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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 17 — Aug. 26, 2013
  • pp: 19951–19965
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VCSEL-based, high-speed, in situ TDLAS for in-cylinder water vapor measurements in IC engines

O. Witzel, A. Klein, C. Meffert, S. Wagner, S. Kaiser, C. Schulz, and V. Ebert  »View Author Affiliations


Optics Express, Vol. 21, Issue 17, pp. 19951-19965 (2013)
http://dx.doi.org/10.1364/OE.21.019951


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Abstract

We report the first application of a vertical-cavity surface-emitting laser (VCSEL) for calibration- and sampling-free, high-speed, in situ H2O concentration measurements in IC engines using direct TDLAS (tunable diode laser absorption spectroscopy). Measurements were performed in a single-cylinder research engine operated under motored conditions with a time resolution down to 100 μs (i.e., 1.2 crank angle degrees at 2000 rpm). Signal-to-noise ratios (1σ) up to 29 were achieved, corresponding to a H2O precision of 0.046 vol.% H2O or 39 ppm·m. The modulation frequency dependence of the performance was investigated at different engine operating points in order to quantify the advantages of VCSEL against DFB lasers.

© 2013 OSA

1. Introduction

The ubiquitous utilization of internal combustion (IC) engines has required automotive engineers to continually improve engine efficiency while meeting strict emission standards. Exhaust-gas recirculation (EGR) is an important component of combustion strategies in modern engines. EGR essentially increases the amount of inert gas in the cylinder, either by returning gas from the exhaust duct to the intake (external EGR) or by retaining residual burnt gas in the cylinder at the end of the exhaust stroke (internal EGR). Depending on the type of engine, EGR has multiple effects on combustion. For example, the lowered peak temperature is beneficial in controlling engine-out nitric oxides, while for premixed combustion ignitability and flame speed are lowered, which can be detrimental. Therefore exact knowledge of the amount as well as the temporal and spatial distribution of the residual gas before ignition is necessary. The rate of steady-state external EGR can be measured with reasonable effort, but this time-average may not represent the residual-gas content in a given single cycle. For the quantification of internal EGR, no satisfactory solution exists yet. Currently available techniques like gas sampling and subsequent analysis with a gas chromatograph or mass spectrometer mostly yield cycle-averaged information [1

1. A. A. Quader and R. F. Majkowski, „Cycle-by-cycle mixture strength and residual-gas measurements during cold starting“, SAE Technical Paper 1999-01-1107 (1999).

,2

2. T. Toda, H. Nohira, and K. Kobashi, „Evaluation of burned gas ratio (BGR) as a predominant factor to NOx“, SAE Technical Paper 760765 (1976).

]. In contrast, optical methods can be applied in situ with high temporal resolution.

Optical methods have been used previously for in situ measurements of the concentration of various species in the compression stroke of IC engines. Coherent anti-Stokes Raman scattering (CARS) [3

3. M. J. Cottereau, F. Grisch, and J. J. Marie, “CARS measurements of temperature and species concentrations in an IC engine,” Appl. Phys. B 51(1), 63–66 (1990). [CrossRef]

] and laser-induced fluorescence (LIF) [4

4. S. A. Kaiser, M. Schild, and C. Schulz, “Thermal stratification in an internal combustion engine due to wall heat transfer measured by laser-induced fluorescence,” Proc. Combust. Inst. 34(2), 2911–2919 (2013). [CrossRef]

,5

5. D. A. Rothamer, J. Snyder, R. K. Hanson, and R. R. Steeper, “Optimization of a tracer-based PLIF diagnostic for simultaneous imaging of EGR and temperature in IC engines,” Appl. Phys. B 99(1-2), 371–384 (2010). [CrossRef]

] enabled point and imaging measurements, respectively, while various absorption-based methods served for line-of-sight-integrated concentration measurements [6

6. C. L. Hagen and S. T. Sanders, “Investigation of multi-species (H2O2 and H2O) sensing and thermometry in an HCCI engine by wavelength-agile absorption spectroscopy,” Meas. Sci. Technol. 18(7), 1992–1998 (2007). [CrossRef]

8

8. N. Kawahara, E. Tomita, A. Ohtsuki, and Y. Aoyagi, “Cycle-resolved residual gas concentration measurement inside a heavy-duty diesel engine using infrared laser absorption,” Proc. Combust. Inst. 33(2), 2903–2910 (2011). [CrossRef]

]. The most important EGR-relevant species are H2O and CO2. However, they cannot be observed directly by fluorescence except at temperatures that are unrealistically high for the compression stroke [9

9. T. Lee, W. G. Bessler, C. Schulz, M. Patel, J. B. Jeffries, and R. K. Hanson, “UV planar laser induced fluorescence imaging of hot carbon dioxide in a high-pressure flame,” Appl. Phys. B 79(4), 427–430 (2004). [CrossRef]

]. On the other hand both species have relatively high absorption cross-sections in the near-infrared spectral range, which makes absorption-based detection interesting.

Numerous absorption methods have been applied to the detection of H2O in engines: Some were based on broadband absorption spectroscopy (BAS) [10

10. L. de Francqueville, B. Thirouard, and V. Ricordeau, „Measurement of Residual Gas Fraction Using IR Absorption“, SAE Technical Paper 2006-01-3337 (2006).

], fixed-wavelength absorption spectroscopy (FWAS) [11

11. E. Tomita, N. Kawahara, M. Shigenaga, A. Nishiyama, and R. W. Dibble, “In situ measurement of hydrocarbon fuel concentration near a spark plug in an engine cylinder using the 3.392 µm infrared laser absorption method: discussion of applicability with a homogeneous methane-air mixture,” Meas. Sci. Technol. 14(8), 1350–1356 (2003). [CrossRef]

], or scanned-wavelength techniques like tunable diode laser absorption spectroscopy (TDLAS) and wavelength modulation spectroscopy (WMS) [12

12. S. Karagiannopoulos, E. Cheadle, P. Wright, S. Tsekenis, and H. McCann, “Multiwavelength diode-laser absorption spectroscopy using external intensity modulation by semiconductor optical amplifiers,” Appl. Opt. 51(34), 8057–8067 (2012). [CrossRef] [PubMed]

,13

13. J. B. Jeffries, J. M. Porter, S. H. Pyun, R. K. Hanson, K. R. Sholes, K. Shouji, and T. Chaya, “An in-cylinder laser absorption sensor for crank-angle-resolved measurements of gasoline concentration and temperature,” SAE Technical Paper 2010–01–2251 (2010). [CrossRef]

]. Methods like BAS and FWAS record a single, for BAS spectrally-integrated signal to determine the absorber-species concentration. This allows the use of simple and inexpensive data acquisition (DAQ) systems. However, these fixed-wavelength or wavelength-integrated methods provide only limited selectivity, require rather complicated, time consuming calibration, while it is difficult to correct for transmission losses or luminous background emissions.

On the other hand, WMS, being a scanning wavelength method, provides selectivity, but uses lock-in-techniques which require additional equipment and in the majority of the cases calibration. Recently calibration-free WMS has been demonstrated, but this technique relies on complicated models for the calculation of the expected WMS signals [14

14. G. B. Rieker, J. B. Jeffries, and R. K. Hanson, “Calibration-free wavelength-modulation spectroscopy for measurements of gas temperature and concentration in harsh environments,” Appl. Opt. 48(29), 5546–5560 (2009). [CrossRef] [PubMed]

], which are rather difficult to parameterize.

