Darbe tirta tiekiamo oro temperatūros, deginių recirkuliacijos ir degalų savybių įtaka suslėgimu uždegamo homogeninio oro – bioetanolio mišinio degimo procesui, variklio darbo ir deginių emisijos rodikliams. Nustatytos variklio darbo homogeniniu oro – bioetanolio mišiniu maksimalios apkrovos zona, kurią riboja slėgio didėjimo greitis ir susidarantis detonacinis degimas. Gautieji rezultatai varikliui veikiant bioetanoliu, lyginti su rezultatais gautais varikliui veikiant baziniais degalais – benzinu. Dyzeliniam varikliui dirbant homogeniniu oro ir bioetanolio mišiniu, keičiant tiekiamo oro temperatūrą, naudojant deginių recirkuliaciją ir įmaišant 20 % reaktyvinių degalų, azoto oksidų emisijos lieka dešimtis kartų mažesnės negu tradicinio dyzelinio variklio. Slegiamo homogeninio oro – bioetanolio mišinio užsiliepsnojimui valdyti pirmą kartą panaudoti reaktyviniai degalai. / It was found that the intake air temperature, exhaust gas recirculation and fuel properties have the influence on the combustion characteristics of homogeneous air – bioethanol mixture, engine performance and emissions. The operating region was found at which critical rate of pressure rise was unacceptably high and engine worked at “knocking” combustion. In order to assess the combustion process in homogeneous charged compression ignition (HCCI) mode fuelled with the air – bioethanol mixture, it was compared whit the combustion process in HCCI mode fuelled with the air – gasoline mixture. When the diesel engine works in HCCI mode fuelled with air – bioethanol mixture, the nitrogen oxides can be reduced more than 10 times compared with normal diesel operation. For the first time, to control the autoignition of compressed homogeneous air and bioethanol mixture, the JET A1 fuel was simultaneously added in to combustible mixture in this study.

Chemical kinetics has been widely acknowledged as a fundamental theory in analysis of chemical processes and the corresponding reaction outputs and rates. The study and application of chemical kinetics thus provide a simulation tool to predict many characteristics a chemical process. Oxidation of hydrocarbon fuels applied in internal combustion engines is a complex chemical process involving a great number of a series of chained reaction steps and intermediate and simultaneous species. Symbolic and Numerical description of such a chemical process leads to the development and application of chemical kinetics models. The up-to-date application of chemical kinetics models is to the simulation of autoignition process in internal combustion engines. Multi-zone thermodynamic combustion modelling has been regarded as a functional simulation approach to studying combustion process in IC engines as a decent compromise between computation accuracy and efficiency. Integration of chemical kinetics models into multi-zone models is therefore a potential modelling method to investigate the chemical and physical processes of autoignition in engine combustion. This research work has been therefore concerned with the development, validation and application of multi-zone chemical kinetic engine models in the simulation of autoignition driven combustion in SI and HCCI engines. The contribution of this work is primarily made to establish a mathematical model based on the underlying physical and chemical principles of autoignition of the fuel-air mixture in SI and HCCI engines. Then, a computer code package has been developed to numerically solve the model. The derived model aims at improving the understanding of autoignition behaviour under engine-like conditions and providing an investigative tool to autoignition characteristics. Furthermore, as part of the ongoing program in the research of free piston engines, the results of this work will significantly aid in the investigation and simulation of the constant volume autoignition applied in free piston engines.

