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Current needs for reduced emissions and improved efficiency have motivated the study of the combustion of novel, oxygenated fuels. The scope of the present work is to experimentally and num...

The present work investigates the effect of fuel injection timing on combustion stratification and soot formation in an optically accessible, single cylinder light duty diesel engine. The engine operated under low load and low engine speed conditions, employing a single injection scheme. The conducted experiments considered three different injection timings, which promoted Partially Premixed Combustion (PPC) operation. The fuel quantity of the main injection was adjusted to maintain the same Indicated Mean Effective Pressure (IMEP) value among all cases considered. Findings were analysed via means of pressure trace and apparent heat transfer rate (AHTR) analyses, as well as a series of optical diagnostics techniques, namely flame natural luminosity, CH* and C2* chemiluminescence high-speed imaging, as well as planar Laser Induced Incandescence (pLII). For the chemiluminescence high-speed imaging a novel methodology was developed to acquire the net spatially resolved chemiluminescence signal, using combustion spectroscopy to remove the background chemiluminescence and thermal radiation signal. The analysis suggests that when the injection timing advanced, the stratification of the combustion became weaker, as indicated by the natural luminosity signal, due to the better pre-mixing between the injected fuel and the surrounding air. The enhanced premixing resulted in lower levels of in-cylinder soot late in the combustion cycle and, presumably the lowest engine-out soot emissions, because of the limited formation of locally fuel-rich areas inside the cylinder. This came at the price of increased fuel consumption.

This work employs a combination of pressure trace analysis, high-speed optical measurements and laser-based techniques for the assessment of the effects of various post-injection schemes on the soot reduction potential in an optical single-cylinder light-duty diesel engine. The engine was operated under a multiple injection scheme of two pilot and one main injection, typical of a partially premixed combustion mode, at the lower end of the load and engine speed range (ca 2.0 bar IMEP at 1200 r/min). Experiments considering the influence of the post-injection fuel amount (up to 15% of the total fuel quantity per cycle) and the post-injection timing within the expansion stroke (5, 10 and 15 CAD aTDC), under a constant total fuel mass per cycle, have been conducted. Findings were analysed via means of pressure trace and apparent rate of heat transfer analyses, as well as a series of optical diagnostic techniques, namely, high-speed flame natural luminosity imaging, CH*, C∗2 and OH* line-of-sight chemiluminescence, as well as planar laser-induced incandescence measurements at 31 and 50 CAD aTDC. The combination of post-injection fuel amount and timing has substantial effects on charge reactivity and soot oxidation potential. The analysis reveals that an amount of fuel (7% of the total fuel mass per cycle) injected more than 10 CAD after the main combustion event leads to higher levels of soot emissions, while a larger amount of fuel (15% of the total fuel mass) injected 5 CAD after the main combustion event appears to have a beneficial effect on the soot oxidation processes. Overall, results indicate that a post-injection scheme close to the main combustion phasing could reduce soot levels and improve engine performance, that is, higher IMEP levels at the same fuel consumption rates, although it could increase engine noise.

The aim of this work is to evaluate the influence of the n-heptane/toluene ratio on the reactivity of binary toluene reference fuels (TRFs), through a combined experimental and numerical work. Novel experimental ignition delay time (IDT) data of three binary TRFs of varying n-heptane/toluene ratios have been obtained in a high-pressure shock tube and in a rapid compression machine at conditions relevant to novel engine operation. Measurements have been performed at two pressures (10 and 30 bar), and at three fuel/air equivalence ratios (0.5, 1.0 and 2.0) for TRF mixtures of 50%, 75% and 90% by volume toluene concentration, over the temperature range of 650-1450 K. It was found that, increasing the n-heptane content, led to an increase in reactivity and shorter measured IDTs. Reduced sensitivity to the equivalence ratio was observed at high temperatures, especially for high toluene content mixtures. A well validated detailed kinetic mechanism for TRF oxidation was utilized to provide further insight into the experimental evidence. The mechanism, which has recently been updated, was also assessed in terms of its validity, contributing thus to its continuous development. Reaction path analysis was performed to delineate critical aspects of toluene oxidation under the considered conditions. Further, sensitivity analysis highlighted the interactions between the chemistry of the two TRF components, revealing toluene's character as a reactivity inhibitor mainly through the consumption of (O) over dotH radicals. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved. The authors are grateful to the EU Marie Curie ITN for the financial support through the ECCO-MATE project (Grant No 607214). The work by authors at LLNL was performed under the auspices of the U.S. Department of Energy (DOE), Contract DE-AC52-07NA27344 and was conducted as part of the Co-Optimization of Fuels & Engines (Co-Optima) project sponsored by the DOE Office of Energy Efficiency and Renewable Energy (EERE), Bioenergy Technologies and Vehicle Technologies Offices. peer-reviewed 2021-11-02

The present work investigates the effect of the injection dwell time and injection pressure on soot reduction potential in an optical single-cylinder light-duty diesel engine. The engine operated under a double-injection scheme under low load and low engine speed conditions. The conducted experiments considered two different dwell times for three different injection pressures. The fuel quantity of the main injection was adjusted to maintain the same indicated mean effective pressure (IMEP) value among all cases considered. Findings were analyzed via means of pressure trace and apparent heat release rate (AHRR) analyses, as well as a series of optical diagnostics techniques, namely high-speed imaging and planar laser-induced incandescence (pLII). The combination of dwell time and injection pressure substantially affects charge reactivity and soot oxidation potential. The analysis suggests that a shorter dwell time combined with a higher injection pressure can lead to an enhanced potential for engine-out particulate reduction by creating an in-cylinder environment that promotes soot oxidation. Overall, results indicate that a close-coupled pilot and main injection scheme can reduce soot levels, albeit while increasing specific fuel consumption by up to 12% to maintain the same engine power output levels.