Direct TDLAS (dTDLAS) [15

15. C. Schulz, A. Dreizler, V. Ebert, and J. Wolfrum, “Combustion diagnostics,” in Handbook of experimental fluid mechanics, C. Tropea, A. L. Yarin, J. F. Foss, ed. (Springer, 2007).

,16

16. V. Ebert and J. Wolfrum, “Absorption spectroscopy,” in Optical measurements-techniques and applications (Springer, 2001).

] combines several advantageous properties: dTDLAS captures DC-coupled signals, which allows for very effective correction of multiplicative transmission losses (e.g., laser attenuation on windows or due to particles) as well as of additive signal components, e.g. luminescence of flames and hot surfaces. dTDLAS also has been shown to allow calibration-free absolute concentration measurements [17

17. B. Buchholz, B. Kühnreich, H. Smit, and V. Ebert, “Validation of an extractive, airborne, compact TDL spectrometer for atmospheric humidity sensing by blind intercomparison,” Appl. Phys. B 110(2), 249–262 (2013). [CrossRef]

]. Calibration-free optical methods are particularly valuable for species like water vapor, whose measurement by extractive sampling is susceptible to artifacts due to strong surface adsorption and possible condensation. Furthermore, traditional H2O sensors are difficult to calibrate since stable calibration gases are not available. In addition, dTDLAS provides high-resolution spectral absorption line-profiles, which also contain information on gas pressure and temperature as well as on background emission. High-speed TDLAS measurements with a scanning rate of several kHz can be implemented with modern data acquisition systems, thus allowing for continuous H2O measurements for extended time series, limited only by the available hard-disk capacity. dTDLAS has been successfully used for in situ measurements in many combustion applications [18

18. J. Wolfrum, T. Dreier, V. Ebert, and C. Schulz, “Laser-based combustion diagnostics,” in Encyclopedia of analytical chemistry, R. A. Meyers, ed. (Chichester, John Wiley & Sons, Ltd, 2006).

], such as flame analysis [19

19. S. Wagner, B. T. Fisher, J. W. Fleming, and V. Ebert, “TDLAS-based in situ measurement of absolute acetylene concentrations in laminar 2D diffusion flames,” Proc. Combust. Inst. 32(1), 839–846 (2009). [CrossRef]

], biomass gasifiers [20

20. P. Ortwein, W. Woiwode, S. Fleck, M. Eberhard, T. Kolb, S. Wagner, M. Gisi, and V. Ebert, “Absolute diode laser-based in situ detection of HCl in gasification processes,” Exp. Fluids 49(4), 961–968 (2010). [CrossRef]

], and full-size coal power plants [21

21. H. Teichert, T. Fernholz, and V. Ebert, “Simultaneous in situ measurement of CO, H2O, and gas temperatures in a full-sized coal-fired power plant by near-infrared diode lasers,” Appl. Opt. 42(12), 2043–2051 (2003). [CrossRef] [PubMed]

].

Vertical-cavity surface-emitting lasers (VCSELs) are quite promising light sources due to their much higher modulation bandwidth and significantly wider wavelength tuning range compared to DFB lasers. In the past, VCSELs have been used for methane concentration measurements at repetition rates up to 5 MHz [23

23. M. Lackner, G. Totschnig, F. Winter, M. Ortsiefer, M. C. Amann, R. Shau, and J. Rosskopf, “Demonstration of methane spectroscopy using a vertical-cavity surface-emitting laser at 1.68 µm with up to 5 MHz repetition rate,” Meas. Sci. Technol. 14(1), 101–106 (2003). [CrossRef]

] and for measurements of oxygen, carbon monoxide, and HCl at pressures up to 1.1 MPa [24

24. P. Ortwein, W. Woiwode, S. Wagner, M. Gisi, and V. Ebert, “Laser-based measurements of line strength, self- and pressure-broadening coefficients of the H35Cl R(3) absorption line in the first overtone region for pressures up to 1 MPa,” Appl. Phys. B 100(2), 341–347 (2010). [CrossRef]

26

26. J. Wang, S. T. Sanders, J. B. Jeffries, and R. K. Hanson, “Oxygen measurements at high pressures with vertical cavity surface-emitting lasers,” Appl. Phys. B 72(7), 865–872 (2001). [CrossRef]

]. However, they have not been used in engine applications since their low output power has been assumed to be insufficient for engine measurements. In this paper we describe what we believe to be the first VCSEL-based, high-speed dTDLAS-based in situ laser hygrometer for in-cylinder measurements in IC engines.

2. Vertical-cavity surface-emitting laser (VCSEL)

The well-characterized absorption line 000-101/110-211 at 7299.431 cm–1 (1369.97 nm) was utilized here. The same line is also used in the DFB-laser-based version of the engine hygrometer [22

22. O. Witzel, A. Klein, S. Wagner, C. Meffert, C. Schulz, and V. Ebert, “High-speed tunable diode laser absorption spectroscopy for sampling-free in-cylinder water vapor concentration measurements in an optical IC engine,” Appl. Phys. B 109(3), 521–532 (2012). [CrossRef]

] and in numerous other TDLAS applications [27

27. S. Hunsmann, K. Wunderle, S. Wagner, U. Rascher, U. Schurr, and V. Ebert, “Absolute, high resolution water transpiration rate measurements on single plant leaves via tunable diode laser absorption spectroscopy (TDLAS) at 1.37 μm,” Appl. Phys. B 92(3), 393–401 (2008). [CrossRef]

29

29. A. Mangold, R. Wagner, H. Saathoff, U. Schurath, C. Giesemann, V. Ebert, M. Krämer, and O. Möhler, “Experimental investigation of ice nucleation by different types of aerosols in the aerosol chamber AIDA: implications to microphysics of cirrus clouds,” Meteorologische Zeitschrift 14(4), 485–497 (2005). [CrossRef]

]. This absorption line was chosen because of the availability of advanced optical components for this wavelength and experimentally verified line parameters [30

30. S. Hunsmann, S. Wagner, H. Saathoff, O. Möhler, U. Schurath, and V. Ebert, “Messung der Temperaturabhängigkeit der Linienstärken und Druckverbreiterungskoeffizienten von H2O-Absorptions-linien im 1,4 µm-Band,” VDI Ber. 1959, 149–164 (2006).

]. The line selection also takes into account that due to significant pressure broadening in the engine application the line must be well isolated from other lines and that absorption needs to be strong enough over the whole temperature and pressure range. During the compression stroke of our research engine we expect gas temperatures ranging from nearly room temperature up to 700 K and total pressures from around 0.05 up to 1 MPa. The H2O vapor concentration – depending on the humidity of the intake air and the recirculated gas concentration – is expected to vary between 0.5 and 5 vol.%. In order to monitor the H2O concentration at typical engine speeds, i.e., at 1000–5000 rpm, a temporal resolution of the analytical system in the range of a few 100 µs is required. Spectral simulations based on HITRAN2008 data [31

31. L. S. Rothman, I. E. Gordon, A. Barbe, D. C. Benner, P. F. Bernath, M. Birk, V. Boudon, L. R. Brown, A. Campargue, J.-P. Champion, K. Chance, L. H. Coudert, V. Dana, V. M. Devi, S. Fally, J.-M. Flaud, R. R. Gamache, A. Goldman, D. Jacquemart, I. Kleiner, N. Lacome, W. J. Lafferty, J.-Y. Mandin, S. T. Massie, S. N. Mikhailenko, C. E. Miller, N. Moazzen-Ahmadi, O. V. Naumenko, A. V. Nikitin, J. Orphal, V. I. Perevalov, A. Perrin, A. Predoi-Cross, C. P. Rinsland, M. Rotger, M. Šimečková, M. A. H. Smith, K. Sung, S. A. Tashkun, J. Tennyson, R. A. Toth, A. C. Vandaele, and J. Vander Auwera, “The HITRAN 2008 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 110(9-10), 533–572 (2009). [CrossRef]

] were used to check for the compatibility of the VCSEL tuning range and the pressure and temperature dependent spectral width of the target line.