In the first part of studies, Controlled Auto-Ignition (CAI) combustion was investigated in a Ricardo E6 single cylinder, four stroke gasoline engine. CAI combustion is achieved by employing positive valve overlap configuration in combination with various compression ratios and intake air temperature strategies. The CAI operational region is limited by engine load due to knock and partial burned boundaries. The combustion characteristics and emissions are studied in order to understand the major advantages and drawbacks of CAI combustion with positive valve overlap. The enlargement of the CAI operational region is obtained by boosting intake air and external EGR. The lean-boosted operation elevators the range of CAI combustion to the higher load region, and the use of external EGR allows the engine to operation with CAI combustion in the mid range of region between boosted and N/A CAI operational range. The results are analyzed and combustion characteristics, performance and emissions are investigated. A Ricardo Hydra single cylinder, four stroke optical gasoline engine with optical access is then experimented to investigate CAI combustion through negative valve overlap configuration and an intake heater. The effects of direct fuel injection timings spark timings and air/fuel ratio are studied by means of simultaneous incylinder heat release study and direct visualization, chemiluminescence techniques which uses full, OH radical and CHO species. Both heat release analysis and chemiluminescence results have identified the pressure of minor combustion during the NVO period. Both the charge cooling and local air/fuel ratio effects are also investigated by varying the quantity of direct air injection.

This thesis is about modelling of the combustion and emissions of dual fuel HCCI engines for design of “engine combustion system”. For modelling the combustion first the laminar flamelet model and a hybrid Lagrangian / Eulerian method are developed and implemented to provide a framework for incorporating detailed chemical kinetics. This model can be applied to an engine for the validation of the chemical kinetic mechanism. The chemical kinetics, reaction rates and their equations lead to a certain formula for which the coefficients can be obtained from different sources, such as NASA polynomials [1]. This is followed by study of the simulation results and significant findings. Finally, for investigation of the knock phenomenon some characteristics such as compression ratio, fuel equivalence ratio, spark timing and their effects on the performance of an engine are examined and discussed. The OH radical concentration (which is the main factor for production of knock) is evaluated with regard to adjustment of the above mentioned characteristic parameters. In the second part of this work the specification of the sample engine is given and the results obtained from simulation are compared with experimental results for this sample engine, in order to validate the method applied in AVL Fire software. This method is used to investigate and optimize the effects of parameters such as inlet temperature, fuels ratio, diesel fuel injection timing, engine RPM and EGR on combustion in a dual fuel HCCI engine. For modelling the dual fuel HCCI engine AVL FIRE software is applied to simulate the combustion and study the optimization of a combustion chamber design. The findings for the dual fuel HCCI engine show that the mixture of methane and diesel fuel has a great influence on an engine's power and emissions. Inlet air temperature has also a significant role in the start of combustion so that inlet temperature is a factor in auto-ignition. With an increase of methane fuel, the burning process will be more rapid and oxidation becomes more complete. As a result, the amounts of CO and HC emissions decrease remarkably. With an increase of premixed ratio beyond a certain amount, NOX emissions decrease. With pressure increases markedly and at high RPM, knock phenomenon is observed in HCCI combustion.

With the depletion of petroleum based fuels and the corresponding concerns of national energy security issues, natural gas as an alternative fuel in IC engine applications has become an attractive option. Natural gas requires minimum mixture preparation, and is chemically stable, both of which make it a suitable fuel for homogeneous charged compression ignition (HCCI) engines. Compared to petroleum based fuels, natural gas produces less green-house emissions. However, natural gas is hard to auto-ignite and therefore requires a higher compression ratio, some amount of intake heating, or some type of pre-ignition. In addition, natural gas usually has large differences in fuel composition from field to field, which adds more uncertainties for engine applications. The current study determines the auto-ignition characteristics, engine performance, and nitric oxides emissions as functions of major operating parameters for a natural gas fueled HCCI engine, and determines differences relative to gasoline fueled HCCI engines which have been studied for many years. These tasks have been done using both zero- and multi-dimensional engine simulations. By zero-dimensional simulation, the effects of varying equivalence ratios, engine speeds, compression ratio, EGR level, intake pressure and fuel compositions are determined and analyzed in detail. To be able to account for the in-cylinder inhomogeneous effect on the HCCI combustion, multi-zone models coupled with cold-flow CFD simulations are employed in addition to the single-zone model. The effects of non-homogeneous temperature and equivalence ratio stratification on the ignition timing, combustion phasing, and emissions formation have been studied and discussed. Finally, the preliminary two-dimensional axial-symmetric CFD simulations have been conducted to study the in-cylinder temperature and the species distributions, which provide better visualization of the natural gas auto-ignition process.

