Hensel, S.

Maas, U.

Sauter, W.

Schiessl, R.

Schubert, A.

Spicher, U.

English

KITopen

Effects of Mixture Quality on Controlled Auto-Ignition in Petrol EnginesControlled auto-ignition in petrol engines is a promising concept for the reduction of ex-haust emissions and fuel consumption. Experimental and numerical investigations have been carried out at the Institute for Reciprocating Engines (IFKM) and the Institute of Technical Thermodynamics (ITT) to investigate the implementation of this combustion process in passenger car engines.RESEARCHATZautotechnology 03I2008 Volume 8 Future Drivetrain EVA-STAR (Elektronisches Volltextarchiv – Scientific Articles Repository) http://digbib.ubka.uni-karlsruhe.de/volltexte/1000010860 First published in:1  IntroductionThe auto-ignition of petrol is achieved by compressing a hot air/fuel mixture until auto-ignition occurs. The high tempera-tures required for auto-ignition are ob-tained by trapping hot residual gas in the combustion chamber. In contrast, combustion in conventional petrol en-gines is induced by spark ignition, which leads to a propagating flame front and the subsequent combustion of the homo-geneous air/fuel mixture. Since the con-ventional combustion of petrol/air mix-tures with low cycle-to-cycle fluctuations leads to high temperatures in the flame front, high nitrogen oxide (NOx) emis-sions cannot be avoided. In conventional diesel engines, non-premixed combus-tion leads to high soot and NOx emis-sions. In Homogeneous Charge Compres-sion Ignition (HCCI) combustion, fuel conversion without flame front propaga-tion or non-premixed combustion is fea-sible. In this mode, it is possible to achieve combustion of very lean mix-tures. Therefore, peak temperatures in the combustion chamber are reduced and NOx formation due to high tempera-tures can be avoided 0. Thermodynamic and numerical analyses of engine opera-tion in HCCI mode were performed based on experimental investigations with an optically accessible single-cylinder en-gine. The adaptation of a new three-di-mensional visualisation system provided insight into the actual combustion proc-ess inside the engine. An efficient com-bustion chemistry model was developed and coupled with a commercial three-di-mensional Computational Fluid Dynam-ics (CFD) code to describe the spatial progress of the reactions. These numeri-cal simulations were used as an aid to interpret the experimental observations in terms of mixing and reaction progress.  Test Engine and Special Experimen-tal Measurement Techniques The engine used, Table, is a modified four-stroke motorcycle engine. Engine modifications include the addition of a vane-type cam phaser and direct fuel in-jection. Auto-ignition is achieved via sym-metrical negative valve overlap 0. The three-dimensional visualisation system 0 consists of three fibre-optic endoscopes and three cameras, Figure 1. The camera system employed here is characterised by a high sensitivity for ultraviolet radia-tion and a high temporal resolution.3  Chemical Kinetic Model to  Describe the Auto-IgnitionThe point of time at which auto-ignition occurs depends neither solely on the av-erage temperature nor solely on the glo-The AuthorsDr.-Ing.  Werner Sauter is a scientific employee at the Institute for Re­ciprocating Engines at the University of Karls­ruhe (TH),  Germany.Dipl.-Ing.  Sebastian Hensel is a scientific employ­ee at the Institute for Reciprocating Engines at the University of Karlsruhe (TH),  GermanyProf. Dr.-Ing.  Ulrich Spicher is head of the Institute for Reciprocating En­gines at the University of Karlsruhe (TH),  Germany.Dipl.-Ing.  Alexander Schubert is a scientific employee at the Institute for Re­ciprocating Engines at the University of Karl­sruhe (TH), Germany.Dr.-Ing.  Robert Schießl is the team leader of the group Laserdiag­nostic Methods at the Institute of Technical Thermodynamics at the University of Karl­sruhe (TH), Germany.Prof. Dr. rer. nat.  Ulrich Maas is head of the Institute of Technical Thermo­dynamics at the Uni­versity of Karlsruhe (TH), Germany.Basic engine BMW / Rotax F650 Engine Type Single­cylinder, four­valve pentroofCompression ratio 11.5Displacement 652 cm3Bore x Stroke 100 mm x 84 mmEngine Speed 2000 rpmMixture formationSwirl injector (sideways located)Multi­hole­injector (centrally located)Injection pressure 125 barMaximum lift, intake valves 2 mm Maximum lift, exhaust valves 1.5 mmOpening duration, intake valves 120 CADOpening duration, exhaust valves 110 CADIntake valve closing 182 CAD BTDC – 140 CAD BTDCExhaust valve closing 272 CAD ATDC – 324 CAD ATDCTable: Technical specifications of the test engineATZautotechnology 03I2008 Volume 8 3 bal mixture composition, but is strongly influenced by the local thermodynamic conditions. Hence, zero-dimensional or one-dimensional tools cannot describe the processes and chemical reactions in-side the combustion chamber of an HCCI engine with sufficient accuracy. Differ-ent approaches exist to realistically in-corporate the auto-ignition and combus-tion processes into three-dimensional simulations. While simple models are computationally inexpensive, their