Figure 1
Fig. 1 Temperature and operation current dependence of the static wavelength tuning of the VCSEL for three different temperatures (red lines). Superimposed is the water-vapor absorption spectrum for T = 500 K, p = 0.5 MPa and c[H2O] = 1 vol.%). The orange bar indicates the significantly narrower tuning range of the previously-used DFB laser.
shows the wide tunability of the VCSEL with current modulation at three different laser temperatures. With very slow, quasi-static current tuning (modulation frequency fmod < 0.01 Hz) the laser continuously scans over 16 cm−1, which is 5–8 times wider than typical current-tuning ranges of a DFB laser (2–3 cm−1, also indicated in Fig. 1 as a yellow bar). For water concentrations from 1 to 5 vol.% and an engine-typical absorption path-length in the order of 10 cm we expect peak absorptions from 6 to 25% at the beginning of the compression stroke, i.e. at 0.1 MPa gas pressure and 300 K gas temperature.

The comparison with the absorption spectrum of water for T = 500 K, p = 0.5 MPa and c[H2O] = 1 vol.% (blue line) shows the advantage of the wider tuning range of the VCSEL (red lines in Fig. 1). At T = 300 K and p = 0.1 MPa the FWHM (full width at half maximum) of the selected absorption line is 0.2 cm−1. This value increases to 0.8 cm−1 at T = 500 K and p = 0.5 MPa. Such wide lines are well covered by a VCSEL but may usually only be incompletely captured by our previously used DFB laser. This makes fitting procedures for DFB lasers more complicated, less stable and may also result in systematic errors, which will also be investigated in this paper.

The problem of incomplete coverage of the absorption line becomes worse and even more important if laser modulation frequencies in the kHz range (or higher), are required in order to achieve high crank-angle resolution. This further complication is caused by the strong modulation frequency dependence of the tuning-range of most DFB laser, as shown in Fig. 2
Fig. 2 Dynamic tuning range versus laser modulation frequency for DFB laser (green) and VCSEL (red). Note the break in the ordinate.
.

Here the DFB tuning range decreases by 50%, i.e., 5% per kHz, when the modulation frequency is increased from 0.1 to 10 kHz. Because of this decrease, we performed our DFB laser measurements previously only with modulation frequencies of up to 4 kHz, which allowed a tuning range of 1.3 cm−1. In comparison, the VCSEL tuning range decreases by only 10% over the same range in modulation frequency. Even with 30 kHz modulation the dynamic tuning range of the VCSEL remains greater than 8 cm−1 and therefore covers the full absorption line in the relevant pressure range.

3. Experiment

The principle of dTDLAS is based on the analysis of the wavelength-dependent attenuation of quasi mono-chromatic light passing through a gas sample. To describe this behavior the Lambert-Beer law can be used [32

32. J. H. Lambert, Lamberts Photometrie: Photometria (Sive de mensura et gradibus luminis, Colorum Et Umbrae (1760), Volume 2. Nabu Press, 2010).

]:
I(ν)=I0(ν)exp(S(T)g(νν0)NL)
(1)
with N being the number density of the molecular absorbers, I0(ν) the initial laser intensity and I(ν) the intensity detected after the probe with an absorption length L. The absorption-line profile is characterized by the temperature-dependent, spectrally-integrated area-normalized (area = 1) line strength S(T), and the shape function g(νν0), which is centered at the wavelength ν0. Two aspects must be considered when acquiring in situ TDLAS measurements in IC engine environments. The first one is a strong, spectrally broad light loss caused by scattering from particles, by window fouling or by laser misalignment occurring over time. The second disturbance is additional light emission, mainly from the combustion process itself or from thermal background emission, which adds to the detected signal. Including transmission losses Tr(t) and background emissions E(t) in Eq. (1) yields an extended version of the Lambert-Beer law [16

16. V. Ebert and J. Wolfrum, “Absorption spectroscopy,” in Optical measurements-techniques and applications (Springer, 2001).

]:
I(ν)=I0(ν)Tr(t)exp(S(T)g(νν0)NL)+E(t).
(2)
Spectrally integrating the absorption line shape g(νν0) over the wavelength [ν] = cm–1 and solving Eq. (2) for the absorber density N leads to:
N=1S(T)Lln(I(ν)E(t)I0(ν)Tr(t))dνdtdt.
(3)
With the ideal-gas law N = p/kBT the concentration can be determined without the need for calibration, solely with knowledge of the line strength S(T), the experimental boundary conditions (p, T, Tr(t), E(t)) and the dynamic laser tuning coefficient dν/dt:
c=kBTS(T)pLln(I(ν)E(t)I0(ν)Tr(t))dνdtdt.
(4)
The use of the ideal gas law especially for higher pressures was considered carefully. Comparisons between the ideal gas law and the virial equation to second order [33

33. J. D. Dymond, R. C. Wilhoit, and K. N. Marsh, Virial Coefficients of Pure Gases and Mixtures (Springer, 2003).

] were performed and showed only differences of below 0.2% for pressures up to 0.5 MPa. As this is much lower than the expected measurement uncertainty the use of the ideal gas law seems appropriate. For the calculations the virial coefficients were determined from the literature for different gas matrixes.

The spectrometer is schematically shown in Fig. 3
Fig. 3 Schematic drawing of laser hygrometer and engine. The function generator modulates the laser-diode current to generate a wavelength scan of the emitted light.
. The VCSEL is temperature-stabilized with an internal Peltier element and coupled into a single-mode fiber. A function generator provides a periodic saw tooth signal modulating the laser current. At the fiber exit, the emitted laser light is collimated and passed directly through the probe volume, i.e., the engine’s combustion chamber. The transmitted light is detected by a 1 mm diameter InGaAs semiconductor photodiode (Hamamatsu), converted into a voltage signal by a low-noise transimpedance amplifier (Femto), subsequently digitized with a 14 bit 100 MS/s A/D converter (National Instruments) and evaluated in our proprietary LabView fitting code. For the complete laser wavelength ramp, also called scan, 2000 (DFB laser) and 3300 (VCSEL) data points or “pixels” (i.e. voltage values) were captured, respectively.

To evaluate the H2O absorption line, a polynomial baseline (containing all disturbance information) as well as a Voigt line-shape [34

34. E. E. Whiting, “An empirical approximation to the Voigt profile,” J. Quant. Spectrosc. Radiat. Transf. 8(6), 1379–1384 (1968). [CrossRef]

] was fitted to the measured H2O spectrum using a non-linear Levenberg-Marquardt algorithm [35

35. J. J. Moré, “The Levenberg-Marquardt algorithm: Implementation and theory,” in Numerical Analysis, (G. A. Watson, Springer, 1978).

