Development of a Single-Ended Laser-Absorption Spectroscopy Sensor for High-Temperature Gases

Tunable diode-laser-absorption spectroscopy (TDLAS) sensors have been one widely-used laser-diagnostic technique that offers great potential for non-intrusive, time-resolved and multi-parameter sensing in combustion systems. These sensors have been used for performance testing, model validation and feedback control of combustors [1–9]. During operation, monochromatic laser light with a specific wavelength is transmitted through the test gas and collected on a photodetector. Gas conditions such as temperature and composition are then inferred by comparing the measured amount of light that is absorbed with that predicted by spectroscopic models. In this thesis, the design and demonstration of a compact single-ended laser-absorption spectroscopy (SE-LAS) sensor for measuring temperature and H2O in high-temperature combustion gases is presented. The primary novelty of this work lies in the design, demonstration, and evaluation of a sensor architecture which uses a single lens to provide single-ended, alignment-free measurements of gas properties in a combustor without windows. The sensor is demonstrated to be capable of sustained operation at temperatures up to at least 625 K and is capable of withstanding direct exposure to high-temperature (≈1000 K) flame gases for long durations (at least 30 min) without compromising measurement quality. The sensor employs a fiber bundle and a 6-mm diameter AR-coated lens mounted in a 1/8” NPT-threaded stainless-steel body to collect laser light that is backscattered off native surfaces (e.g., a combustor wall). Distributed-feedback (DFB) tunable diode lasers (TDLs) with a wavelength near 1392 nm and 1343 nm were used to interrogate well characterized H2O absorption transitions using wavelength-modulation spectroscopy (WMS) techniques. The sensor is demonstrated with measurements of gas temperature and H2O mole fraction in a propane-air burner with a measurement bandwidth up to 25 kHz. In addition, this work presents an improved wavelength-modulation-spectroscopy spectral-fitting technique which reduces computational time by a factor of 100 compared to previously developed techniques

Optical Gas Sensing: Media, Mechanisms and Applications

Optical gas sensing is one of the fastest developing research areas in laser spectroscopy. Continuous development of new coherent light sources operating especially in the Mid-IR spectral band (QCL—Quantum Cascade Lasers, ICL—Interband Cascade Lasers, OPO—Optical Parametric Oscillator, DFG—Difference Frequency Generation, optical frequency combs, etc.) stimulates new, sophisticated methods and technological solutions in this area. The development of clever techniques in gas detection based on new mechanisms of sensing (photoacoustic, photothermal, dispersion, etc.) supported by advanced applied electronics and huge progress in signal processing allows us to introduce more sensitive, broader-band and miniaturized optical sensors. Additionally, the substantial development of fast and sensitive photodetectors in MIR and FIR is of great support to progress in gas sensing. Recent material and technological progress in the development of hollow-core optical fibers allowing low-loss transmission of light in both Near- and Mid-IR has opened a new route for obtaining the low-volume, long optical paths that are so strongly required in laser-based gas sensors, leading to the development of a novel branch of laser-based gas detectors. This Special Issue summarizes the most recent progress in the development of optical sensors utilizing novel materials and laser-based gas sensing techniques

Tunable diode-laser absorption-based sensors for the detection of water vapor concentration, film thickness and temperature

