A computational study of the use of hydrogen peroxide as pilot fuel for a homogeneous mixture of ammonia/hydrogen in a compression ignition engine

We report a computational investigation of a compression ignition (CI) engine (compression ratio: 17.6, displacement volume: 1.3 L) where the main fuel is a homogeneous mixture of ammonia (70-60% vol%) and hydrogen (30-40% vol%), with a global equivalence ratio varying between 0.44 and 0.5, depending on the H2/NH3 ratio. The novelty of this study is that it employs a pilot injection of hydrogen peroxide to initiate ignition, without the use of any air or charge preheating (the temperature/pressure at intake BDC are 330 K/1.4 atm, representing mild boost an defficeint intervooling). Hydrogen peroxide (H2O2) has the attributes that it can be produced from renewable sources, and it is already widely manufactured, distributed, and stored, with diverse applications (as an aqueous solution of H2O2 with shares up to 30 vol%). The main advantage of using H2O2 as pilot fuel, as opposed to other more conventional ones (e.g., diesel), is that it contains no carbon, and hence produces no CO2 and particulate matter (PM). The computational investigation was conducted with an advanced commercial stochastic engine model that has been previously validated. The investigation was primarily focused on assessing the effects of using hydrogen peroxide in aqueous solution as pilot fuel on engine efficiency, combustion phasing and NOx emissions. These three aspects were investigated over a range of engine speeds (750-1,750 rpm) in view of: (i) the variation of the mass of the directly injected (DI) aqueous solution (0.1-10 mg); (ii) the variation of the H2O2 share (15-50%) in the directly injected (DI) aqueous solution; (iii) the variation of the start of injection (from -20 to -4 CAD aTDC) and injection duration (1-8 CAD). There is a strong effect of the peroxide in advancing combustion timing (CAD50 is advanced by up to 15 CAD) and in decreasing combustion duration (CAD90-CAD10 decreases up to 10 CAD), and peroxide readily enables medium engine loads which are the ones investigated in this work. Indicated thermal efficiencies above 50% were readily achieved at all engine speeds and, with a 30 vol% peroxide share in the solution, the pressure rise rate was always below 30 bar/ms. However, the NOx emissions in all cases exceeded the IMO’s Tier III standard. Possible ways to tackle this would be either the use of exhaust gas recirculation, or optimisation of the injection strategy, or the use of aftertreatment. For most cases, on a volume basis, the required aqueous H2O2 amount is 3% of the main fuel while on an energy basis this translates to 1.1% of the main fuel blend. The results reported in this initial study are promising because it is possible to ignite a premixed charge on the basis of a small pilot volume of commercially available hydrogen peroxide solutions

A computational study of the use of hydrogen peroxide as pilot fuel for a homogeneous mixture of ammonia/hydrogen in a compression ignition engine

We report a computational investigation of a compression ignition (CI) engine (compression ratio: 17.6, displacement volume: 1.3 L) where the main fuel is a homogeneous mixture of ammonia (70-60% vol%) and hydrogen (30-40% vol%), with a global equivalence ratio varying between 0.44 and 0.5, depending on the H2/NH3 ratio. The novelty of this study is that it employs a pilot injection of hydrogen peroxide to initiate ignition, without the use of any air or charge preheating (the temperature/pressure at intake BDC are 330 K/1.4 atm, representing mild boost an defficeint intervooling). Hydrogen peroxide (H2O2) has the attributes that it can be produced from renewable sources, and it is already widely manufactured, distributed, and stored, with diverse applications (as an aqueous solution of H2O2 with shares up to 30 vol%). The main advantage of using H2O2 as pilot fuel, as opposed to other more conventional ones (e.g., diesel), is that it contains no carbon, and hence produces no CO2 and particulate matter (PM). The computational investigation was conducted with an advanced commercial stochastic engine model that has been previously validated. The investigation was primarily focused on assessing the effects of using hydrogen peroxide in aqueous solution as pilot fuel on engine efficiency, combustion phasing and NOx emissions. These three aspects were investigated over a range of engine speeds (750-1,750 rpm) in view of: (i) the variation of the mass of the directly injected (DI) aqueous solution (0.1-10 mg); (ii) the variation of the H2O2 share (15-50%) in the directly injected (DI) aqueous solution; (iii) the variation of the start of injection (from -20 to -4 CAD aTDC) and injection duration (1-8 CAD). There is a strong effect of the peroxide in advancing combustion timing (CAD50 is advanced by up to 15 CAD) and in decreasing combustion duration (CAD90-CAD10 decreases up to 10 CAD), and peroxide readily enables medium engine loads which are the ones investigated in this work. Indicated thermal efficiencies above 50% were readily achieved at all engine speeds and, with a 30 vol% peroxide share in the solution, the pressure rise rate was always below 30 bar/ms. However, the NOx emissions in all cases exceeded the IMO’s Tier III standard. Possible ways to tackle this would be either the use of exhaust gas recirculation, or optimisation of the injection strategy, or the use of aftertreatment. For most cases, on a volume basis, the required aqueous H2O2 amount is 3% of the main fuel while on an energy basis this translates to 1.1% of the main fuel blend. The results reported in this initial study are promising because it is possible to ignite a premixed charge on the basis of a small pilot volume of commercially available hydrogen peroxide solutions