]. Significant overlap of the absorption lines from 7294 cm−1 to 7306 cm−1 occurs for pressures above 0.1 MPa. Therefore, a multi-line Voigt fit with 21 lines (with line strength larger than 10−23 (cm−1)/(molecule·cm−2)) in the fitted laser scan range was applied. Further 8 lines outside this area with line strength larger than 10−22 (cm−1)/(molecule·cm−2) were considered during the fit process. The width of all lines was calculated from the measured gas pressure and the derived temperature using HITRAN data. For the target line at 7299,431 cm−1, which is used to determine the H2O concentration, the air broadening coefficient and the temperature broadening measured by Hunsmann et al. [30

30. S. Hunsmann, S. Wagner, H. Saathoff, O. Möhler, U. Schurath, and V. Ebert, “Messung der Temperaturabhängigkeit der Linienstärken und Druckverbreiterungskoeffizienten von H2O-Absorptions-linien im 1,4 µm-Band,” VDI Ber. 1959, 149–164 (2006).

] was used.

4. Single-cylinder research engine

In-cylinder in situ TDLAS was performed in a single-cylinder research engine. The engine is designed to provide optical access through a transparent quartz cylinder and an extended slotted piston for viewing through a piston window. However, neither of these large-diameter optical ports was used here. For the DFB-laser-based version of the hygrometer we had developed two compact and minimally-invasive fiber-optical access ports denoted as FOAP and FOAP-2, respectively [22

22. O. Witzel, A. Klein, S. Wagner, C. Meffert, C. Schulz, and V. Ebert, “High-speed tunable diode laser absorption spectroscopy for sampling-free in-cylinder water vapor concentration measurements in an optical IC engine,” Appl. Phys. B 109(3), 521–532 (2012). [CrossRef]

]. For measurements presented here we used the FOAP-2 port, which completely replaces the quartz cylinder liner with a metal liner and two wedged sapphire windows (16 mm diameter). The laser collimator and a photodiode are each positioned on opposite sides of the cylinder. Possible ambient humidity in the gaps between laser collimator and the window as well as between photodiode and window was minimized to near zero by purging with dry N2 to suppress unwanted H2O absorption.

Engine parameters and operating conditions are presented in Table 1

Table 1. Engine Specifications; Compression TDC Equals 0 CAD

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.

The position of the piston is measured in crank angle degrees (CAD). One engine cycle (four strokes) consists of two complete crankshaft turns i.e. 720 CAD. The crank angle convention used here is that this cycle corresponds to –360 < CAD < + 360, where 0 CAD is called top dead center (TDC), which corresponds to the upper-most piston position in the compression stroke, i.e. the smallest volume in the combustion chamber.

TDLAS measurements were possible until −43 CAD, at which point the piston blocked (in our engine) the laser beam and prevented further measurements in the compression stroke. To synchronize engine and spectrometer, we recorded with an additional 60 MS/s A/D converter (National Instrument) a trigger signal in parallel to the TDLAS signal (see section 3) at every TDC position as well as a trigger from the opto-electronic crank-shaft encoder every 0.1 CAD. In order to determine the in-cylinder H2O concentration from the laser absorption, gas pressure and temperature must be known (see Eq. (4). The in-cylinder pressure was measured with a piezo-capacitive transducer, which was pegged to the absolute pressure recorded by a piezo-resistive pressure transducer in the intake manifold. For the motored measurements a pressure trace averaged over 300 engine cycles was used. The in-cylinder temperature during compression was determined from the measured intake-air temperature and the averaged cylinder-pressure trace.

5. Results

All measurements shown below were performed in a motored single-cylinder engine at a speed of 2000 rpm. Figure 4
Fig. 4 In situ H2O absorption profiles (measured data [circles] and fitted spectral profile [solid red lines]) measured with the DFB laser in the motored engine at −180 CAD (left) and −50 CAD (right) corresponding to cylinder pressures of 0.054 and 0.312 MPa, respectively.
shows two typical single-shot water-vapor absorption profiles (at cylinder pressures of 0.054 MPa and 0.312 MPa) measured with the DFB laser during the compression stroke with a temporal resolution of 250 µs (fmod = 4 kHz). All signals are filtered with a moving-average filter over 5 pixels = 0.0065 cm−1 (DFB laser) or 0.016 cm−1 (VCSEL) to minimize the effect of electrical noise, mainly caused by the large dynamometer of the engine test bed. The plots’ abscissae each cover a 70% subsection selected for data evaluation at the end of the total wavelength scan.

The strong increase in Lorentzian width with increasing pressure can be seen clearly. With increasing pressures it became more and more important to compute the line width using the measured pressure, instead of fitting with the width as a free parameter. With the line width computed, the scan-averaged optical residual (i.e. the standard deviation of the difference between measured and fitted data) could be reduced to 8.4 × 10−4 OD (1σ). This converts into an optical signal-to-noise ratio (SNR, 1σ) of 124. At 0.05 MPa this corresponds to a precision of 0.011 vol.% H2O, which is equivalent to a length-normalized precision of 9 ppm·m. At 0.3 MPa (−50 CAD), the optical SNR decreased to 64, i.e. a precision of 0.023 vol.% or 19 ppm·m, respectively. Especially at −50 CAD it can be seen that the profile of the absorption line is significantly wider than the DFB laser’s tuning range. In the fit of the DFB laser signals this causes – above approximately 0.3 MPa – increasing ambiguity between line profile and baseline and hence increased uncertainty in the baseline determination. More robust fitting algorithms will be developed in the future in order to be able to use DFB lasers at higher pressures.

Figure 5
Fig. 5 Spectral H2O absorption profiles measured with the VCSEL at −180, −130, −90, −65, −50, and −45 CAD corresponding to pressures between 0.074 and 0.42 MPa. The organization of each plot is the same as in Fig. 4.
depicts typical single-shot H2O line spectra measured with the VCSEL under the same engine operating conditions as in Fig. 4. Here a 10 kHz laser modulation was used, corresponding to a temporal resolution of 1.2 CAD and a laser scan time of 100 µsec.

With this temporal resolution and the same digital signal filters as used for the DFB laser signals, the absorption resolution achieved with in a single scan ranged from 2.5 × 10−3 (SNR = 28) to 3.4 × 10−3 (SNR = 16) at −180 CAD and −50 CAD, respectively. At −180 CAD and an in-cylinder pressure of 0.074 MPa, this results in a precision of 0.046 vol.% H2O or a normalized sensitivity of 39 ppm·m. For −50 CAD (0.42 MPa), the precision is 0.081 vol.% or 69 ppm·m. This is only slightly worse than in DFB laser scans and still fully sufficient for the intended application of measuring EGR rates. In motored engine operation, from which the current results are, the H2O concentration is about a factor of 5 lower than in fired operation, where parts of the exhaust gas with typical H2O concentrations around 130 000 ppm are mixed with the intake air [36

36. G. P. Merker, Combustion Engines Development: Mixture Formation, Combustion, Emissions and Simulation (Springer, 2012).

]. For measurements in the compression stroke until ignition the pressure and pressure rise rates are very similar only the temperature calculation becomes more complex. The presented measurements can thus be seen as a test case for the spectrometer sensitivity. The advantage of the VCSEL becomes visible in the later part of the compression stroke (e.g. at −50 and −45 CAD), where – despite pressures of up to approximately 0.5 MPa – the entire absorption-line shape still can be captured due to the VCSEL’s wider tuning range.