The increasing need for more efficient and less emissive technologies has shifted the focus of combustion research towards technologies involving combustion in porous inert media. Although burners of this type have started to be examined and characterized over the past years, nonintrusive experimental methods are needed to describe the actual processes taking place inside the porous structure. In the present work, the technique of laser induced fluorescence (LIF) is employed to visualize the flame stabilization process inside porous media combustion, utilizing the excitation and subsequent detection of the hydroxyl radical (OH). In order to perform planar measurements inside the porous combustion zone, optical access along the porous structure had to be accomplished. Optical access along the porous structure allowed the laser beam to reach the probe volume and to detect sufficient fluorescence amount. This was realized by creating a thin gap of similar width as the porous cavities. The optical gap size and positioning were chosen so as to yield the most suitable configuration for a sufficient signal to noise ratio without significant disturbance of the combustion process in the porous reaction zone. The experiments were conducted for various thermal loads and excess air ratios for methane/air combustion. The main scope of this work is to demonstrate how the flame structure is influenced by the thermal load and the equivalence ratio over a wide operational range. The flame stabilization in the porous inert medium at fixed position, which is almost independent of the flow rate in some cases, is also observed. Moreover, an experimental characterization of the wide, stable operating conditions of the porous burner is given, along with the description of the flame zone inside the porous matrix.

Porous media combustion constitutes a particularly attractive technology, allowing operation in ultra-lean combustion regimes with excellent fuel interchangeability and low levels of pollutant emissions. This work presents a comprehensive experimental characterization of a state-of-the-art porous burner fuelled with a simulated biogas mixture, in terms of thermal efficiency and pollutant emissions. The combustor is a rectangular two-layer porous burner with an Al 2 O 3 flame trap and a 10 ppi (pores per inch) SiSiC foam. The burner was operated with a mixture of 60% methane and 40% carbon dioxide. An extensive stability mapping was performed in order to establish the range of operation in terms of thermal loads and mixture equivalence ratios. Gas and solid phase temperature profiles were measured using thermocouples and infrared thermography respectively, and gaseous emissions were quantified using an online gas analyser sampling system and a gas chromatographer. The results revealed wide stability with respect to thermal loads, low NO x and CO levels, and negligible hydrocarbon emissions. The effects of CO 2 addition on burner operation and efficiency, the relative impact of thermal load on temperature and emission values with respect to equivalence ratio were investigated. The comparably strong physical effect of the CO 2 addition was discussed against its chemical impact on the processes.

Homogeneous charge compression ignition (HCCI) combustion encompasses the advantages of both diesel and gasoline fuel engines. However, shortcomings mainly associated with the rapid pressur...

Low-temperature combustion concepts for transport and power generation employ mixture dilution techniques, such as exhaust gas recirculation (EGR), that offer the potential of fuel flexibility, reduced pollutant emissions, and improved efficiency. These combustion modes, however, display a higher sensitivity to the compositional changes brought about by dilution, which in turn may have an adverse influence on the overall system performance. A fundamental study on the interactions between methane and simulated EGR was carried out on a swirl-stabilized, stoichiometric flame. The effects of varying levels and composition of diluents and preheating temperatures on flame structure and exhaust emissions, were experimentally investigated. Reductions of up to 90% and over 95% in NOx and CO emissions, respectively, where observed for higher levels of added diluents, whereas an increase in preheating temperature resulted in the opposite trends. It has been further demonstrated that, depending on fuel and th...

Porous media combustion offers significant advantages against free flame burners, concerning pollutant emissions, power density, turn down ratio, combustion stability and the potential to operate in ultra-lean combustion regimes. The objective of the present work was to perform a comprehensive experimental characterization of a state-of-the-art porous burner in terms of thermal efficiency and pollutant emissions and assess its operating limits. The combustor was a rectangular two-layer porous burner with an Al 2 O 3 flame trap and a 10 ppi (pores per inch) SiSiC foam. The burner was operated with methane and LPG. An extensive stability mapping was performed in order to establish its range of operation in terms of thermal loads and mixture equivalence ratios. Gas phase temperature profiles were measured using thermocouples and the solid phase temperature distribution was obtained using an IR camera. Gaseous emissions were quantified using an online gas analyser sampling system. The results revealed a homogeneous temperature distribution, low NO x and CO emissions and wide flexibility with respect to fuels and thermal loads. The effects of fuel interchange on efficiency and emissions were also analysed. Finally, the relative impact of thermal load on temperature and emission values, with respect to equivalence ratio or fuel type, is discussed.