accu-racy is quite poor. Chemical source terms can be computed accurately using de-tailed reaction mechanisms, but this method is computationally prohibitive. An efficiently reduced method for mod-elling the chemical reactions in a CFD code was developed. The model is based on a detailed reaction mechanism 0. The link between the CFD code and the chemical kinetics model is made by de-fining a chemical progress variable whose source term is tabulated as a func-tion of the local conditions. This chemi-cal progress variable is defined and the rate of this variable is pre-calculated for a homogenous reactor and then tabulated, depending on the physical and chemical conditions, using detailed chemistry. The values of the progress variable range from zero (fresh mixture of fuel and air before the start of any reactions) and one (chemical equilibrium after combustion). The arc length of the reaction trajectory based on detailed kinetics in the state space was selected as the progress varia-ble χ. Figure  shows an example of such a reaction trajectory. The development of ignition and combustion are shown in the carbon dioxide (CO2), carbon monox-ide (CO) and formaldehyde (CH2O) mass fraction subspace. At the start of the re-action, the mass fractions are zero for all three species. Shortly before the begin-ning of ignition, the CH2O mass fraction reaches its maximum. CO is an interme-diate species that is transformed into CO2 as the process continues. The mass frac-tion of CO2 increases during the combus-tion process. The colour of the reaction trajectory represents the temperature during the combustion process. The im-plementation of the chemistry model in the CFD code StarCD requires the solu-tion of additional governing equations for the progress variable and for the mass fractions of various species (CO2, CO, CH2O, fuel). Given values of the pressure, air/fuel ratio, enthalpy of the gas mix-ture and chemical progress variable, the chemical rate of the progress variable and the chemical mixture are looked up in pre-calculated tables of the detailed chemistry and passed to the CFD code. A more detailed description of the combus-tion model can be found in 0.4  Effect of Injection TimingIt was determined from the experimen-tal investigations that the start of injec-tion (SOI) has a major effect on the oper-Figure : Reaction trajectory in the CO2, CO and CH2O mass fraction subspaceFigure 3: MFB50, calculated temperature 10 CAD BTDC (T10 CAD BTDC) as a func-tion of SOI, IMEP=2 bar, 2000 rpmFigure 1: Images of the combustion chamber captured by endoscopes 1-3RESEARCHATZautotechnology 03I2008 Volume 84 Future Drivetrain ating behaviour in HCCI mode, Figure 3. By varying SOI, the point of 50 % mass fraction burned (MFB50) can be changed. As a result of this investigation, it was discovered that the shifting of MFB50 is primarily related to the temperatures in the combustion chamber at the end of compression (T10CAD BTDC in Figure 3). Because of the charge cooling effect, a late injection leads to a higher amount of cool fresh air in the cylinder at intake valve closing. Hence, the mixture has lower temperatures that lead to later au-to-ignition. For early SOI, the effects of negative valve overlap have to be consid-ered. The required amount of hot residu-al gas is trapped in the cylinder by early exhaust valve closing and late intake valve opening (negative valve overlap). This leads to compression of the residual gas with maximum temperatures of ap-proximately 1400 K and maximum pres-sures of approximately 15 bar in the cyl-inder. If an early SOI is chosen, the fuel is injected into the hot residual gas, which may lead to pre-reactions. By detecting the radiation emitted during the nega-tive valve overlap, it has been shown that, in operating points with lean air/fuel ra-tios and early fuel injection, a small amount of fuel reacts during the com-pression after the exhaust valves close 0. On the other hand, later injection tim-ings result in poorer mixture prepara-tion. For very late SOIs, it was observed that the temperatures calculated for the end of compression no longer correlate with the start of combustion. To deter-mine the influence of the mixture qual-ity on the combustion progress, the spa-tial and temporal progress of the auto-ig-nition was analysed for three different SOIs (380, 320 and 260 computer-aided design before top dead centre, CAD BT-DC) with the three-dimensional visuali-sation technique and the three-dimen-sional simulation. In all cases, the same air/fuel ratio (λ=1.15) was maintained. With early SOI at 380 CAD BTDC, the fuel was injected during the compression of the hot residual gas after the exhaust valves had closed. The middle injection was set to 320 CAD BTDC before intake valve opening and the late injection was set to 280 CAD BTDC during the intake stroke. Figure 4 shows a comparison of the measured and simulated pressure curves. Comparable results are achieved by using the chemical kinetic model de-veloped in this work. The images in Fig-ure 5, which were taken with the three-dimensional visualisation system, show the ignition