Temperature and species concentration are fundamental parameters in combustion-related systems. For optimizing the operation and minimizing the pollutant emissions of combustion devices and to provide validation data for simulations, quantitative measurement techniques of these parameters are required. Laser-based diagnostic techniques are an advantageous tool for in-situ non-intrusive measurement in combustion related systems, e.g. flame reactors, combustors, and shock tubes. Fiber-based multiplexed tunable diode laser absorption spectroscopy (TDLAS) is attractive and employed in this thesis because of compact, rugged packaging, low cost, reliability and relative ease of use. In the present work, water (H2O) is chosen as the target species for the technique, since it has a rich absorption spectrum in the vapor-phase and a broad-band absorption spectrum for the liquid-phase in the near infrared region (NIR). TDLAS two-line thermometry is used to determine the temperature in gas-phase systems with homogenous temperature distribution. However, in many practical environments, temperature varies along the beam path. For this case the temperature-binning technique is used for retrieving non-uniform temperature distributions from line-of-sight (LOS) absorption data with multiplexed five-color absorbance areas. In this thesis, TDLAS was applied to determine the spatially-resolved temperature information inside a low-pressure nanoparticle flame synthesis reactor. The temperature distribution was obtained by assuming the temperature to be constant in variable lengths along the LOS. The length fractions for the temperature values along the LOS are determined using postulated temperature bins. Quantitative knowledge of liquid film thickness is important in many industrial applications. One example is Diesel engine exhaust gas aftertreatment, where NOx reduction via selective catalytic reduction (SCR) is accomplished in the exhaust using sprays of water/urea solutions. In this thesis a novel TDLAS sensor was developed to simultaneously measure the water film thickness, film temperature and vapor-phase temperature above the film. For this sensor four individual NIR wavelengths were selected for optimized sensitivity of the technique. The sensor was first validated using a calibration tool providing known film thicknesses and temperature, and then applied to open liquid water films deposited on a transparent quartz plate. In a collaborative project the technique was also compared with imaging measurements based on laser-induced fluorescence and Raman scattering, respectively. Furthermore, the TDLAS sensor was applied to determine time series data of liquid water film thickness resulting from impinging water jets and subsequent film evaporation on the wall of a gas flow channel.Temperatur und Spezies-Konzentration sind elementare Kenngrößen in Verbrennungssystemen. Um den Betrieb von Verbrennungs- und Reaktionsprozessen zu optimieren, die Schadstoffemission zu minimieren und außerdem Validierungsdaten für Simulationen zu generieren, sind quantitative Messungen dieser Kenngrößen notwendig. Laserbasierte Diagnostik-Methoden sind nützliche Verfahren für die berührungslose in-situ Messung innerhalb von Verbrennungssystemen wie z.B. Brenner, Flammenreaktoren und Stoßwellenrohren. Absorptionsspektroskopie mit mehreren faserbasierten und abstimmbaren Laserdioden (tunable diode laser absorption spectroscopy, TDLAS) wurde in dieser Arbeit wegen des kompakten, robusten Aufbaus, der kostengünstigen Komponenten und der Zuverlässigkeit aufgrund der optischen Fasern verwendet. In der vorliegenden Arbeit wurde Wasser (H2O) als Untersuchungssubstanz für diese Methode ausgewählt, da es in zahlreichen technisch relevanten Prozessen, im nahen Infrarot-Bereich (NIR) in der Gasphase ein schmalbandiges und in der flüssigen Phase ein breitbandiges Absorptionsspektrum besitzt. Die TDLAS-zwei-Linien-Thermometrie wird zur Temperaturbestimmung in Verbrennungssystemen mit homogener Temperaturverteilung benutzt. In anwendungsnahen Systemen jedoch ändert sich die Temperatur entlang des Strahlweges. In diesem Fall ist ein Temperatur-binning-Verfahren nötig, um aus einer Absorptionsmessung entlang einer Sichtlinie auch auf ungleichförmige Temperaturverteilungen rückschließen zu können. In der vorliegenden Arbeit wurde TDLAS mit einer Kombination von fünf Wellenlängen eingesetzt, um räumlich aufgelöst Temperaturen innerhalb eines Niederdruck-Nanopartikel-Synthesereaktors zu bestimmen. Dabei wurden Temperaturen bestimmt, indem diese in variablen Längen entlang der Sichtlinie als konstant angesehen wurde. Die Längenanteile dieser Wegstrecken mit verschiedenen Temperaturen wurden für vordefinierte Temperaturbereiche bestimmt. Die quantitative Kenntnis der Filmdicke von flüssigen Filmen ist wichtig für zahlreiche industrielle Anwendungen, z.B. die NOx-Reduktion mittels einer Wasser/Harnstoff-Lösung in selektiv-katalytischer Reduktion (selective catalytic reduction, SCR) im Abgas von Dieselmotoren. In der vorliegenden Arbeit wurde ein neuartiger TDLAS-Sensor entwickelt, um gleichzeitig Filmdicke, Filmtemperatur und Wasserdampftemperatur oberhalb des Films zu messen. Die vier eingesetzten NIR-Wellenlängen wurden hierbei auf optimale Empfindlichkeit hin ausgewählt. Der Sensor wurde zuerst in einer Kalibrationszelle mit bekannter Filmdicke und Filmtemperatur validiert und dann an einem freien Film auf einer transparenten Quarzglas-Platte getestet. Zusätzlich wurde der TDLAS-Sensor verwendet, um zeitaufgelöst die Filmdicke während der Einspritzung- und Verdampfungsprozesse innerhalb eines Strömungskanals zu bestimmen

