., Hueble, M. S., Bauah, P. C., Khalighi, B. and Wesslau, M., 00, Modifying an Intae Manifold to Impoe Cylinde-to-Cylinde EGR Distibution in a DI Diesel Engine Using Combined CFD and Engine Expeiments, SAE Pape, Zheng, M., Reade, G. T. and Hawley, J. G., 004, Diesel engine eshaust gas eciculation a Reiew on adanced and noel concepts, Enegy Conesion and Management,. 45, n. 6, pp RESPONSIBILITY NOTICE The authos ae the only esponsible fo the pinted mateial included in this pape.