Spectroscopy – An Engine Diagnostic Tool
In natural gas engines used for stationary power generation, the fuel cost alone amounts to more than 70% of the total operational cost spread over the life of the system. As a result, engine misfires could prove costly. Further, demands for improving efficiency while reducing NOx emissions warrant understanding NOx formations and its reduction at the source. To address such issues, in-cylinder optical and spectroscopic diagnostics are being developed to monitor successful ignition and its transformation to a combustion flame front. The in-cylinder optical signal monitoring can also be used for engine tuning, as well as, for real-time feedback.
Picture in figure 1(left) shows a laser mounted on a single cylinder natural gas research engine and an optical-fiber bundle (Blue cable in the figure) and figure 1(right) is schematic of laser plug and spectra collection optics. Laser is used as the ignition source to ignite (via laser induced gas breakdown) the fuel-air mixture. The radiation from the spark kernel and the combustion gases are collected by the focusing lens and directed to an optical fiber. The signal is imaged by a spectrometer coupled to an ICCD camera. A typical spectral scan for natural –air combustion is shown below. The radiant emissions from Laser Induced Breakdown (LIB) that immediately follows the laser pulse and flame chemiluminescence are used for developing useful diagnostics.
Figure 1. (left) Picture of laser ignition and spectra collection setup. (right) Schematic of the optical arrangement to collect spectral signals. 1: mirror with central hole, 2: cylinder head, 3: laser plug, 4: f=13 mm sapphire lens, 5: f=50 mm quartz lens, 6: optical fiber
As natural gas is clean burning, the radiant emission is predominantly due to chemiluminescence. Figure 2 shows a typical combustion spectrum obtained from a natural gas reciprocating engine. The chemiluminescence in the visible and near UV is primarily associated to be from four main electronically excited species formed through chemical reactions: OH* (306.4 nm); CH* (431.5 nm); C2* (516.5 nm) and CO2* (broadband emission 340 to over 650 nm). In the case of natural gas, the emission from CH* and C2* are insignificant, whereas, that from CO2* alone accounts to more than 90% of the signal integrated over the entire spectrum.
Figure 2. Crank angle resolved spectra obtained from the laser-ignited single cylinder natural gas engine.
Temperature Measurements from OH*:
OH* chemiluminescence can be used to have an estimate of temperature in and around reaction zone in the cylinder by comparing experimental spectrum with a simulated spectrum, as shown in the figure 3.
Figure 3. OH* chemiluminescence can provide in cylinder temperature.
Laser Induced Breakdown Spectroscopy (LIBS):
Laser induced gas breakdown creates high temperature plasma and the molecules in the region breakdown into constituent atomic species. As these atomic species cool down they emit characteristic spectral lines. By monitoring the ratios of spectral signatures from a multitude of species the local fuel-air ratio (equivalence ratio) can be estimated. For example, figure 4 shows the LIBS signal obtained from Argonne’s single cylinder natural gas engine after 1 μs from the laser pulse and exposure time of 5 μs. The spectrum in the figure 4 atomic emissions lines corresponding to Hydrogen, Nitrogen and Oxygen are identified as: Ha at 656.28 nm, N atom at 742.36, 744.22 and 746.83 nm, and by the O atom at 777 nm. The engine was operated for a range of conditions, 0.6 < f < 1.0 and EGR (Exhaust Gas Recirculation) up to 29%, and the intensities of Ha, N and O line intensities were obtained by integrating under the corresponding peaks.
Figure 4. LIBS signal obtained from the engine after 1 μs from the laser pulse and exposure time of 5 μs.
The correlations of the intensity ratios (Ha/O) (Ha/N_746) and (Ha/N_sum) with local equivalence ratio, based on local fuel to oxygen ratio, are shown in figure 5 for different levels of EGR.
Figure 5. Correlation of line intensity ratios (Hα/O), (H/N_746) and (H/N_sum) with y for EGR ≥ 0.
- Sreenath Gupta, Bipin Bihari, Munidhar Biruduganti and Raj R. Sekar, “In-Cylinder Equivalence Ratio Measurements in an EGR Equipped Engine,” submitted as ASME ICEF 2010-35072, ASME-ICED Fall Technical conference, Sept. 12-15, 2010.
- Bihari B., Gupta S.B., Biruduganti M. S., M. V. Johnson and R. R. Sekar, “Spectroscopic Diagnostics for Combustion Metrics in a Natural Gas Fired Engine” B., Proceedings of the 2010 Technical Meeting of the Central States of The Combustion Institute.
Bipin Bihari Bihari@anl.gov; Sreenath Gupta, firstname.lastname@example.org