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Abstract. In recent decades, large wildfires have inflicted considerable damage on valuable Natura 2000 regions in Belgium. Despite these events and the general perception that global change will exacerbate wildfire prevalence, this has not been studied yet in the Belgian context. Therefore, the national government initiated the national action plan on wildfires in order to evaluate the wildfire risk, on the one hand, and the materials, procedures, and training of fire services, on the other hand. This study focuses on the spatial distribution of the ignition probability, a component of the wildfire risk framework. In a first stage, we compile a historical wildfire database using (i) newspaper articles between 1994 and 2016 and (ii) a list of wildfire interventions between 2010 and 2013, provided by the government. In a second stage, we use a straightforward method relying on Bayes' rule and a limited number of covariates to calculate the ignition probability. It appears that most wildfire-prone areas in Belgium are located in heathland where military exercises are held. The provinces that have the largest relative areas with a high or very high wildfire risk are Limburg and Antwerp. Our study also revealed that most wildfire ignitions in Belgium are caused by humans (both arson and negligence) and that natural causes such as lightning are rather scarce. Wildfire prevention can be improved by (i) excluding military activity in fire-prone areas during the fire season, (ii) improving collaboration with foreign emergency services, (iii) concentrating the dedicated resources in the areas that display the highest ignition probabilities, (iv) improving fire detection methods, and (v) raising more awareness among the public.

[EN] The ignition characteristics of six different fuels have been correlated as a function of the temperature, pressure, equivalence ratio and oxygen molar fraction in this investigation. More specifically, the ignition delay referred to cool flames, the high-temperature ignition delay and the critical concentrations and ignition times of HO2 and CH2O have been parameterized for n-dodecane, PRFO, PRF25, PRF50, PRF75 and PRF100. To do so, a wide database of ignition data of the aforementioned fuels has been generated by means of chemical simulations in CHEMKIN, solving a detailed mechanism for PRF mixtures and a reduced mechanism for n-dodecane. In fact, in cylinder engine-like conditions reached in a Rapid Compression Expansion Machine (RCEM) have been replicated. The mathematical correlations have shown a relative deviation around 20% with the database in the low-temperature, low-pressure zone, which is the typical accuracy of usual correlations for the ignition delay. Finally, the ignition delay under transient conditions measured in the RCEM has been predicted by means of different integral methods coupled to both the proposed correlations and the generated database. It has been found that deviations between the predictions obtained with the correlations or with the database are lower than 1%. This means that the correlations are accurate enough to predict the ignition time in spite of showing high deviation with the database, since the low-temperature, low-pressure zone has a minor contribution to the ignition delay. ; The authors would like to thank different members of the CMT-Motores Termicos team of the Universitat Politecnica de Valencia for their contribution to this work. The authors would also like to thank the Spanish Ministry of Education for financing the PhD. Studies of Dario Lopez-Pintor (grant FPU13/02329). This research has been partially funded by FEDER and the Spanish Government through project TRA2015-67136-R. ; Desantes, J.; Bermúdez, V.; López, JJ.; López-Pintor, D. (2017). Correlations ...

This paper reviews the fundamental requirements of liquid hydrocarbon fuels for spark-ignition engines, namely that the fuel should vaporize satisfactorily and burn in a controlled manner. The phenomenon of knock and the development of the octane scale are discussed. The variation in pressure time histories between engines is discussed, along with how this leads to different fuel requirements. The difference in the octane rating tests, and how engine down-sizing exacerbates these differences in the pressure time histories is discussed. The applicability of RON and MON to modern engines is reviewed, along with the phenomenon of low speed preignition and mega-knock. The effect of hydrocarbon fuel distillation characteristics on drivability and emissions is reviewed, and placed in a historical context and the current legislative requirements. Brief mention is made of other fuel requirements such as density, gum content and aromatic content.