In motored operation, the in-cylinder water-vapor concentration is solely due to the outside air humidity, which allows assessing the accuracy of the TDLAS measurement under the strong pressure and temperature variations of the compression cycle. Such an assessment can be made on the basis of Fig. 6
Fig. 6 H2O concentration measured within a single engine cycle from −190 to −43 CAD using the VCSEL spectrometer (top). Measured pressure and calculated temperature (bottom).
, which shows the water concentration measured over a single cycle (top) as well as pressure and temperature for the same cycle (bottom). As for Fig. 5 the laser modulation frequency was 10 kHz. The humidity of the intake air is defined by the humidity of the room air (approximately (14000 ± 1000) ppm, measured in the engine test stand with a simple room humidity sensor) and assumed to be constant over the few milliseconds that are represented in Fig. 6; therefore, the measured cycle-resolved water vapor concentration is also expected to be constant. Analyzing Fig. 6, the H2O concentration over the entire engine cycle was quite constant – as expected – with an average of 13220 ppm and a statistical standard deviation (SD) of 264 ppm (i.e. 2% relative deviation) over the cycle. The slight decrease in the concentration in the beginning can be caused by an uncertainty in pressure ( ± 2 kPa) or temperature ( ± 5 K).

To investigate the stability of the spectrometers, but also to show the sensors’ potential to study possible cycle-to-cycle variations of the engine, we captured, evaluated, and compared 80 consecutive compression cycles using both laser types and two modulation frequencies. The data from these 80 cycles are plotted in Fig. 7, with every colored point indicating an individual laser-based H2O measurement. The temporally local, crank-angle dependent statistics of the H2O concentration scatter was investigated by binning the data in narrow time bins of 3 CAD (fmod = 4 kHz) or 1.2 CAD (fmod = 10 kHz) width. For each bin we calculated the mean (white rectangles in Fig. 7) and the standard deviation (error bars, indicating 2σ standard deviation). On the right side of Fig. 7 a histogram of the H2O concentration at −46 CAD is shown for each configuration. Since the engine was motored, the measured H2O concentration is expected to be constant over the whole compression stroke. The evolution of laser-based H2O concentration vs. detection time in Fig. 7 and in particular the deviations from the expected constant H2O concentration therefore allow evaluating the suitability of the laser type and modulation frequency for high-speed H2O measurements in engines.

At later detection times, i.e. higher pressure and higher pressure rise-rate, increased scatter in the H2O concentration appears, in particular for the DFB laser: This is the case for the 4 kHz-measurement (Fig. 7: A and B) after about −65 CAD. Here, from −190 to −65 CAD (fmod = 4 kHz) the average standard deviation within each bin is quite similar, i.e. 123 ppm for the DFB laser and 193 ppm for the VCSEL. After −65 CAD the measured concentration from both spectrometers with 4 kHz modulation frequency shows much greater fluctuations. At −43 CAD the standard deviation is 2970 ppm for the DFB and 1155 ppm for the VCSEL.

The fact that the scatter is a problem common to both spectrometers (i.e. VCSEL and DFB laser) and thus appears to be relatively independent of the individual width of the laser scan indicates that the scatter is mainly caused by the high rate of change in pressure and temperature and not by a baseline-fitting problem. Using the given engine and spectrometer arrangement we can deduce that in a single 4 kHz-laser-scan the maximum pressure change can be as great as 0.02 MPa, i.e., a relative change of 6%, by calculating the difference in the pressure at the beginning and end of the scan. Towards the later part of the compression stroke this effect, causing a varying absorption profile during the scan together with electrical noise can result in relative H2O-concentration fluctuations of up to 50%.

Hence, faster modulation frequencies were studied to minimize variations in pressure and temperature during a single laser scan. Indeed, Fig. 7C shows that VCSEL measurements with fmod = 10 kHz yield much less scatter. During the late compression stroke (−65 to −43 CAD) scatter amounts to only 13% of that incurred at 4 kHz, which supports the hypothesis that the scatter is caused by instationary pressure conditions.

Table 2

Table 2. Average H2O Concentration and Standard Deviation in Each Bin during the Compression Stroke from −190 to −43 CAD for Both Laser Systems and Different Modulation Frequencies

table-icon
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summarizes the statistics of measured H2O concentrations over a range from −190 to −43 CAD. All three independent measurements show good agreement in average H2O concentrations. This is a significant finding since none of the TDLAS spectrometers was ever calibrated against an external humidity reference, i.e. all measurements derive their absolute character solely from the underlying spectroscopic data evaluation. The agreement thus shows the ability of dTDLAS to measure absolute concentrations without the need for calibration even under engine conditions.

The influence of the temporally local CAD-dependent pressure rate of change dp/dt can be investigated further by systematic variations of that parameter via the engine’s throttle valve. Throttling varies intake pressure pintake and thus in-cylinder pressure. Figure 8
Fig. 8 In-cylinder pressure traces for three throttle positions and hence different intake pressures.
shows the pressure traces over an engine cycle for three different intake pressures.

Varying intake pressure from 0.048 to 0.083 MPa yields maximum in-cylinder pressure-rise rates from 0.011 MPa/CAD (pintake = 0.048 MPa) to 0.018 MPa/CAD (pintake = 0.083 MPa). This increasing rate should result in larger measurement fluctuations, especially for lower modulation frequencies.

The results of this study are shown in Fig. 9
Fig. 9 Intra-cycle H2O concentration from −190 to −43 CAD measured with DFB laser (fmod = 4 kHz, left) and VCSEL (fmod = 10 kHz, right) at varying intake pressure.
. Shown on the on the left side are the H2O concentrations traces measured with the DFB laser at fmod = 4 kHz. As expected increasing intake pressure and therefore higher pressure-rise rate lead to significantly increased H2O fluctuations at the end of the compression stroke. The situation is completely different for the VCSEL at fmod = 10 kHz, for which results are on the right side of Fig. 9. Here, due to rapid and wide tunability, a nearly constant, absolute concentration of about 13200 ppm could be measured over the entire compression stroke without larger systematic variations or a rapid increase in H2O scatter towards late crank angles.

Table 3

Table 3. Average H2O Concentration and Single-cycle Precision During the Compression Stroke from the Data Represented in Fig. 9, i.e., Measured with DFB Laser and VCSEL for Different Intake Pressures*

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compares the H2O concentrations (from Fig. 9) measured with both laser systems for the three intake pressures. The H2O concentration measured in all three experiments was about 13200 ppm (corresponding to a relative humidity = 46% at T = 296 K), i.e. we could demonstrate excellent absolute accuracy of both spectrometer types, without the need for sensor calibration. Independent of the laser type, modulation frequency, and throttle position, all absolute average values are in agreement within the current uncertainty range (8.5% relative). Table 3 also correlates the spectrometer’s temporally local precision and absolute accuracy in the early and late stage of the compression (i.e. −130 and −46 CAD) with the local average pressure p and the pressure rise-rate Δp/ΔCAD. The resulting numbers are graphically presented in Fig. 10
Fig. 10 Ensemble-averaged H2O concentration at −130 (left) and −46 CAD (right) for different intake pressures and pressure-rise rates measured with DFB laser (fmod = 4 kHz, red circles) and VCSEL (fmod = 10 kHz, black rectangles). The error bars correspond to 2σ. The cross-hatched band corresponds to the averaged H2O concentration (over all measurements) with its uncertainty. Noted at the bottom is the pressure-rise rate, which is low early in the compression (−130 CAD) but much higher later in the compression stroke (−46 CAD).
.

