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Zugl.: München, Techn. Univ., Diss., 2007

Zugl.: München, Techn. Univ., Diss., 2007

The improvement of the indicated thermal efficiency of an argon power cycle (replacing nitrogen with argon in the combustion reaction) is investigated in a CFR engine at high compression ratios in homogeneous charge compression ignition (HCCI) mode. The study combines the two effects that can increase the thermodynamic efficiency as predicted by the ideal Otto cycle: high specific heat ratio (provided by argon), and high compression ratios. However, since argon has relatively low heat capacity (at constant volume), it results in high in-cylinder temperatures, which in turn, leads to the occurrence of knock. Knock limits the feasible range of compression ratios and further increasing the compression ratio can cause serious damage to the engine due to the high pressure rise rate caused by advancing the combustion phasing. The technique proposed in this study in order to avoid intense knock of an argon cycle at high compression ratios is to cool the intake charge to subzero temperatures which leads to lower in-cylinder temperatures and hence, less possibility of having knock. The main variable in this study was the intake temperature which was investigated at 40.0 °C and -6.0 °C which corresponded to low and high compression ratios, respectively. Emission analysis shows that the low in-cylinder temperature of the cooled case led to less complete combustion, and so, lower combustion efficiency. Since nitrogen is replaced with argon, NOx was only formed in negligible amounts due to some nitrogen traces in the used gasses cylinders. Furthermore, the cooled charge required more work to be done in the gas exchange process due to the decrease in the intake pressure caused by cooling the intake which deteriorated the gas exchange efficiency. The heat losses factor was found to be the main parameter that dictated the improvement of the thermodynamic efficiency and it was found that the indicated thermal efficiency was deteriorated for the cooled case as a result of all the aforementioned factors. Although the values of the thermodynamic efficiency at high compression ratios did not meet the expectations based on the ideal Otto cycle due to the assumptions of the ideal cycle, the obtained values, in general, are relatively high.

Dans le but de réduire la consommation en carburant, les émissions de CO2 et les polluants tout en maintenant le haut rendement des moteurs, de nouveaux modes de combustions ont été étudiés et sont d’excellents candidats pour remplacer les moteurs conventionnels. En particulier, le mode HCCI a montré une excellente aptitude pour répondre à ces objectifs. Néanmoins, en dépit de ses avantages, de nombreux challenges sont à surmonter avant de permettre le développement de tels moteurs. Parmi eux, obtenir un contrôle efficace de la totalité de ce processus de combustion sur un large domaine d’utilisation demeure le principal défi. Ces travaux de thèse s’intéressent à l’utilisation des espèces chimiques oxydantes comme un moyen robuste de contrôle de la combustion HCCI. En raison de ces fortes propriétés oxydantes, l’ozone a été la principale molécule étudié. De plus, son intérêt est renforcé par le fait que l’ozone peut être produit au sein d’un véhicule au moyen de petits générateurs, mais cela peut aussi produire des oxydes d’azote. Ces recherches ont été effectuées au moyen d’un banc moteur monocylindre HCCI et couplées avec des simulations de cinétique chimique. Les deux principaux objectifs ont été : (1) Evaluer le potentiel d’utilisation d’un générateur d’ozone pour contrôler la combustion HCCI. L’impact de plusieurs espèces chimiques oxydantes, ozone and NOx, a été étudié sur la combustion de l’isooctane. De plus, un contrôle dynamique a été mis en place avec succès. (2) Comparer l’influence de l’ozone sur la combustion de l’isooctane et de carburants alternatifs. Des carburants à forte teneur en méthane et des alcools ont été étudiés en raison de leur forte résistance à l’autoinflammation et de leur structure chimique. / To reduce the fuel consumption, CO2 emissions and pollutant emissions while keep improving thermal efficiency of engines, alternative combustion modes are being investigated as good candidates to replace spark-ignited and diesel engines. In particular, Homogeneous Charge Compression Ignition (HCCI) engines have proven their potential to meet these requirements. However, despite of these advantages, several challenges remain to be addressed prior to the widespread implementation of HCCI engines. Among them, the control of the overall combustion process in such an engine over the full operating range is still considered as the main challenge to overcome. The present work introduces the use of oxidizing chemical species seeded in the intake system as a robust control technique for HCCI combustion process. In particular, ozone was examined due to its strong oxidizing characteristics. Moreover, ozone can be easily produced on-board a real vehicle from the intake oxygen thanks to small ozone generators, but can also lead to the production of NOx. Investigations were carried out using a single-cylinder HCCI engine and kinetics computation analysis. The two main objectives of this work are: (1) Evaluate the potential of using ozone generator to control the HCCI combustion. Along these lines, the interaction between NOx and ozone was investigated for isooctane as fuel and a real time control of the HCCI combustion was implemented and successfully tested. (2) Compare the influence of ozone on the combustion of isooctane and alternative fuels. Methane-based fuels (methane/propane and methane/hydrogen mixtures) and alcohols (methanol, ethanol, n-butanol) were selected due to their higher resistance to autoignition and their different chemical structure.