Laser diagnostics of C2H4 and CH4 from n-butane pyrolysis

Combustion of fossil fuels remains the dominant source of energy which enables us sustain thrive in this planet. Meanwhile, the negative effects of burning fossil fuels, however, are devastating our climate and environment. Eliminating those negative effects while attaining energy supply from fossil fuels becomes urgent and prominent. It is, nevertheless, impossible without a thorough understanding of the combustion process. Experimental approach remains one of the dominating approaches to study combustion despite the growing interest in numerical approach. The development of workstations and massive supercomputers is providing the computation ability that one has never imaged. Nevertheless, it still appears difficult to catch up with the ever-increasing computational power demand, especially in the area of combustion. Not only the intermediate species need to be studied experimentally, but also the reactions need to be verified using experimental approach. Due to the nature of laser and Laser diagnostics, which conducts the diagnosis by measuring the responses of laser illumination, it is incredibly suitable for combustion research. Moreover, laser based diagnostics techniques provide the capabilities of remote, non-intrusive, in situ measurements with spatial and temporal accuracy that has never been achieved. In current study, two laser based diagnostics techniques are explored: Coherent microwave scattering from resonance enhanced multiphoton ionization (Radar REMPI) and Tunable diode laser absorption spectroscopy (TDLAS). Combustion of heavy hydrocarbons is a complex process, which can be roughly divided into two sub-processes: pyrolysis and burning of lighter hydrocarbons. Ethylene and methane are two common products of heavy hydrocarbon pyrolysis, e.g. n-butane. Their detection under harsh environment, i.e. higher temperature and pressure, are explored using Radar REMPI and TDLAS. Radar REMPI is used to detect ethylene under high temperature and pressure. The results obtained justified Radar REMPI as a promising detection technique for ethylene under combustion. On the other hand, TDLAS is used to detect methane in current study. A numerical absorption spectroscopic model is built which predicts methane’s concentration under different pressure and temperature. Methane from n-butane pyrolysis is detected and quantified using TDLAS

Pathlength calibration of integrating sphere based gas cells

Integrating sphere based multipass cells, unlike typical multipass cells, have an optically rough reflective surface, which produces multiple diffuse reflections of varying lengths. This has significant advantages, including negating scattering effects in turbid samples, removing periodicity of waves (often the cause of etalon fringes), and simple cell alignment. However, the achievable pathlength is heavily dependent on the sphere wall reflectivity. This presents a challenge for ongoing in-situ measurements as potential sphere wall contamination will cause a reduction in mean reflectivity and thus a deviation from the calibrated pathlength. With this in mind, two techniques for pathlength calibration of an integrating sphere were investigated. In both techniques contamination was simulated by creating low reflectivity tabs e.g. ≈5x7mm, that could be introduced into the sphere (and removed) in a repeatable manner. For the first technique, a four beam configuration, adapted from a turbidity method used in the water industry, was created using a 5cm diameter sphere with an effective pathlength of 1m. Detection of methane gas was carried out at 1650nm. A mathematical model was derived that corrected for pathlength change due to sphere wall contamination in situ, thus enabling gas measurements to continue to be made. For example, for a concentration of 1500ppm of methane where 1.2% of the sphere wall was contaminated with a low reflectivity material, the absorption measurement error was reduced from 41% to 2% when the model was used. However some scenarios introduced errors into the correction, including contamination of the cell windows which introduced errors of, for example, up to 70% if the particulate contamination size was on the order of millimetres. The second technique used high frequency intensity modulation with phase detection to achieve pathlength calibration. Two types of modulation were tested i.e. sinusoidal modulation and pulsed modulation. The technique was implemented using an integrated circuit board which allowed for generation of modulation signals up to 150MHz with synchronous signal processing. Pathlength calibration was achieved by comparison of iii the phase shift for a known length with the measured phase shift for the integrating sphere with unknown pathlength over a range of frequencies. The results for both modulation schemes showed that, over the range of frequencies detected, 3-48MHz, the resultant phase shift varied as an arctangent function for an integrating sphere. This differed from traditional single passes where frequency and phase have a linear relationship