Algorithmic asymptotic analysis of igniting gases

Using algorithmic tools derived from the methodology of Computational Singular Perturbation various autoignition problems are investigated. In the first problem, the autoignition kinetics of DME/air and EtOH/air stoichiometric mixtures are compared both at low and high initial temperatures. DME and EtOH are two isomer fuels, with the potential for production from renewable sources, that have virtually identical thermochemistry. These isomer fuels have drastically different ignition delays because of their different kinetics. In particular, at sufficiently high initial temperatures, the first and largest part of the ignition delay in the DME and EtOH cases is dominated by two different sets of components of carbon chemistry, while the last and shortest part is dominated by the same hydrogen chemistry. In the DME case the time scale that characterizes autoignition in the first part is promoted by single-carbon chemistry, while in the EtOH case the two-carbon chain retains its bond in that part, therefore, the hydrogen chemistry plays an important role in promoting the autoignition. These features generate a substantially shorter ignition delay for EtOH. At sufficiently low initial temperatures, in the DME case, it is shown that the low-temperature oxidation is dominated by reactions involving heavy carbonaceous species. Moreover, it is demonstrated that the outcome of the competition between two specific reactions is the cause for the exhibited negative temperature coefficient (NTC). In the EtOH case, the analysis points to the importance of carbonaceous species and in particular acetaldehyde. In the second problem, the effect of selected additives on the ignition delay of EtOH/air and DME/air mixture is investigated. CSP tools are utilized in an effort to determine algorithmically which species to select as additives and it is established that CSP can identify species whose addition to the mixture can affect ignition delay. In the third autoignition problem, the reactions via which H2O-dilution influences ignition delay and chemical paths that generate NO, are examined, in the context of isochoric homogenous CH4/air autoignition. Both, the thermal and chemical effects of dilution are examined and it is concluded that the thermal effects result in a lower temperature at the end of the explosive stage, while among the most notable chemical effects are (i) the increased OH production throughout the explosive stage and (ii) the lower levels of O after this stage. Finally, the homogeneous autoignition dynamics of a stoichiometric H2/air mixture around the third explosion limit is investigated using CSP tools, on the basis of two detailed chemical kinetics mechanisms; one that includes surface radical loses and one that does not. It is shown that very close to the third limit both the gas phase and the surface reactions contribute to its development.Χρησιμοποιώντας αλγοριθμικά εργαλεία προερχόμενα από τη μέθοδο Computational Singular Perturbation (CSP), μελετώνται διάφορα προβλήματα αυτανάφλεξης στη βάση των βιοκαυσίμων DME και EtOH. Η παρούσα μελέτη επικεντρώνεται σε τέσσερα προβλήματα ισόχωρης αδιαβατικής αυτανάφλεξης. Στο πρώτο πρόβλημα, συγκρίνεται η κινητική αυτανάφλεξης στοιχειομετρικών μιγμάτων DME/ αέρα και EtOH/αέρα, τόσο σε υψηλές όσο και σε χαμηλές αρχικές θερμοκρασίες. Ο DME και η EtOH είναι δύο ισομερή καύσιμα, με δυνατότητα παραγωγής από ανανεώσιμες πηγές, τα οποία έχουν πρακτικά ίδια θερμοχημεία. Όμως, τα εν λόγω δύο ισομερή καύσιμα έχουν τελείως διαφορετικούς χρόνους ανάφλεξης, εξαιτίας της διαφορετικής κινητικής. Πιο συγκεκριμένα, σε υψηλές αρχικές θερμοκρασίες, το πρώτο και μεγαλύτερο τμήμα του χρόνου ανάφλεξης