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. / Emission standards tightening as well as economical needs urge to study newcombustion modes for engines. Low-temperature homogeneous charge auto-ignition offersgood prospects for NOx, soot, and CO2 emissions. However, its control remains sharp for it isextremely influenced by temperature and fuel chemistry. Assisting non-equilibrium plasmascould provide a solution. Experiments are RCM managed with lean isooctane/air mixtures andprototype Renault ignition devise. Combustion occurs in a two steps mode known as SICI:flame propagation compresses the remaining gas to auto-ignition. The experimental settemperature rise is computed in order to measure the SICI effect compared to pure autoignition.The plasma seems to act mainly through the energy dropped, albeit its effect quicklyreaches a maximum, no matter how early it starts. This asymptomatic high energy behaviorrelies on the streamers overheating, as underlined by the look-like SICI effect from a regulararc discharge. On the contrary, minimal required energy appears to be linked to the capabilityof generating a sustainable flame kernel, making it closer to a standard ignition issue in roughconditions. Flame propagation sets auto-ignition start, according to an astonishingly linearcharacteristic not even influenced by charge’s thermodynamic conditions. Cool flame is putforward through formaldehyde PLIF imaging. Prereaction seems to enhance front propagationspeed.

Pour franchir les prochaines étapes réglementaires, une des solutions adoptées par les constructeurs automobiles est la dépollution à la source par des nouveaux concepts de combustion. Une piste d’étude est le moteur à charge homogène allumé par compression, le moteur HCCI. Le défi majeur est de contrôler le phasage de la combustion lors des transitions. Or, l’ozone est un additif prometteur de la combustion. La première partie de ce travail est consacrée au développement d’un modèle 0D physique de la combustion dans le moteur HCCI à l’aide d’une approche statistique basée sur une fonction de densité de probabilité (PDF) de la température. Pour cela, un modèle de variance d’enthalpie est développé. Après la validation expérimentale du modèle, il est utilisé pour développer des cartographies du moteur HCCI avec et sans ajout de l’ozone afin d’évaluer le gain apporté par cet actuateur chimique en terme de charge et régime. La deuxième partie porte sur le contrôle du phasage de combustion par ajout d’ozone. Une étude de simulation est effectuée où des lois de commandes sont appliquées sur un modèle orienté contrôle. Les résultats montrent que l’ajout d’ozone permet de contrôler cycle-à-cycle le phasage de la combustion. En parallèle, une étude expérimentale sur un banc moteur est facilitée grâce à un système d’acquisition des paramètres de combustion (Pmax, CA50) en temps réel, développé au cours de cette étude. En intégrant les lois de commande par ajout d’ozone dans le calculateur du moteur (ECU), les résultats expérimentaux montrent la possibilité de contrôler non seulement cycle-à-cycle le phasage de la combustion par ajout d’ozone lors des transitions mais aussi de stabiliser le phasage de la combustion d’un point instable. / To pass the next legislator steps, one of the alternative solutions proposed for the depollution at the source by new concepts of combustion. One of proposed solution is the Homogeneous Charge Compression Ignition (HCCI) engine. The major challenge is to control combustion phasing during transitions. Ozone is promising additive to combustion. During this work, a 0D physical model is developed based on temperature fluctuations inside the combustion chamber by using Probability Density Function (PDF) approach. For this, an enthalpy variance model is developed to be used in Probability Density Function (PDF) of temperature. This model presents a good agreement with the experiments. It is used to develop HCCI engine map with and without ozone addition in order to evaluate the benefit of using ozone in extending the map in term of charge-speed. The second part deals with control the combustion phasing by ozone addition. A Control Oriented Model (COM) coupled with control laws demonstrates the possibility to control combustion phasing cycle-to-cycle. Thereafter, an experimental test bench is developed to prove this possibility. A real time data acquisition system is developed to capture combustion parameters (Pmax, CA50). By integrating control laws into Engine Control Unit (ECU), results demonstrate not only the controllability of combustion phasing cycle-to-cycle during transitions but also to stabilize it for an instable operating point.