Development of laser spectroscopy for scattering media applications

Laser spectroscopy for both large and small spatial scales has been developed and used in various applications ranging from remote monitoring of atmospheric mercury in Spain to investigation of oxygen contents in wood, human sinuses, fruit, and pharmaceutical solids. Historically, the lidar group in Lund has performed many differential absorption lidar (DIAL) measurements with a mobile lidar system that was first described in 1987. During the years the lidar group has focused on fluorescence imaging and mercury measurements in the troposphere. Five lidar projects are described in this thesis: fluorescence imaging measurement outside Avignon, France, a unique lidar project at a mercury mine in Almadén, Spain, a SO2 flux measurement at a paper mill in Nymölla, Sweden, and two fluorescence imaging projects related to remote monitoring of vegetation and building facades characterization. A new method to measure wind speed remotely in combination with DIAL measurements is presented in this thesis. The wind sensor technique is called videography and is based on that images of plumes are grabbed continuously and the speed is estimated by the use of image processing. A technique that makes it possible to measure a gas in solids and turbid media, non-intrusively, is presented in this thesis. The technique is called gas in scattering media absorption spectroscopy (GASMAS) and has been used since 2001. The GASMAS concept means that a traditional spectroscopy instrument, based on tunable diode lasers, is used but the gas cell or optical path is replaced by a material that strongly scatters light. Mostly, wavelength modulation spectroscopy has been utilized. Four projects using the GASMAS technique to measure gases in fruit, wood, pharmaceutical solids, and human tissue are presented. Two applications have shown a great potential so far; to be able to diagnose the health of human sinuses and gas ventilation in sinuses, and to measure gas inside pharmaceutical solids. A performance analysis of the GASMAS technique is included. This thesis also presents a technique to suppress optical noise in fiber lasers and how to construct a compact tunable diode laser spectroscopy system based on plug-in boards for a standard computer

Mid-infrared absorption sensor for CO concentration and temperature measurements for pyrolysis and oxidation behind reflected shock waves