From this comparison across different rates of change in pressure and hence also in temperature the advantages of the wider scan range and higher modulation frequency possible with the VCSEL can be seen clearly. As Table 3 and Fig. 10 show, the DFB-spectrometer at 4 kHz modulation frequency performs very well in the early compression stroke, where an average H2O scatter of only 200 ppm at −46 CAD was demonstrated. But in the late compression stroke and with higher intake pressures, using the DFB laser resulted in pronounced decrease of precision, with a scatter band of 1230 to 6490 ppm (i.e. a maximum relative scatter of almost 50%). In contrast, the 10 kHz-VCSEL-system had nearly constant precision of 260 ppm at −130 CAD and 400 ppm at −46 CAD.

The absolute accuracy of the in-cylinder H2O concentration measurements is affected by numerous contributions, the most important of which we list in Table 4

Table 4. Contributions to the Total Uncertainty in H2O Concentration at −50 CAD

table-icon
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. The table shows all measurands that were used for the uncertainty calculation, as well as their value, their estimated uncertainty and the corresponding influence on the total uncertainty of the derived H2O concentration for a measurement at −50°CAD. The greatest influence in the total uncertainty has the uncertainty of the calculated in-cylinder gas temperature with over 74%.

The uncertainty in temperature is low at the beginning of compression, because the charge temperature is assumed to be between the intake air and the engine inlet temperature, leading to an total uncertainty in the H2O concentration of 7.6%. The temperature uncertainty increases towards the end of the compression, because it becomes more difficult to account for the increasing heat transfer, which influences the temperature derived from the in-cylinder pressure. This leads to an increased uncertainty in the concentration at the end of the compression stroke of 8.5%. The uncertainty in the averaged concentrations was also estimated to be 8.5%. Clearly, most beneficial for improving uncertainty in future measurements would be more accurate temperature information. This could come from a more sophisticated engine simulation with thermodynamic model or from simultaneous TDLAS-based temperature measurements.

6. Conclusions and outlook

A new high-speed laser hygrometer for sampling- and calibration-free, in situ gas analysis in the compression cycle in internal combustion engines has been developed, successfully demonstrated and validated. This fully fiber-coupled hygrometer is based on direct in situ TDLAS and uses for the first time a 1.4 µm VCSEL with a rather low ex-fiber optical output power of only 0.17 mW. The laser was fiber-coupled, which simplified the hygrometer optics, in particular the connection to the optical interface developed for single-cylinder research engines with exchangeable quartz/steel cylinders. By scanning the laser at up 10 kHz across an isolated H2O absorption line at 7299.4 cm−1 we were able to extract absolute H2O concentrations with temporal resolutions of up to 100 µs, i.e. a crank angle resolution of 1.2 CAD at 2000 rpm. The spectrometer was successfully used to measure H2O concentrations within the compression stroke of a single-cylinder research engine motored at 2000 rpm, thereby covering an in-cylinder temperature and pressure range from 300 to 600 K and 0.05–0.5 MPa, respectively. Motored operation of the engine with room air of constant humidity allowed validating the spectrometer under engine conditions.

In the best spectrometer configuration, at H2O concentrations around 1.3 vol.%, we could demonstrate a measurement precision of 0.046 vol.% H2O, which indicates a sensitivity and dynamic range well suited for engine applications. Additionally, we also quantified the influence of laser scan speed (i.e. laser modulation frequency) and of the laser scan range on the accuracy and precision of the spectrometer. Furthermore, we compared the suitability of VCSEL and DFB lasers. For this engine experiment, the VCSEL-based spectrometer, modulated at 10 kHz, proved to be the best choice and significantly superior to a DFB-laser-based spectrometer, despite the more than 100 times higher DFB output power. Both systems (DFB laser and VCSEL) showed equal absolute accuracy during the early compression phase, i.e. at rather low in-cylinder pressures (less than 0.2 MPa). However, later in the compression, at higher pressures and more rapid pressure changes, the VCSEL hygrometer showed much better H2O stability and much less systematic errors over the course of the compression stroke.

In conclusion we could demonstrate (to our knowledge) the first VCSEL- and direct TDLAS-based hygrometer for sampling- and calibration-free, high-speed H2O concentration measurements inside an IC engine. This opens up new possibilities for measuring EGR, for which water vapor can serve as a proxy. Due to its “single-scan, single-cycle” capability the new spectrometer is well suited to study intra- and inter-cycle variations of EGR in engines, which is of great practical interest. In the future we intend to further validate the spectrometer in other engines and operating conditions. We also aim to expand its measurement capabilities to other species like CO or CO2 and to simultaneous measurements of gas temperature and gas concentrations. This would be particularly useful in fired engine operation.

Acknowledgments

The IGF project 15970 N of the research association Forschungskuratorium Maschinenbau e.V.FKM, Lyoner Straße 18, 60528 Frankfurt, was funded via the AiF within the scope of the program for the promotion of the Industrielle Gemeinschaftsforschung und -entwicklung (IGF) by the Federal Ministry of Economy and Technology on the basis of a decision of the German Federal Parliament. The authors also thank Dennis Bensing from the Institute for Combustion and Gas Dynamics at the University of Duisburg-Essen for his work in the construction and operation of the engine.

References and links

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A. A. Quader and R. F. Majkowski, „Cycle-by-cycle mixture strength and residual-gas measurements during cold starting“, SAE Technical Paper 1999-01-1107 (1999).

2.

T. Toda, H. Nohira, and K. Kobashi, „Evaluation of burned gas ratio (BGR) as a predominant factor to NOx“, SAE Technical Paper 760765 (1976).

3.

M. J. Cottereau, F. Grisch, and J. J. Marie, “CARS measurements of temperature and species concentrations in an IC engine,” Appl. Phys. B 51(1), 63–66 (1990). [CrossRef]

4.

S. A. Kaiser, M. Schild, and C. Schulz, “Thermal stratification in an internal combustion engine due to wall heat transfer measured by laser-induced fluorescence,” Proc. Combust. Inst. 34(2), 2911–2919 (2013). [CrossRef]

5.

D. A. Rothamer, J. Snyder, R. K. Hanson, and R. R. Steeper, “Optimization of a tracer-based PLIF diagnostic for simultaneous imaging of EGR and temperature in IC engines,” Appl. Phys. B 99(1-2), 371–384 (2010). [CrossRef]

6.

C. L. Hagen and S. T. Sanders, “Investigation of multi-species (H2O2 and H2O) sensing and thermometry in an HCCI engine by wavelength-agile absorption spectroscopy,” Meas. Sci. Technol. 18(7), 1992–1998 (2007). [CrossRef]

7.

D. W. Mattison, J. B. Jeffries, R. K. Hanson, R. R. Steeper, S. De Zilwa, J. E. Dec, M. Sjoberg, and W. Hwang, “In-cylinder gas temperature and water concentration measurements in HCCI engines using a multiplexed-wavelength diode-laser system: Sensor development and initial demonstration,” Proc. Combust. Inst. 31(1), 791–798 (2007). [CrossRef]

8.

N. Kawahara, E. Tomita, A. Ohtsuki, and Y. Aoyagi, “Cycle-resolved residual gas concentration measurement inside a heavy-duty diesel engine using infrared laser absorption,” Proc. Combust. Inst. 33(2), 2903–2910 (2011). [CrossRef]

9.