στις περιπτώσεις του DME και της EtOH κυριαρχείται από δύο διαφορετικούς μηχανισμούς χημείας του άνθρακα, ενώ το τελικό τμήμα της διεργαίας ανάφλεξης κυριαρχείται από την ίδια χημεία υδρογόνου. Στην περίπτωση του DME, η χρονοκλίμακα που χαρακτηρίζει το αρχικό (χρονικά μεγαλύτερο) τμήμα της διεργασίας ανάφλεξης ενισχύεται από είδη που περιέχουν ένα άτομο άνθρακα ενώ, στην περίπτωση της EtOH, η αλυσίδα μεταξύ των δύο ατόμων άνθρακα διατηρείται στο συγκεκριμένο τμήμα της διεργασίας, και κατά συνέπεια η χημεία του υδρογόνου διαδραματίζει σημαντικό ρόλο, ευνοώντας την αυτανάφλεξη. Ως απόρροια, προκύπτει στην περίπτωση της EtOH ένας σημαντικά μικρότερος χρόνος ανάφλεξης. Σε αρκετά μικρές αρχικές θερμοκρασίες, στην περίπτωση του DME, η διεργασία κυριαρχείται από αντιδράσεις που περιέχουν είδη με μεγάλο αριθμό ατόμων άνθρακα. Επιπλέον, αποδεικνύεται ότι το αποτέλεσμα της ανταγωνιστικής δράσης δύο αντιδράσεων είναι μια από τις βασικές αιτίες για τη συμπεριφορά αρνητικού συντελεστή θερμοκρασίας (Negative Temperature Coefficient - NTC). Στην περίπτωση της EtOH, η ανάλυση δεικνύει τον κρίσιμο ρόλο οργανικών ενώσεων όπως η ακεταλδεΰδη. Στο δεύτερο πρόβλημα, διερευνάται η επίδραση επιλεγμένων πρόσθετων στον χρόνο ανάφλεξης μιγμάτων DME/αέρα και EtOH/αέρα. Χρησιμοποιούνται εργαλεία της μεθόδου CSP με σκοπό να προσδιοριστούν αλγοριθμικά υποψήφια πρόσθετα. Από την ανάλυση προκύπτει ότι συγκεκριμένα είδη έχουν σημαντική επίδραση στον χρόνο ανάφλεξης, και συνεπώς συνιστούν υποψήφια πρόσθετα. Στο τρίτο πρόβλημα, μελετώνται οι επιδράσεις της αραίωσης με υδρατμό (H2O) στη χημεία του μίγματος μεθανίου (CH4) και αέρα. Η συγκεκριμένη επιλογή οφείλεται στο ότι το μεθάνιο αποτελεί τον απλούστερο υδρογονάθρακα, και η χημεία του περιλαμβάνεται στη χημεία της καύσης οποιουδήποτε υδρογονάνθρακα, και κατά συνέπεια καί στη χημεία των DME και EtOH. Εδώ, μελετώνται οι αντιδράσεις μέσω των οποίων η αραίωση του μίγματος (με H2O) επηρεάζει τον χρόνο ανάφλεξης, καθώς και οι χημικές οδοί της παραγωγής ρύπων ΝΟx. Μελετώνται τόσο η θερμική όσο και η χημική επίδραση της αραίωσης στη διεργασία της έναυσης. Από τα αποτελέσματα προκύπτει ότι η θερμική επίδραση έχει ως αποτέλεσμα τη μείωση της τελικής θερμοκρασίας του μίγματος, ενώ η χημική επίδραση έχει ως αποτέλεσμα: (i) την αυξημένη παραγωγή ρίζας υδροξυλίου (ΟΗ) στο εκρηκτικό τμήμα της διεργασίας (μείωση του χρόνου ανάφλεξης), και (ii) χαμηλότερα επίπεδα συγκέντρωσης ρίζας ατομικού οξυγόνου (Ο) μετά το πέρας του εν λόγω τμήματος (μείωση της παραγωγής NOx). Στο τέταρτο πρόβλημα, μελετάται η δυναμική στο τρίτο εκρηκτικό όριο του μίγματος H2/αέρα. Κίνητρο για τη συγκεκριμένη μελέτη αποτελεί το γεγονός ότι η χημεία του υδρογόνου είναι κυρίαρχη στο τελικό στάδιο της διεργασίας ανάφλεξης μιγμάτων DME/αέρα και EtOH/αέρα. Στην παρούσα εργασία, χρησιμοποιείται ένας πλήρης χημικός μηχανισμός ο οποίος περιλαμβάνει αντιδράσεις καταστροφής ειδών στα τοιχώματα. Επίσης, χρησιμοποιείται ένας λιγότερο εκτενής μηχανισμός, ο οποίος δεν περιλαμβάνει τις αντιδράσεις καταστροφής ειδών στα τοιχώματα. Από τη σχετική ανάλυση προκύπτουν οι αντιδράσεις οι οποίες είναι σημαντικές για την πορεία της διεργασίας της έναυσης. Συμπερασματικά, στο πλαίσιο της παρούσας έρευνας, χαρακτηρίστηκε η δυναμική της ανάφλεξης σημαντικών πρότυπων μιγμάτων σε αντιπροσωπευτικές συνθήκες, εξήχθησαν συμπεράσματα σχετικά με την επίδραση πρόσθετων στη διεργασία της έναυσης, καθώς και συμπεράσματα για την επίδραση της χημικής κινητικής στην παραγωγή ρύπων NOx. Τα αποτελέσματα της παρούσας έρευνας επιβεβαιώνουν την καταλληλότητα της αλγοριθμικής μεθόδου CSP για τη μελέτη προβλημάτων έναυσης για τα καύσιμα της παρούσας μελέτης, σε μεγάλο εύρος αρχικών συνθηκών και στοιχειομετρίας του προαναμεμιγμένου μίγματος

