Harald Schlüter

Fluorescence imaging inside an internal combustion engine using tunable excimer lasers

Tunable excimer lasers are used to obtain 2-D images of molecular (and some state-specific) density distributions inside a cylinder of a modified four-cylinder in-line engine that has optical access. Natural fluorescence (i.e., without a laser) is used for some OH pictures, normal laser-induced fluorescence (LIF) for those of NO and of the isooctane fuel, and laser-induced predissociative fluorescence (LIPF) for other OH pictures and for those of O(2). Relevant spectroscopy is done to find the laser and fluorescence frequencies needed to measure isolated species. LIPF works well at high pressures, is state specific, and is ideally suited to follow turbulent processes. No similar measurements in engines have been previously reported. Pictures are taken in succeeding engine cycles. Their sequence is either at a particular point of the engines cycle to show cyclic fluctuations, or at succeeding portions of the cycle to illustrate the progress of the gasdynamics or of the combustion.

Predissociation of o2 in the b-state

Dispersed LIF spectra of O2 in the Schumann–Runge band were measured with a modified tunable ArF laser in a flame. Spin‐state selective predissociation of the B state was directly observed in fluorescence excitation spectra, revealing the relative coupling matrix elements of the triplet components to the manifold of repulsive states. Such data determines the symmetries of the important predissociating curves for each observed B‐state vibrational level and shows that past interpretation of absorption linewidth data is in error. Due to the fast predissociation, quench‐free emission spectra arising from laser prepared single rovibronic levels in the B state were observed even in an atmospheric flame. Fluorescence to X‐state vibrational levels as high as v‘=35 was observed and relative emission probabilities were derived.

Crank-angle-resolved laser-induced fluorescence imaging of NO in a spark-ignition engine at 248 nm and correlations to flame front propagation and pressure release

Inside the combustion chamber of a spark-ignition engine, NO fluorescence is excited with a narrow-band tunable KrF excimer laser. The fluorescence light is detected by an intensified CCD camera that yields images of the NO distributions. Rotational-vibrational transitions of NO are excited by the A(2)Σ+ ? X(2)Π (0, 2) band system around 248 nm. Single laser shot planar NO distributions are obtained with good signal-to-noise ratio at all crank angles and allow us to locate areas of NO formation during combustion. The pressure within the combustion chamber is measured simultaneously with the NO distributions, which allows the evaluation of correlations between indicated work and NO formation. The crank-angle-resolved sequences of two-dimensional NO distributions and averaged pressure traces are presented for different engine-operating conditions. In addition, laser-induced predissociation fluorescence of OH excited by the same laser source is measured in order to visualize the corresponding flame front propagation and to compare the time of formation of NO relative to that of OH.

State-selective detection of CO using a tunable ArF excimer laser

CO molecules in their electronic and vibrational ground state were state selectively detected using a tunable ArF excimer laser. Rotational state selectivity is due to the one‐photon resonance of the spin‐forbidden a 3Π, v’=2←X 1∑+,v‘=0 transition. Subsequent absorption of two additional ArF laser photons yields, via a dissociative continuum, electronically excited C(3 1P) atoms, whose fluorescence is detected.

Planar imaging of a laboratory flame and of internal combustion in an automobile engine using UV rayleigh and fluorescence light

Rayleigh scattering of tunable excimer laser light (193 nm and 248 nm) is used to obtain 2-D images of the distribution of total densities in a laboratory flame and in a cylinder of an automobile engine. Because the UV light is very strongly scattered, there is ample signal and there is excellent contrast of Rayleigh light against surface scattered light, even in the small volume of the engine cylinder. The laboratory flame data are converted to an image of the temperature field. The Rayleigh images are compared with those from planar laser induced predissociative fluorescence, which yield state-specific densities of selected molecules. The experimental arrangement is the same except for the selection of laser wavelength and the filtering of the radiated light.

Simple way to improve a tunable argon fluoride laser

The locking efficiency and stability of a Lambda Physik EMG150 tunable ArF laser was significantly improved in a simple way. The active gain volume of the oscillator laser was increased by a factor of 5 by replacing the 1‐mm circular holes supplied with the original laser by 1 mm×5 mm rectangular apertures. The resulting rectangular output of the oscillator was condensed with a cylindrical telescope and used for injection locking. The scanning range, locking efficiency, and stability of the laser were significantly improved.

Operation of KrF and ArF tunable excimer lasers without Cassegrain optics

A commercial tunable excimer laser consists of an oscillator-amplifier combination. The oscillator produces high-quality light that is sent to the amplifier and is distributed throughout the amplifier cavity via Cassegrain optics. We describe here two alternative approaches, a “single-pass” configuration for use with KrF and a “triple-pass” configuration with ArF, both of which do away with the Cassegrain optics. In each approach, the beam energy is the same as with Cassegrain optics. For KrF, the changes provide better locking, a higher degree of linear polarization, and a better spatial beam homogeneity, but a poorer beam divergence. For ArF, there is also better beam homogeneity, but the locking efficiency and divergence are not as good as with Cassegrain optics.

Spatially selective chemistry control in an oil-burning furnace: A new method to reduce NOx emissions developed through combined three-dimensional laser diagnostics

In contrast to many laser diagnostic activities in combustion that are dealing with more fundamental problems, the results presented here lead to a considerable improvement of combustion in a commercial oil-burning furnace (i.e., to an NO reduction of roughly, 50%). New methods are combined to yield a laser diagnostic method that allows a more intelligent design of the fuel/air injection system. Planar laser-induced fluorescence (PLIF) in combination with intensified 12-bit CCD camera technology is used to find positions in the mixing device in which small protions of exhaust gas are guided directly to localized hot areas in the flame where NO formation is highly enhanced to achieve a spatially selective cooling of the flame. All PLIF measurements are carried out by averaging over several hundred laser shots using a pulsed tunable KrF6 excimer laser operating at 248 nm. These averaged data offer the possibility to interpolate between different two-dimensional images to obtain three-dimensional results of the qualitative distribution of OH, pyrolized fuel, and NO. Spatially selective NO-tracer seeding is applied in order to follow the gas flow across the flame front. By pulsing the NO gas tracer, the flow velocity is determined.