Due to the increasing energy demand and the related environmental issues, combustion effi-ciency and reduction of pollutants emission have become a major concern for combustion ap-plications. In combustion, pollutant formation and fuel ignition are controlled by chemical kinetics, therefore, the design and optimization of combustion systems heavily relies on an accurate understanding of the underlying chemical processes. While combustion processes are governed by an interaction of chemical kinetics and transport processes, for gaining funda-mental understanding it is beneficial to separate both processes. To this end, shock tubes are frequently applied to generate a uniform gas phase environment for a wide range of tempera-tures and pressures that is suited for initiating reactions with subsequent time-resolved detec-tion. The combination of shock tube technique and laser absorption spectroscopy provides the platform for accurate chemical kinetic studies. Infrared laser absorption diagnostics have been widely applied in combustion research for example for in situ, fast, and sensitive measure-ments of temperature, pressure, and species concentrations. In the present study, laser absorption spectroscopy of carbon monoxide (CO) near 4.7 µm has been developed for the sensing of temperature and CO concentration behind the reflected shock wave. The sensor was further developed to enable fiber-based thermometry for more flexible applications in harsh environments. The oxidation of fuel-rich CH4/O2 mixtures, the thermal decomposition of anisole (C6H5OCH3), and the pyrolysis of acetylene (C2H2) and benzene (C6H6) were investigated by monitoring the CO concentration and temperature based on two-line absorption thermometry. The experimental data were applied for validation of reaction mechanisms covering different kinetics conditions such as single elementary reaction, partial oxidation, and soot formation. The oxidation of fuel-rich CH4/O2 mixtures was investigated to validate reaction mechanisms for reaction conditions that are important for polygeneration processes where partial oxidation allows to convert natural gas to higher-value chemicals. With the presences of dimethyl ether (DME) and n-heptane, the initial reaction temperature is significantly reduced because they promote the production of additional OH radicals. Anisole has recently been identified as fluorescence tracer for fuel/air mixing studies, but its decomposition kinetics were not yet fully understood. In the investigation of thermal decom-position of anisole at elevated temperatures, the literature model was found to strongly under-estimate the CO formation. As main reaction path for CO formation, the unimolecular decom-position of phenoxy radical (C6H5O) was investigated independently and new rate constants were determined. Soot formation from combustion is of high scientific interest. The temperature dependence as well as the influence of H2, O2, and CH4 on soot formation in the pyrolysis of C2H2 and C6H6 were investigated. The temperature dependence of the soot yield and the particle formation induction time were found to be in a good agreement with literature data. The presence of H2 led to a depletion of the particle formation in both systems whereas the opposite trend yield was observed in the presence of CH4 and O2.Aufgrund des steigenden Energiebedarfs durch die industrielle Entwicklung ist der Schutz der Atmosphäre die wichtigste Aufgabe unserer Zeit. Das Interesse in modernen, umwelt-freundlichen Verbrennungssystemen hat zu Verbesserung der Verbrennungseffizienz und zur Verringerung der Schadstoffemissionen geführt. Das Design und die Optimierung von Ver-brennungssystemen beruht auf einer genauen Modellierung elementarer, chemischer Prozesse. Infrarot-Laserabsorptionsdiagnostik ist eine hochentwickelte Diagnostik in der Verbrennungs-forschung für schnelle, hochsensitive in-situ Messungen von Temperatur, Druck, und Spezies-konzentrationen. Stoßwellenrohre sind einfache und robuste Instrumente, die eine homogene Gasphasenumgebung in einem großen Temperatur- und Druckbereich erzeugen. Die Kombina-tion aus Stoßwellentechnik und Laserabsorptionsspektroskopie bietet eine Plattform für akku-rate, chemische kinetische Untersuchungen. In der vorliegenden Arbeit wurde die Laserabsorptionsspektroskopie von Kohlenmonoxid (CO) nahe 4.7 μm für die Erfassung von Temperatur und CO-Konzentration hinter den reflek-tierten Stoßwellen entwickelt. Des Weiteren wurde der Sensor als faserbasiertes Thermometer für flexible und robuste Anwendungen weiterentwickelt. Die Oxidation von fetten Methan (CH4)/Sauerstoff (O2) Gemischen, der thermische Zerfall von Anisol (C6H5OCH3) sowie die Pyrolyse von Acetylen (C2H2) und Benzol (C6H6) wurden in der Gasphase im Stoßwellenrohr in Kombination mit der CO-Thermometrie untersucht. Die experimentellen Daten wurden mit Simulationen auf der Basis ausgewählter Reaktions-mechanismen verglichen. Bei der Oxidation von fetten CH4/O2 Mischungen senkten die Addi-tive Dimethylether (DME) und n-Heptan die anfängliche Reaktionstemperatur signifikant, indem sie zusätzliche OH-Radikale erzeugen. Keiner der Mechanismen ist für chemische Umwandlungsreaktionen optimiert. Die CO-Bildung wird beim thermischen Zerfall von Anisol nur schlecht vom Modell wiedergegeben. Die Reaktionsgeschwindigkeit des unimole-kularen Zerfalls des Phenoxyradikals (C6H5O) wurde experimentell bestimmt. Bei der Pyrolyse von C2H2 und C6H6 wurde die Temperaturabhängigkeit der Rußbildung und deren Induktionszeit mittels Laserlicht-Extinktion gemessen und eine gute Übereinstimmung mit der Literatur gefunden. Die Anwesenheit von Wasserstoff (H2) führte zu einer Reduktion der Partikelbildung in beiden Systemen, wohingegen ein entgegengesetztes Verhalten bei der Rußbildung in Anwesenheit von CH4 und O2 bei C2H2 und C6H6 beobachtet wurde

Fasergekoppelte In-situ-Laserhygrometer auf Basis der direkten Absorptions- und Wellenlängenmodulations-Spektroskopie für minimale Messstrecken