This doctoral thesis is about controlling the combustion timing of the combustion concept Homogeneous Charge Compression Ignition, HCCI, by means of variable valve timings. The HCCI research usually is regarded to have started in Japan during the later part of the 1970´s. The world of HCCI has since grown and HCCI is of today researched worldwide. Of particular interest from a Swedish point of view is that Lund Institute of Technology has emerged as one of the world leading HCCI laboratories. The idea with HCCI is to combine the Otto and Diesel engine. As in an Otto engine the charge is premixed but as in a Diesel engine the operation is unthrottled and the compression heat causes the ignition. The combustion that follows the ignition takes place homogeneously and overall lean. The result is ultra low NOx and particulate emissions combined with high total efficiency. A difficulty with the HCCI-concept is that it only works in a narrow area and that there is no direct way to control the Start Of Combustion, SOC. Out of this follows that timing/phasing of the combustion is one of the main difficulties with HCCI combustion concepts. This is particularly emphasized during transient operation and calls for feedback control of the combustion timing. This work investigates one method, the variable valve timing, to achieve feedback control of the combustion phasing. From the work it can be concluded that the variable valve timing can control the combustion phasing during engine transients. In order to improve the performance a non-linear compensation from ignition delay to valve timings has been suggested, incorporated in a control structure and tested in engine test. The engine test has been performed in a single cylinder engine based on a Scania truck engine. The speed range from 500 to 1750 rpm and the load range 1.26 and 10.5 bar of netIMEP has been covered with fair transient performance. / QC 20100629

Depuis plusieurs années, l'une des problématiques sociétales est de diminuer les émissions de polluants et de gaz à effet de serre dans l'atmosphère. Le secteur du transport terrestre est directement concerné par ces considérations. Le moteur Diesel semble promis à un bel avenir grâce à son rendement supérieur à celui du moteur à allumage commandé, conduisant à de plus faibles rejets de CO2. Cependant, sa combustion génère des émissions d'oxyde d'azote (NOx) et de particules dans l'atmosphère. Les normes anti-pollution étant de plus en plus sévères et les incitations à diminuer les consommations de carburant de plus en plus fortes, le moteur Diesel est confronté à une problématique NOx/particules/consommation toujours plus difficile à résoudre. Une des voies envisagées consiste à modifier le mode de combustion afin de limiter les émissions polluantes à la source tout en conservant de faibles consommations. La voie la plus prometteuse est la combustion HCCI (Homogeneous Charge Compression Ignition) obtenue par injections directes précoces. Plusieurs limitations critiques doivent cependant être revues et améliorées : le mouillage des parois par le carburant liquide et le contrôle de la combustion à forte charge. Le but de cette thèse est ainsi de mieux comprendre les phénomènes mis en jeu lors de la combustion HCCI à forte charge obtenue par des multi-injections directes précoces. Une méthodologie a été mise au point afin de détecter le mouillage des parois du cylindre, ce qui a permis de comprendre l'effet du phasage et de la pression d'injection sur cette problématique. Une stratégie optimale de multi-injections permettant d'atteindre une charge élevée sans mouiller les parois a ainsi été développée et choisie. Nous avons ensuite pu mettre en évidence le potentiel de la stratification par la dilution en tant que moyen de contrôle de la combustion en admettant le diluant dans un seul des 2 conduits d'admission. Des mesures réalisées en complémentarité sur le même moteur mais en version 'optique', ont permis, à partir de la technique de Fluorescence Induite par Laser, de montrer que concentrer le diluant dans les zones réactives où se situe le carburant permet un meilleur contrôle de la combustion, ce qui permet d'amener le taux de dilution a des niveaux faisables technologiquement.