"Propensity Score Matching bezeichnet eine Klasse nicht- bzw. semiparametrischer Verfahren zur statistischen Abschätzung kausaler Effekte, die ursprünglich im Bereich der Biometrie entwickelt wurden und in den letzten Jahren zunehmend auch in den Sozialwissenschaften Anwendung finden. Neben ihren vergleichsweise sparsamen Modellannahmen und einer methodologisch entsprechend transparenten Vorgehensweise zeichnen sich Matchingverfahren auch dadurch aus, dass die resultierenden Parameterschätzungen leicht interpretierbar sowie direkt auf zentrale Parameter des so genannten Rubin Causal Models bzw. des kontrafaktischen Kausalitätsverständnisses zurückzuführen sind. Die empirische Abschätzung kausaler Effekte mittels Propensity Score Matching erfolgt in drei Analyseschritten: der Entwicklung eines Zuweisungsmodells zur statistischen Kontrolle der antezedierenden Bedingungen des beobachteten Treatmentstatus, die Verwendung eines Matchingalgorithmus zur Balancierung der Hintergrundkovariaten zwischen den Vergleichsgruppen der Untersuchung sowie die eigentliche Schätzung der interessierenden kausalen Effekte durch den Vergleich der Ereignisverteilungen in der Experimental- und Kontrollstichprobe. Für jeden Analyseschritt stehen unterschiedliche statistische Verfahren und Algorithmen zur Verfügung, um die allgemeine Schätzstrategie einer Matchinganalyse im konkreten Einzelfall effektiv umzusetzen. Der vorliegende Überblicksbeitrag kann diese vielfältigen Analysevarianten nicht im Einzelnen darstellen, sondern beschränkt sich darauf, die grundsätzliche Vorgehensweise einer Matchinganalyse anhand der Abschätzung der empirischen Karrierefolgen von Arbeitslosigkeit mittels paarweisem (nearest-neighbor) Propensity score matching beispielhaft zu illustrieren." (Autorenreferat)

Despite the expected upsurge of hybrid and electric cars in the coming decades, internal combustion will remain the main power supply for (long-distance) transport. Buses, trucks, ships and airplanes will still rely on combustion engines. Nevertheless, emission legislation is becoming more stringent and the oil price continues to rise. Consequently, there still exists a serious interest in new developments that may improve combustion efficiency and fuel flexibility, and reduce emissions; both fundamental and applied research in this field is thriving. Recently, a lot of research has focused on advanced compression ignition combustion strategies, like premixed charge compression ignition (PCCI). These strategies aim at combining the advantages of gasoline and diesel combustion within the same engine. As a result, engines will be more efficient and have extremely low exhaust emissions enabling to meet emission legislations without after treatment. To gain understanding of the physical processes occurring in the 'black box' of an engine, engines can be made optically accessible, thus allowing a look inside the engine during the combustion process. In these so-called "optical engines" various optical diagnostics can be used to visualize in-cylinder processes, and to supply data on e.g. flow fields and temperature. As a next step, empirically measured data can be used to optimize numerical calculations, which, in turn, hopefully will improve to such extent that they can be used to replace testing of real manufactured prototypes. One of the main topics in the area of internal combustion research is to identify and explain phenomena around the onset of ignition, including the injection of fuel itself. In the short time period between the start of fuel injection and ignition (typically a few milliseconds), all events of interest take place. Large cycle-to-cycle variations in real engines require fast diagnostic techniques to resolve these short relevant time spans in single combustion cycles. Therefore, fast optical diagnostic techniques are of major importance. In this thesis, novel optical diagnostics and improved combustion strategies for compression ignition internal combustion engines are presented. Each chapter focuses on a specific optical measurement technique, which can be used to characterize the different aspects of compression ignition combustion in detail. Velocities. To investigate mixing of fuel and air, the in-cylinder gas-flow velocities were measured using particle image velocimetry (PIV) in chapter 3 and 4. From these velocities, the turbulence intensity was derived, which gives an indication of the degree of mixing of the charge. These measurements have been performed in a 2D plane parallel to and a few mm below the cylinder head. The 2D measurements indicate that the turbulence is homogeneous and that the swirl center shifts towards the center of the combustion chamber during the compression stroke due to squish motion. Increasing the recording rate by using a high-speed laser in combination with a high-speed camera enables to investigate the flow evolution in a single cycle, and to record more data before window fouling occurs. To achieve enough contrast between the particles and the environment, the relatively low laser power of the high-speed laser system needs to be compensated by using hollow microspheres as tracers which have a larger size compared to the more generally used oil droplets. The spatial resolution of the presented in-cylinder velocity fields in chapter 4 is shown to be lower for the high-speed measurements than for the low-speed measurements presented in chapter 3. High-speed