In der angewandten- und der Grundlagenforschung hat die Absorptionsspektroskopie mit abstimmbaren Diodenlasern (TDLAS) vielfachen Einsatz gefunden. Die hervorragenden spektralen Eigenschaften der Diodenlaser sowie die mögliche schnelle Abstimmung der Wellenlänge erlauben eine zuverlässige In-situ-Bestimmung absoluter Gasspezieskonzentrationen und -temperaturen mit hoher Sensitivität, Selektivität und Skalierbarkeit. Ein Ziel dieser Arbeit war die Entwicklung und Validierung eines Absorptionsspektrometers zur Kombination der beiden meist genutzten TDLAS-Techniken - der direkten Absorptions- und der Wellenlängenmodulations-Spektroskopie. Ein schnelles Zeitmultiplex-Verfahren ermöglicht beide Methoden in einem einzigen Aufbau simultan zu verwenden. Die hierfür aufgebauten Spektrometer nutzen die direkte online Kalibrierung des WMS-Signals durch die mit dTDLAS gleichzeitig ermittelte absolute Spezieskonzentration. Hierdurch konnte die Nachweisgrenze und die Präzision um das Fünffache von 150 nmol/mol*m*Hz1/2 auf 34 nmol/mol*m*Hz1/2 verbessert werden. Dies ermöglicht Messungen absoluter Gaskonzentrationen ohne vorherige Kalibrierung gegen einen bekannten Gasstandard. Die ausgezeichneten Eigenschaften der TDLAS für die innermotorische Gasanalyse wurden in dieser Arbeit genutzt für die Entwicklung und Validierung eines Laserhygrometers für eine kalibrierungsfreie In-situ H2O-Bestimmung mit fasergekoppelten Sensorkopf für den minimal-invasiven Einsatz mit nur einem 12 mm kleinen Zugang zur Brennkammer des Verbrennungsmotors, um damit prinzipiell eine zyklusaufgelöste Analyse der Abgasrückführung zur Emissionsreduktion in modernen PKWs zu ermöglichen. Für die Entwicklung des Spektrometers wurde zunächst eine passende Absorptionslinie selektiert und die spektralen Molekülparameter metrologisch charakterisiert, was die Unsicherheit des Spektrometers massiv verringerte. Speziell die Linienstärke der gewählten Absorptionslinie bei 2,551 [my]m konnte mit einer sehr kleinen Unsicherheit von ± 1,15 % vermessen werden. Der Sensor erreichte eine Zeitauflösung von 0,9° Kurbelwinkel bei 1500 U/min des Motors (100 [my]s). Durch die stabile und kompakte Optik des Sensorkopfes war die optische Auflösung in der betrachteten Kompressionsphase über 130 Motorzyklen stabil bei 3,7*10-3. Dies führte zu einem SNR von 34 bei 15000 [my]mol/mol bei 1500 U/min des geschleppten Motors. Die H2O-Konzentration für den AGR-Rate relevanten Bereich, konnte absolut und kalibrierungsfrei mit ± 570[my]mol/mol bestimmt werden. Die Mittelung der Konzentration über den Kompressionsbereich von mehr als achtzig aufeinanderfolgenden Motorzyklen, ergab eine H2O-Konzentration von 13340 [my]mol/mol mit einer nur geringen Schwankung von 170 [my]mol/mol. Dieses Ergebnis bestätigt die ausgezeichnete Stabilität des Spektrometers und die damit verbundene Möglichkeit die H2O-Konzentration für eine AGR-Analyse innerhalb des Motors zyklusaufgelöst zu bestimmen.Absorption spectroscopy with tunable diode laser (TDLAS) is widely used in applied and fundamental research. The spectral characteristics of the diode lasers and their possibility for fast wavelength tuning enable reliable in situ measurements of absolute gas species concentrations and temperatures with excellent selectivity, sensitivity and scalability. One objective of this work was the development and validation of an absorption spectrometer based on a tunable diode laser which combines the two most commonly used TDLAS-techniques – direct absorption- and wavelength modulation spectroscopy. A newly developed approach using fast time division multiplexed laser modulation allows simultaneous use of both methods in a single setup. The new approach utilizes in process calibration of the WMS-2f/1f signal from absolute species concentration obtained by dTDLAS. Thereby, decreasing the detection limit and enhancing the precision by a factor of five, from 150 nmol/mol*m*Hz1/2 to 34 nmol/mol*m*Hz1/2. This enables absolute measurements of gas concentrations with improved sensitivity, and without prior calibration against a known gas standard. The proven method of TDLAS for in-cylinder gas analysis was used to develop and validate a laser hygrometer for calibration-free in situ H2O measurements in modern car engines. A fiber coupled minimally invasive sensor head was designed to fit an existing 12 mm access point within the combustion chamber, which can be used for a cycle resolved analysis of the exhaust gas recirculation to reduce the pollution. The uncertainty of the spectrometer was greatly reduced by metrological characterizing the spectral molecule parameter. Especially the strength of the chosen absorption line at 2,551 µm was determined with a very low uncertainty of ± 1,15 %. The achieved time resolution of the sensor is 0,9° crank angle at 1500 rpm (100 µs). A stable optical resolution during compression of the engine of 3,7*10-3 was achieved, due to the compact and very robust optics of the minimal invasive sensor head. This led to a signal to noise ratio of 34 at 15000 µmol/mol and 1500 rpm in motored engine operation. The H2O-conecentration could be determined to ± 570 µmol/mol during the relevant compression part for characterizing the exhaust gas recirculation. Averaging of more than eighty successive compression cycles resulted in a water vapor concentration of 13340 µmol/mol with a very low variance of 170 µmol/mol. This results display the excellent stability of the developed spectrometer and hence the possibility to measure the H2O-concentration with crank angle resolution to analyze the exhaust gas recirculation process