Le durcissement des normes d'émission, tout autant que l'impératif d'économie,poussent à étudier de nouveaux modes de combustion pour les moteurs. L'autoallumage decharges homogènes à basse température offre de bonnes perspectives quant au rejet de NOx,suies, et CO2. Cependant son control reste délicat, car il est extrêmement sensible à latempérature et la cinétique de l'hydrocarbure. L'assistance par plasma hors-équilibre pourraitfournir une solution. Les expériences sont menées dans une MCR avec des mélanges pauvresd'isooctane/air et un prototype d'allumeur Renault. La combustion obtenue identifiée commeSICI se déroule en deux phases: la propagation d'une flamme comprime les gaz restantjusqu'à leur autoallumage. Le réchauffement du système expérimental est intégré dans leprotocole d'exploitation, afin de quantifier l'effet SICI relativement à l'autoallumage pur.L'effet du plasma semble avant tout dépendre de l'énergie déposée, bien qu'il convergerapidement, quel que soit l'avance du déclenchement. Le comportement asymptotique à hauteénergie s'explique par la thermalisation des filaments, soulignée par comparaison avec l'effetSICI d'un arc classique. A l'inverse, le seuil minimal d'énergie nécessaire semble lié à lacapacité à générer un noyau de flamme viable, rapprochant le phénomène d'un problèmeclassique d'allumage en conditions difficiles. La propagation de la flamme détermine ledéclenchement de l'autoallumage selon une caractéristique linéaire particulièrementremarquable, car indépendante des conditions thermodynamiques du mélange. L'existenced'une flamme froide est mise en avant par des acquisitions de PLIF formaldéhyde. Lapréréaction semble accélérer la propagation du front de flamme.

The Homogeneous Charge Compression Ignition (HCCI) engine is a promising engine concept that emits low concentrations of NOx and particulates and still has a high efficiency. Since the charge is auto-ignited, the auto-ignition quality of the fuel is of major importance. It has been shown in several studies that neither of the classical measures of auto-ignition quality of gasoline-like fuels, RON and MON, can alone describe this in all conditions in HCCI combustion. However, even in such cases it is possible to combine RON and MON into an octane index, OI, that describes the auto-ignition quality well in most conditions. The octane numbers are combined into the OI with the variable K according to the following equation: OI = (1-K)RON + K MON = RON – K S The OI of a sensitive fuel is the equivalent of the octane number of a primary reference fuel with the same resistance to auto-ignition in the tested condition. The K-value is dependent on the temperature and pressure history. A generic parameter Tcomp15, the temperature at 15 bar during the compression, was introduced to describe the temperature and pressure history. It was found that the K-value increases with increasing Tcomp15 and two linear equations have been suggested to describe this relationship. At high or low Tcomp15 it has been found that the sensitivity of the fuel octane quality on combustion phasing is small and the auto-ignition quality defined by the OI scale does no longer play a big role. NO affects the combustion phasing of gasoline-like fuels. This effect is most significant at low concentration where it advances the combustion phasing considerably. At higher conditions its influence is different for different fuels. A sensitive fuel is considered a good HCCI fuel since its OI changes in the same direction as the octane requirement of the engine, which would make the engine management easier. It is also likely that a sensitive fuel will enable a wider operating range. The auto-ignition quality of diesel-like fuels was studied in tests with three different strategies of mixture formation. In these tests it was found that the ignition delay increased with lower cetane number and that the cetane number described the auto-ignition quality well, even for fuels of significantly different physical properties. The experiments were, however, made at a limited range of operating conditions and low load. A good diesel-like HCCI fuel should be easy to vaporize to facilitate homogeneity. It should have a high resistance to auto-ignition, not necessarily the highest, one that allows both high and low loads at a given compression ratio. Finally, it should also function well with the injection system without a significant decrease in injection system life length. / QC 20100917

Zugl.: Karlsruhe, Univ., Diss., 2009