process for early, middle and late injection timings. The different starts of combustion for the three cases can be seen. For all three cases, an igni-tion spot in the centre of the combustion chamber can be identified. It can be clearly seen that the ignition of the air/fuel mixture does not occur simultane-ously throughout the entire combustion chamber. Due to the local mixture and temperature distribution, auto-ignition occurs in some regions earlier and in other regions with a delay. By analysing the images captured by the visualisation system, the combustion process can be three-dimensionally reconstructed. The so-called “ignition sites” are not located close to the combustion chamber wall, but rather in the centre of the combus-tion chamber. Starting from these igni-tion sites, a sequential auto-ignition process can be seen. This process ceases in the outer regions of the combustion chamber under the valves. In Figure 6, the experimentally determined locations of ignition sites are shown. A variation of SOI leads to the displacement of these ignition sites. With an early SOI, the igni-tion sites are located on the opposite side Figure 4: Comparison of the measured (pCyl,exp) and simulated (pCyl,sim) pressure curvesATZautotechnology 03I2008 Volume 8 5 of the injector. Retarding the SOI shifts the preferred ignition sites to the centre of the combustion chamber. Results from the three-dimensional simulation con-firm this trend. The simulations of the different injection timings show that the differences regarding the residual gas distribution are negligible. Regarding the fuel distribution, the early and mid-dle SOIs lead to more uniform fuel distri-butions than the late SOIs. A late SOI leads to a highly inhomogeneous local air/fuel ratio with very rich and very lean regions. The temperature distribution in the combustion chamber shows that the highest temperatures occur in regions with low mass fractions of fuel and high mass fractions of residual gas. Late SOI leads to the most inhomogeneous tem-perature distribution, in which the re-gions with high amounts of fuel have the lowest temperatures. For early SOI, the regions with high amounts of fuel and high mass fractions of residual gas over-lap partially or are located side-by-side. Thus, a homogeneous temperature dis-tribution is achieved. The comparison of the reaction progress in Figure 7 for early, middle and late SOI shows that the ho-mogeneous temperature and fuel distri-bution in the case of the early injection timing leads to very rapid combustion, whereas the inhomogeneous mixture in the case of late SOI leads to a slower reac-tion progress. In all cases, the ignition sites are located in the regions of highest temperature. With late injection timing, the combustion is retarded so drastically that, in lean regions, complete fuel con-version is not achieved. By simulating dif-ferent injection timings, it was shown that the progress of the combustion is influenced by specific variations in the fuel distribution. As a result of fuel strati-fication, an increased ignition stability, shifted ignition timing and control of the combustion duration can be achieved.5  ConclusionThe process of auto-ignition in an HCCI engine was investigated with a combina-tion of engine experiments and CFD simulations. The development of a chem-ical kinetic model enabled an analysis of the engine behaviour with three-dimen-sional simulations. For the first time, ig-Figure 6: Experimentally determined locations of the ignition sites with early,  middle and late SOIFigure 5: Selected single cycles for early, middle and late SOIRESEARCHATZautotechnology 03I2008 Volume 86 Future Drivetrain nition sites have been located three-di-mensionally by the application of an en-doscopic fibre-optic visualisation system. Extensive investigations of the effect of SOI on the combustion were also ana-lysed and injection timing was deter-mined to be a feasible control parameter for the start of combustion. References[1] Günthner, M.; Sauter, W.; Schwarz, F.; Velji, A.; Spicher, U.: A Study of the Ignition and Combustion Process in a Gasoline HCCI Engine Using Port and Direct Fuel Injection. 6th International Symposium on Diagnostics and Modeling of Combustion in In­ternal Combustion Engines, 2004 [2] Han, K.­M.; Velji, A.; Spicher U.: A New Approach for Three­Dimensional High­Speed Combustion  Diagnostics in Internal Combustion Engines. SAE Technical Paper 2006­01­3315[3] Sauter, W.; Hensel, S.; Spicher, U.; Schubert, A.; Maas, U.: Untersuchung der Selbstzündungsmech­anismen für einen HCCI­Benzinbetrieb im Hinblick auf NOX und HC­Rohemissionen unter Berücksichti­gung der Kennfeldtauglichkeit. (in German) Ab­schlussbericht FVV­Vorhaben Nr. 831, 2007Figure 7: Reaction progression with early, middle and late SOI; CO mass fraction (wCO) and fuel concentration (wfuel) after combustionATZautotechnology 03I2008 Volume 8 7 

Effects of Mixture Quality on Controlled Auto-Ignition

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Effects of Mixture Quality on Controlled Auto-Ignition

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