Η διατριβή στοχεύει στην ανάπτυξη διαγνωστικών εργαλείων για τη μέτρηση εκπομπών από φαινόμενα καύσης και την εφαρμογή τους σε συστήματα καύσης προανάμιξης βαθμωτής πολυπλοκότητας. Αρχικά, αναδεικνύεται η αναγκαιότητα και το πεδίο εφαρμογής των διαγνωστικών μεθόδων καύσης και οι χρησιμοποιούμενες τεχνικές αναλύονται και σχολιάζονται. Το πρώτο μέρος της διατριβής επεξηγεί τις αρχές λειτουργίας, τη θεμελιώδη θεωρία και τα βασικά συστατικά των χρησιμοποιηθέντων τεχνικών. Σε αυτές περιλαμβάνονται το σύστημα της αέριας χρωματογραφίας, το σύστημα συνεχούς ανάλυσης καυσαερίων, καθώς επίσης και το σύστημα για φασματοσκοπία φθορισμού επαγόμενου από διέγερση με χρήση ακτινοβολίας laser. Το πρώτο μέρος της διατριβής παρέχει το απαραίτητου υπόβαθρο για το σχεδιασμό, την εκτέλεση και την κατανόηση των αποτελεσμάτων κι υπό αυτό το πρίσμα, καθίσταται απαραίτητο συστατικό της Στο δεύτερο σκέλος της διατριβής, τα διαγνωστικά εργαλεία χρησιμοποιούνται για τη μελέτη διατάξεων χαμηλής πολυπλοκότητας, στοχεύοντας αφενός στην αξιολόγηση των διαγνωστικών μεθόδων και αφετέρου στη διεξαγωγή πρωτότυπης έρευνας, συνδεόμενης με υπαρκτά ζητήματα στο πεδίο των συστημάτων καύσης. Υπό αυτό το πρίσμα, η τεχνική της αέριας χρωματογραφίας επιστρατεύτηκε για την ποσοτικοποίηση αέριων αλλά και επιλεγμένων πολυκυκλικών αρωματικών υδρογονανθράκων, που ευρίσκονται στα προϊόντα της πυρόλυσης μιγμάτων μεθανίου σε αντιδραστήρα εμβολικής ροής. Τα αποτελέσματα αναπαρήχθησαν με δυο μηχανισμούς λεπτομερούς χημείας, για την πληρέστερη κατανόηση των διεργασιών, με ικανοποιητική συμφωνία υπολογιστικών προλέξεων και πειραματικών δεδομένων. Εν συνεχεία, η τεχνική της φασματοσκοπίας φθορισμού επαγόμενου από διέγερση laser εφαρμόστηκε για τη μελέτη φλόγας προανάμιξης εγκάρσιας ροής. Η ψυχρή ροή μελετήθηκε ιχνηλατώντας μόρια ακετόνης στη ροή του καυσίμου και τα χαρακτηριστικά του σχήματος της φλόγας αναλύθηκαν μέσω της παρακολούθησης της ρίζας του υδροξυλίου στη ζώνη καύσης. Μέσω της εκμετάλλευσης συμβατικών πηγών ενέργειας, όπως το φυσικό αέριο και ο γαιάνθρακας, δύνανται μέσω της διεργασίας Fischer-Tropsch να προκύψουν εναλλακτικά καύσιμα, συμβατά με τις υπαρκτές υποδομές. Στο έκτο κεφάλαιο της διατριβής, αεροπορικά καύσιμα της ανωτέρω προέλευσης, εξετάζονται με τη χρήση ενός εργαστηριακού καυστήρα προανάμιξης. Μετρήσεις θερμοκρασίας και καυσαερίων, σε συνδυασμό με μια αναλυτική μεθοδολογία για τον υπολογισμό των θερμοχημικών ιδιοτήτων βάσει της αναλυτικής χημικής σύστασης, κατέστησαν εφικτή την αξιολόγηση των καυσίμων και τη συσχέτιση της απόδοσης, με τα περιεχόμενα συστατικά τους. Τελικώς, οι προαναφερθείσες παρεμβατικές και μη τεχνικές, συνδυάστηκαν για τον χαρακτηρισμό ενός καυστήρα υψηλής πολυπλοκότητας (καυστήρας πορώδους αδρανούς μέσου). Η οπτικοποίηση της φλόγας μέσα στο πορώδες, μέσω της τεχνικής φασματοσκοπίας φθορισμού επαγόμενου από διέγερση laser και συγκεκριμένα της παρακολούθησης της ρίζας του υδροξυλίου στη ζώνη καύσης, ανέδειξε της περιοχές λειτουργίας του συναρτήσει του θερμικού φορτίου και της στοιχειομετρίας. Εν κατακλείδι, διεξήχθη πλήρης πολυπαραμετρικός χαρακτηρισμός του πορώδους καυστήρα και αναλύθηκε η ικανότητα του για λειτουργία με συμβατικά και εναλλακτικά καύσιμα.