T. Lee, W. G. Bessler, C. Schulz, M. Patel, J. B. Jeffries, and R. K. Hanson, “UV planar laser induced fluorescence imaging of hot carbon dioxide in a high-pressure flame,” Appl. Phys. B 79(4), 427–430 (2004). [CrossRef]

10.

L. de Francqueville, B. Thirouard, and V. Ricordeau, „Measurement of Residual Gas Fraction Using IR Absorption“, SAE Technical Paper 2006-01-3337 (2006).

11.

E. Tomita, N. Kawahara, M. Shigenaga, A. Nishiyama, and R. W. Dibble, “In situ measurement of hydrocarbon fuel concentration near a spark plug in an engine cylinder using the 3.392 µm infrared laser absorption method: discussion of applicability with a homogeneous methane-air mixture,” Meas. Sci. Technol. 14(8), 1350–1356 (2003). [CrossRef]

12.

S. Karagiannopoulos, E. Cheadle, P. Wright, S. Tsekenis, and H. McCann, “Multiwavelength diode-laser absorption spectroscopy using external intensity modulation by semiconductor optical amplifiers,” Appl. Opt. 51(34), 8057–8067 (2012). [CrossRef] [PubMed]

13.

J. B. Jeffries, J. M. Porter, S. H. Pyun, R. K. Hanson, K. R. Sholes, K. Shouji, and T. Chaya, “An in-cylinder laser absorption sensor for crank-angle-resolved measurements of gasoline concentration and temperature,” SAE Technical Paper 2010–01–2251 (2010). [CrossRef]

14.

G. B. Rieker, J. B. Jeffries, and R. K. Hanson, “Calibration-free wavelength-modulation spectroscopy for measurements of gas temperature and concentration in harsh environments,” Appl. Opt. 48(29), 5546–5560 (2009). [CrossRef] [PubMed]

15.

C. Schulz, A. Dreizler, V. Ebert, and J. Wolfrum, “Combustion diagnostics,” in Handbook of experimental fluid mechanics, C. Tropea, A. L. Yarin, J. F. Foss, ed. (Springer, 2007).

16.

V. Ebert and J. Wolfrum, “Absorption spectroscopy,” in Optical measurements-techniques and applications (Springer, 2001).

17.

B. Buchholz, B. Kühnreich, H. Smit, and V. Ebert, “Validation of an extractive, airborne, compact TDL spectrometer for atmospheric humidity sensing by blind intercomparison,” Appl. Phys. B 110(2), 249–262 (2013). [CrossRef]

18.

J. Wolfrum, T. Dreier, V. Ebert, and C. Schulz, “Laser-based combustion diagnostics,” in Encyclopedia of analytical chemistry, R. A. Meyers, ed. (Chichester, John Wiley & Sons, Ltd, 2006).

19.

S. Wagner, B. T. Fisher, J. W. Fleming, and V. Ebert, “TDLAS-based in situ measurement of absolute acetylene concentrations in laminar 2D diffusion flames,” Proc. Combust. Inst. 32(1), 839–846 (2009). [CrossRef]

20.

P. Ortwein, W. Woiwode, S. Fleck, M. Eberhard, T. Kolb, S. Wagner, M. Gisi, and V. Ebert, “Absolute diode laser-based in situ detection of HCl in gasification processes,” Exp. Fluids 49(4), 961–968 (2010). [CrossRef]

21.

H. Teichert, T. Fernholz, and V. Ebert, “Simultaneous in situ measurement of CO, H2O, and gas temperatures in a full-sized coal-fired power plant by near-infrared diode lasers,” Appl. Opt. 42(12), 2043–2051 (2003). [CrossRef] [PubMed]

22.

O. Witzel, A. Klein, S. Wagner, C. Meffert, C. Schulz, and V. Ebert, “High-speed tunable diode laser absorption spectroscopy for sampling-free in-cylinder water vapor concentration measurements in an optical IC engine,” Appl. Phys. B 109(3), 521–532 (2012). [CrossRef]

23.

M. Lackner, G. Totschnig, F. Winter, M. Ortsiefer, M. C. Amann, R. Shau, and J. Rosskopf, “Demonstration of methane spectroscopy using a vertical-cavity surface-emitting laser at 1.68 µm with up to 5 MHz repetition rate,” Meas. Sci. Technol. 14(1), 101–106 (2003). [CrossRef]

24.

P. Ortwein, W. Woiwode, S. Wagner, M. Gisi, and V. Ebert, “Laser-based measurements of line strength, self- and pressure-broadening coefficients of the H35Cl R(3) absorption line in the first overtone region for pressures up to 1 MPa,” Appl. Phys. B 100(2), 341–347 (2010). [CrossRef]

25.

J. Chen, A. Hangauer, R. Strzoda, and M.-C. Amann, “VCSEL-based calibration-free carbon monoxide sensor at 2.3 μm with in-line reference cell,” Appl. Phys. B 102(2), 381–389 (2011). [CrossRef]

26.

J. Wang, S. T. Sanders, J. B. Jeffries, and R. K. Hanson, “Oxygen measurements at high pressures with vertical cavity surface-emitting lasers,” Appl. Phys. B 72(7), 865–872 (2001). [CrossRef]

27.

S. Hunsmann, K. Wunderle, S. Wagner, U. Rascher, U. Schurr, and V. Ebert, “Absolute, high resolution water transpiration rate measurements on single plant leaves via tunable diode laser absorption spectroscopy (TDLAS) at 1.37 μm,” Appl. Phys. B 92(3), 393–401 (2008). [CrossRef]

28.

A. R. Awtry, B. T. Fisher, R. A. Moffatt, V. Ebert, and J. W. Fleming, “Simultaneous diode laser based in situ quantification of oxygen, carbon monoxide, water vapor, liquid water in a dense water mist environment,” Proc. Combust. Inst. 31(1), 799–806 (2007). [CrossRef]

29.

A. Mangold, R. Wagner, H. Saathoff, U. Schurath, C. Giesemann, V. Ebert, M. Krämer, and O. Möhler, “Experimental investigation of ice nucleation by different types of aerosols in the aerosol chamber AIDA: implications to microphysics of cirrus clouds,” Meteorologische Zeitschrift 14(4), 485–497 (2005). [CrossRef]

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S. Hunsmann, S. Wagner, H. Saathoff, O. Möhler, U. Schurath, and V. Ebert, “Messung der Temperaturabhängigkeit der Linienstärken und Druckverbreiterungskoeffizienten von H2O-Absorptions-linien im 1,4 µm-Band,” VDI Ber. 1959, 149–164 (2006).

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33.

J. D. Dymond, R. C. Wilhoit, and K. N. Marsh, Virial Coefficients of Pure Gases and Mixtures (Springer, 2003).

34.

E. E. Whiting, “An empirical approximation to the Voigt profile,” J. Quant. Spectrosc. Radiat. Transf. 8(6), 1379–1384 (1968). [CrossRef]

35.

J. J. Moré, “The Levenberg-Marquardt algorithm: Implementation and theory,” in Numerical Analysis, (G. A. Watson, Springer, 1978).

36.

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37.