PIV is therefore not a substitute for high-resolution low-speed PIV, but an additional measurement method to track fast changes. The high-speed PIV results during and after injection results show a sudden change of air motion at the start of injection as a result of air entrainment at the core of the spray. Furthermore, as expected, spray injection causes a considerable increase in the cycle-to-cycle fluctuations of the flow pattern, the more so for longer injection durations. In the case of multiple injections in a single engine cycle the air-entrainment during the first injection is consistent, and fluctuations between consecutive cycles are small. When fuel vapor from previous injections re-enters the investigated plane via impingement on the cylinder wall or the top of the piston, the flow structure changes drastically and loses coherence, compensating the inward motion due to air entrainment by subsequent injections. The spray induced flow can evolve into various structures, which might influence the actual mixing of fuel and air, causing differences in local fuel/air ratios between different injection strategies and therefore change the ignition delay in PCCI combustion. The high-speed PIV data has been subjected to proper orthogonal decomposition analysis (POD). The results confirm the observed changes in flow structures during and after injection. POD might be a good tool for comparison of experimental PIV data with future CFD results. However, in highly unsteady flow, as observed during injection, a good representation of the instantaneous velocity field using only a few POD modes is not possible and therefore comparison between PIV and CFD data needs careful selection of the amount of modes. Phase-invariant POD was found not to be an appropriate method to represent highly fluctuating flows during and after fuel injections. Because of the lack of resemblance between flow fields of consecutive crank angles, the use of separate basis functions for each CAD is more appropriate; alternatively one could use separate sets of base functions before and during injection. Temperature gradients Temperature gradients occurring before, during and after fuel injection were measured using a 2-color laser induced fluorescence (LIF) technique with toluene as a fuel tracer (chapter 5). The toluene fluorescence signal was recorded simultaneously in two disjunct wavelength bands by a two-camera setup. After calibration, the LIF signal ratio is a proxy for the local temperature. Good agreement has been found between our new calibration curves and previously presented results by other researchers. A detailed measurement procedure is presented to minimize measurement inaccuracies and to improve precision. N-heptane was used as the base fuel and 10% of toluene was added as tracer. The toluene LIF method is capable of measuring temperatures up to 700 K; above that the signal becomes too weak. The precision of the spray temperature measurements is 4% of the temperature and the spatial resolution 1.3 mm. The fluorescence signal was also used as a fuel tracer to investigate the fuel distribution in the optical engine. . The technique was found to be very sensitive to disturbances by fluorescence of the base fuel in the wavelength range of interest. However after calibration, comparison of the temperature gradients inside the spray with Large-Eddy Simulation (LES) shows similar results. Combustion visualization Controlling ignition delay is the key to successfully enable partially premixed combustion in diesel engines. Chapter 6 presents experimental results of partially premixed combustion in an optically accessible engine, using primary reference fuels in combination with artificial exhaust gas recirculation. By varying the fuel composition and oxygen concentration, the ignition delay was adjusted. OH chemiluminescence is a useful tool to measure flame lift-off of burning fuel sprays. A similar approach has been presented using a high-speed spectral measurement setup to measure the position of the flame after injection when running in partially premixed mode. In general, increased ignition delay results in a longer lift-off length of quasi-steady flames. When combustion starts after fuel injection is completed, the longer ignition delay results in flame fronts closer to the injector due to "reflection" of the fuel against the piston wall. The mixing of fuel and air during the ignition delay period defines the local equivalence ratio. To investigate the influence of the ignition delay on the gas volume involved in combustion and the corresponding local equivalence ratio, the position of the flame is determined using high-speed visualization of OH-chemiluminescence. This enables a cycle resolved analysis of the location of OH formation, i.e. the flame position. A clear correlation was observed between ignition delay and flame location, proving that a longer ignition delay increases mixing. Emission measurements using fast-response analyzers of CO, HC and NOX confirmed the decrease in local equivalence ratio as a function of increasing ignition delay. Furthermore, multiple injection strategies were investigated, applying pilot as well as post injections, in combination with a main injection at constant load. From these results it is concluded that both pilot and post injections result in an increase of unburned hydrocarbon and CO emission and a slight decrease of nitric oxide emissions.