Comparative investigation of homogeneous autoignition of DME/air and EtOH/air mixtures at low initial temperatures

The dynamics of homogeneous isochoric explosions of dimethylether (DME)/air and ethanol (EtOH)/air mixtures were studied and compared at relatively low initial temperatures (≤ 700 K) using algorithmic tools derived from the methodology of computational singular perturbation. In the DME case, it is shown that the low-temperature oxidation is dominated by reactions involving heavy carbonaceous species, in contrast to the high-temperature case where oxidation is dominated by reactions involving light carbonaceous species. Moreover, it is demonstrated that the outcome of the competition between two specific reactions is the cause of the exhibited negative temperature coefficient (NTC). In the EtOH case, the analysis points to the importance of carbonaceous species and in particular acetaldehyde, which is different from what happens at elevated initial temperatures (above 1000 K), where hydrogen chemistry dominates the entirety of the oxidation process. The autoignition dynamics of both mixtures are shown to be pretty much independent of initial pressure, with the exception of the NTC behaviour of DME

Development of a reduced four-component (toluene/n-heptane/iso-octane/ethanol) gasoline surrogate model

The prospect of blending gasoline fuel with ethanol is being investigated as a potential way to improve the knock residence of the base gasoline. However, one of the drawbacks is a lack of proper understanding of the reason for the non-linear response of blending ethanol and gasoline. This non-linearity could be better understood by an improved knowledge of the interactions of these fuel components at a molecular level. This study proposed a highly reduced four-component (toluene/n-heptane/iso-octane/ethanol) gasoline surrogate model containing 59 species and 270 reactions. The model was reduced using the direct relation graph with expert knowledge (DRG-X) (Lu and Law, 20015; Lu et al., 2011) and isomer lumping method. The computational singular perturbation (CSP) analysis were performed to reduce the potential stiffness issues by accordingly adjusting the Arrhenius coefficients of the proper reactions. The model has been comprehensively validated against wide range of ignition delay times (IDT) and flame speed (FS) measurement data as well as compared against two representative literature models from Liu et al. (2013) and Wang et al. (2015). Overall, good agreements were observed between model predictions and experimental data across the entire research octane number (RON), equivalence ratio, pressure and temperature range. In addition, the model has also been coupled with the computational fluid dynamic (CFD) models to simulate the experimental data of constant volume reacting spray of a low-octane gasoline (Haltermann straight-run naphtha), and in-cylinder pressures and temperatures of a high-octane gasoline (Haltermann Gasoline) combustion in a heavy duty compression ignition engine. The coupled model can qualitatively predict the experimentally obtained data with an improved performance for PRF, TPRF, and TPRF-ethanol surrogates

Q fever and early pregnancy failure: a Scottish case control study

Q fever is a bacterial disease that passes between animals and humans and causes disease in both. The disease has been associated with pregnancy complications including miscarriage. This study was undertaken to identify if Q fever exposure was correlated with miscarriage in 369 women attending a pregnancy support unit in Edinburgh. The women in the study were in two groups, the miscarriage group with 251 women who had experienced a miscarriage and a control group of 118 women who had not experienced miscarriage. Three women were found to be positive for Q fever antibodies, suggesting that they had previously been exposed to the infection and all of them were from the group who had experienced miscarriage. The study indicates that Q fever is relatively rare in women attending an urban Scottish hospital suggesting that the infection is not a major cause of miscarriage in this population. However, as Q fever antibodies could only be found in women within the miscarriage group, it suggests that the infection cannot be ruled out as a potential cause of miscarriage in individual cases