R. A. Toth, “Linelists of water vapor parameters from 500 to 8000 cm−1,” see http://mark4sun.jpl.nasa.gov/ h2o.html.

OCIS Codes
(120.1740) Instrumentation, measurement, and metrology : Combustion diagnostics
(300.6260) Spectroscopy : Spectroscopy, diode lasers

ToC Category:
Instrumentation, Measurement, and Metrology

History
Original Manuscript: June 5, 2013
Revised Manuscript: July 15, 2013
Manuscript Accepted: July 15, 2013
Published: August 16, 2013

Citation
O. Witzel, A. Klein, C. Meffert, S. Wagner, S. Kaiser, C. Schulz, and V. Ebert, "VCSEL-based, high-speed, in situ TDLAS for in-cylinder water vapor measurements in IC engines," Opt. Express 21, 19951-19965 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-17-19951


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References

  1. A. A. Quader and R. F. Majkowski, „Cycle-by-cycle mixture strength and residual-gas measurements during cold starting“, SAE Technical Paper 1999-01-1107 (1999).
  2. T. Toda, H. Nohira, and K. Kobashi, „Evaluation of burned gas ratio (BGR) as a predominant factor to NOx“, SAE Technical Paper 760765 (1976).
  3. M. J. Cottereau, F. Grisch, and J. J. Marie, “CARS measurements of temperature and species concentrations in an IC engine,” Appl. Phys. B51(1), 63–66 (1990). [CrossRef]
  4. S. A. Kaiser, M. Schild, and C. Schulz, “Thermal stratification in an internal combustion engine due to wall heat transfer measured by laser-induced fluorescence,” Proc. Combust. Inst.34(2), 2911–2919 (2013). [CrossRef]
  5. D. A. Rothamer, J. Snyder, R. K. Hanson, and R. R. Steeper, “Optimization of a tracer-based PLIF diagnostic for simultaneous imaging of EGR and temperature in IC engines,” Appl. Phys. B99(1-2), 371–384 (2010). [CrossRef]
  6. C. L. Hagen and S. T. Sanders, “Investigation of multi-species (H2O2 and H2O) sensing and thermometry in an HCCI engine by wavelength-agile absorption spectroscopy,” Meas. Sci. Technol.18(7), 1992–1998 (2007). [CrossRef]
  7. D. W. Mattison, J. B. Jeffries, R. K. Hanson, R. R. Steeper, S. De Zilwa, J. E. Dec, M. Sjoberg, and W. Hwang, “In-cylinder gas temperature and water concentration measurements in HCCI engines using a multiplexed-wavelength diode-laser system: Sensor development and initial demonstration,” Proc. Combust. Inst.31(1), 791–798 (2007). [CrossRef]
  8. N. Kawahara, E. Tomita, A. Ohtsuki, and Y. Aoyagi, “Cycle-resolved residual gas concentration measurement inside a heavy-duty diesel engine using infrared laser absorption,” Proc. Combust. Inst.33(2), 2903–2910 (2011). [CrossRef]
  9. T. Lee, W. G. Bessler, C. Schulz, M. Patel, J. B. Jeffries, and R. K. Hanson, “UV planar laser induced fluorescence imaging of hot carbon dioxide in a high-pressure flame,” Appl. Phys. B79(4), 427–430 (2004). [CrossRef]
  10. L. de Francqueville, B. Thirouard, and V. Ricordeau, „Measurement of Residual Gas Fraction Using IR Absorption“, SAE Technical Paper 2006-01-3337 (2006).
  11. E. Tomita, N. Kawahara, M. Shigenaga, A. Nishiyama, and R. W. Dibble, “In situ measurement of hydrocarbon fuel concentration near a spark plug in an engine cylinder using the 3.392 µm infrared laser absorption method: discussion of applicability with a homogeneous methane-air mixture,” Meas. Sci. Technol.14(8), 1350–1356 (2003). [CrossRef]
  12. S. Karagiannopoulos, E. Cheadle, P. Wright, S. Tsekenis, and H. McCann, “Multiwavelength diode-laser absorption spectroscopy using external intensity modulation by semiconductor optical amplifiers,” Appl. Opt.51(34), 8057–8067 (2012). [CrossRef] [PubMed]
  13. J. B. Jeffries, J. M. Porter, S. H. Pyun, R. K. Hanson, K. R. Sholes, K. Shouji, and T. Chaya, “An in-cylinder laser absorption sensor for crank-angle-resolved measurements of gasoline concentration and temperature,” SAE Technical Paper 2010–01–2251 (2010). [CrossRef]
  14. G. B. Rieker, J. B. Jeffries, and R. K. Hanson, “Calibration-free wavelength-modulation spectroscopy for measurements of gas temperature and concentration in harsh environments,” Appl. Opt.48(29), 5546–5560 (2009). [CrossRef] [PubMed]
  15. C. Schulz, A. Dreizler, V. Ebert, and J. Wolfrum, “Combustion diagnostics,” in Handbook of experimental fluid mechanics, C. Tropea, A. L. Yarin, J. F. Foss, ed. (Springer, 2007).
  16. V. Ebert and J. Wolfrum, “Absorption spectroscopy,” in Optical measurements-techniques and applications (Springer, 2001).
  17. B. Buchholz, B. Kühnreich, H. Smit, and V. Ebert, “Validation of an extractive, airborne, compact TDL spectrometer for atmospheric humidity sensing by blind intercomparison,” Appl. Phys. B110(2), 249–262 (2013). [CrossRef]
  18. J. Wolfrum, T. Dreier, V. Ebert, and C. Schulz, “Laser-based combustion diagnostics,” in Encyclopedia of analytical chemistry, R. A. Meyers, ed. (Chichester, John Wiley & Sons, Ltd, 2006).
  19. S. Wagner, B. T. Fisher, J. W. Fleming, and V. Ebert, “TDLAS-based in situ measurement of absolute acetylene concentrations in laminar 2D diffusion flames,” Proc. Combust. Inst.32(1), 839–846 (2009). [CrossRef]
  20. P. Ortwein, W. Woiwode, S. Fleck, M. Eberhard, T. Kolb, S. Wagner, M. Gisi, and V. Ebert, “Absolute diode laser-based in situ detection of HCl in gasification processes,” Exp. Fluids49(4), 961–968 (2010). [CrossRef]
  21. H. Teichert, T. Fernholz, and V. Ebert, “Simultaneous in situ measurement of CO, H2O, and gas temperatures in a full-sized coal-fired power plant by near-infrared diode lasers,” Appl. Opt.42(12), 2043–2051 (2003). [CrossRef] [PubMed]
  22. O. Witzel, A. Klein, S. Wagner, C. Meffert, C. Schulz, and V. Ebert, “High-speed tunable diode laser absorption spectroscopy for sampling-free in-cylinder water vapor concentration measurements in an optical IC engine,” Appl. Phys. B109(3), 521–532 (2012). [CrossRef]
  23. M. Lackner, G. Totschnig, F. Winter, M. Ortsiefer, M. C. Amann, R. Shau, and J. Rosskopf, “Demonstration of methane spectroscopy using a vertical-cavity surface-emitting laser at 1.68 µm with up to 5 MHz repetition rate,” Meas. Sci. Technol.14(1), 101–106 (2003). [CrossRef]
  24. P. Ortwein, W. Woiwode, S. Wagner, M. Gisi, and V. Ebert, “Laser-based measurements of line strength, self- and pressure-broadening coefficients of the H35Cl R(3) absorption line in the first overtone region for pressures up to 1 MPa,” Appl. Phys. B100(2), 341–347 (2010). [CrossRef]
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