<p>The residual gas fraction is an important parameter to get good performance with high efficiency and low emissions in the HCCI-engine. </p><p>The goal in this thesis is to formulate an algorithm for estimation of the residual gas fraction based on the cylinder pressure. The estimation is improved if also the exhaust gas temperature is used together with the cylinder pressure. </p><p>The formulated algorithm has then been tested on data from a single cylinder engine running in HCCI-mode during steady state conditions. An error of 4% was noted compared with the residual gas fraction obtained from simulations. </p><p>The thesis also investigates the effects of some possible error sources.</p>

Homogeneous Charge Compression Ignition (HCCI) operation is desirable for its high thermal efficiency and low emissions of NOx and particulates. Difficulty with cold starting and maximum achievable speed/load highlight the desire for mode-switching to traditional spark ignition (SI) operation. Mode-switching between SI and HCCI is investigated using only actuation of throttle, CNG injector pulse width, and CNG injection timing on a single cylinder CFR engine. Open-loop control achieves a one cycle mode-switch between two adjustable IMEP levels. Sequences are repeatable as demonstrated by 10 mode-switches with the same inputs. Performance is evaluated using a developed mode-switch performance criterion (MSPC) by considering duration between steady-states of operation, smoothness of IMEP, and knock based on maximum rate of pressure rise. Comparing the results with subjective analysis (the current standard) reveals good correlation. Throughout development, mode-switching performance is shown to improve by a factor of 60.

Homogeneous Charge Compression Ignition (HCCI) Engines hold promises of being the next generation of internal combustion engines due to their ability to produce high thermal efficiencies and low emission levels. HCCI combustion is achieved through the auto-ignition of a compressed homogenous fuel-air mixture, thus making it a "fusion" between spark-ignition and compression-ignition engines. The main challenge in developing HCCI engines is the absence of a combustion trigger hence making it difficult to control its combustion timing. The aim of this research project is to model and control a natural gas HCCI engine. Since HCCI depends primarily on temperature and chemical composition of the mixture, Exhaust Gas Recirculation (EGR) is used to control ignition timing. In this research, a thermodynamical, physics-based nonlinear model is developed to capture the main features of the HCCI engine. In addition, the Modified Knock Integral Model (MKIM), used to predict ignition timing, is optimized. To validate the nonlinear model, ignition timing under varying conditions using the MKIM approach is shown to be in accordance with data acquired from a model developed using a sophisticated engine simulation program, GT-Power. Most control strategies are based on a linear model, therefore, the nonlinear model is linearized using the perturbation method. The linear model is validated by comparing its performance with the nonlinear model about a suitable operating point. The control of ignition timing can be defined as a regulation process where the goal is to force the nonlinear model to track a desired ignition timing by controlling the EGR ratio. Parameters from the linear model are used to determine the gains of the LQR controller. The performance of the controller is validated by implementing it on the nonlinear model and observing its ability to track the desired timing with 0.5% error within a certain operating range. To increase the operating range of the controller and reduce steady-state error, an integrator is added to the LQR. Finally, it is shown that the LQR controller is able to successfully reject disturbance, parameter variation, as well as noise.