Werner Hentschel

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.

Spatially Resolved Raman Scattering for Multi-Species and Temperature Analysis in Technically Applied Combustion Systems: Spray Flame and Four-Cylinder In-Line Engine

Spatially resolved Raman scattering is used to measure the single shot stoichiometry before ignition inside a realistic internal combustion engine with high single shot precision of l%–4% (depending on the extent of spatial averaging). The high precision results from the simultaneous detection of fuel and N2 (O2), which yields stoichiometry via a relative measurement. The cycle-to-cycle fluctuations of stoichiometry are clearly resolved. The feasibility of averaged spatially resolved simultaneous multi-species detection is demonstrated in a commercial oil-burning furnace as well. The limited precision that is usually obtained in Raman scattering by interfering emissions is highly improved using the fact that the interfering emission is unpolarized whereas Raman scattering is highly polarized. Therefore, Raman measurements provided good signal-to-noise ratios in the spray flame even in the area where fuel droplets occur and during combustion in the engine. The optical multichannel analyzer yields one-dimensional spatial resolution, and offers the capability to easily combine Raman scattering with Rayleigh scattering and laser-induced fluorescence detection of minority species.

Laser metrology : a diagnostic tool in automotive development processes

Abstract Laser measurement techniques are widely used in automotive development processes. Applications at Volkswagen are presented where laser metrology works as a diagnostic tool for analysing and optimising complex coupled processes inside and between automotive components and structures such as the reduction of a vehicles interior or outer acoustic noise, including brake noise, and the combustion analysis for diesel and gasoline engines to further reduce fuel consumption and pollution. Pulsed electronic speckle pattern interferometry (ESPI) and holographic interferometry are used for analysing the knocking behaviour of modern engines and for correct positioning of knocking sensors. Holographic interferometry shows up the vibrational behaviour of brake components and their interaction during braking, and allows optimisation for noise-free brake systems. Scanning laser vibrometry analyses structure-born noise of a whole car body for the optimisation of its interior acoustical behaviour.Modern engine combustion concepts such as in direct-injection (DI) gasoline and diesel engines benefit from laser diagnostic tools which permit deeper insight into the in-cylinder processes such as flow generation, fuel injection and spray formation, atomisation and mixing, ignition and combustion, and formation and reduction of pollutants. The necessary optical access inside a cylinder is realised by so-called ‘transparent engines’ allowing measurements nearly during the whole engine cycle. Measurement techniques and results on double-pulse particle image velocimetry (PIV) with a frequency-doubled YAG laser for in-cylinder flow analysis are presented, as well as Mie-scattering on droplets using a copper vapour laser combined with high-speed filming, and laser-induced fluorescence (LIF) with an excimer laser for spray and fuel vapour analysis.

Optical diagnostics for combustion process development of direct-injection gasoline engines

This paper describes the operating principle and the combustion process of spark-ignition engines operating with direct injection (DI) of gasoline and used in passenger cars. This new type of engine has an advantage in efficiency on the order of 15% compared with current port-injected gasoline engines using a three-way catalyst and has a high potential for further improvements. Current knowledge of the combustion process of DI gasoline engines benefits from the application of optical diagnostic techniques which permit deep insights into the in-cylinder processes such as flow development, fuel injection, and spray-air interaction. This paper explains the optical techniques used in research and development (R&D) and shows how laser diagnosties can contribute to the analysis and optimization of various phenomena associated with this promising combustion concept.

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.

Polarization separated spatially resolved single laser shot multispecies analysis in the combustion chamber of a realistic SI engine with a tunable KrF excimer laser

For precise spontaneous Raman scattering measurements of species densities in hydrocarbon fueled combustion, it is vital to separate the Raman scattered light from interfering (broadband) fluorescence light. This paper describes the application of a detection scheme for separated and simultaneous acquisition of both polarization components of the laser-induced emissions (i.e., perpendicular and parallel to the polarization of the incident laser light). Using highly polarized incident laser light, the Raman scattered light maintains almost this polarization, whereas broadband fluorescence and most laser-induced predissociation fluorescence (LIPF) emissions are strongly depolarized. Therefore, it is possible to extract the Raman signal by subtracting the signals separately obtained for the two polarization directions (i.e., vertical polarized light consists of fluorescence and parallel polarized light consists of fluorescence and Raman scattering). This detection scheme is used to acquire high-precision single laser shot multispecies, polarization separated, and crank angle-resolved spectra in the combustion chamber of a firing spark-ignition (SI) engine. In addition, a combinative approach for cycle-resolved mixture composition and exhaust gas analysis is lemonstrated. In this approach, the air-fuel ratio before ignition and the NO LIF intensity in the burned gas are measured within the same engine cycle using the different polarization properties of Raman scattered and fluorescence light. The measurements are carried out in a standard production, four-cylinder SI engine that is modified only slightly to allow optical access. The engine data have not been changed by these modifications (e.g., the compression ratio is maintained at 10).

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.

Modern tools for diesel engine combustion investigation

This paper describes the operating principle and the current development status of high-speed direct-injection diesel engines used in passenger cars. This type of engine has an advantage in efficiency of the order of 30% compared with current gasoline engines with three-way catalyst and has a high potential for further improvement, such as reduction of NO and soot emissions. The present state of knowledge of diesel engine combustion has benefited from optical diagnostic techniques that permit deeper insights into the in-cylinder processes such as flow generation, fuel injection and spray formation, atomization and mixing, autoignition and combustion, and the formation and reduction of pullutants. Part of the work described was carried out within the European (diesel) engine research programs IDEA and IDEA EFFECT. Finally, a summary is given of future trends in combustion diagnostics and engine developments, with emphasis on the significance of the D1 diesel engine as a power plant for passenger cars both inside and outside of Europe.

Time-resolved emission spectroscopy for the combustion analysis of series production engines

This paper presents a device that detects light emerging from the combustion inside a series production automotive engine. Simulta- neous time and wavelength resolution is achieved by this system and it can be applied in a simple manner to either diesel or spark ignition (SI) engines without any geometrical modification of the combustion cham- ber. An optical probe is inserted into spark plug or glow plug. A fiber is connected to the probe and leads the light to a spectrograph, which provides spectral analysis in the UV and visible wavelength ranges. An intensified streak camera with time resolution in the microsecond range completes the detection unit. This measuring system enables time- resolved emission spectroscopy applied to the light emitted during the combustion in a series production engine. Time-resolved emission spec- tra are presented from both a diesel and an SI engine. The time behavior of the internal temperature in a diesel engine combustion chamber and its dependence on engine speed and load are measured with this setup using a multiple two-color method. In an SI engine, the time behavior of the emissions of specific molecules or radicals is detected. Thus, differ- ences in the combustion process are demonstrated to be caused by operation with different fuels.

Flow, spray and combustion analysis by laser techniques in the combustion chamber of a direct-injection diesel engine

Abstract The purpose of this paper is to show how the analysis of in -cylinder flow, fuel injection, and combustion by means of state-of-the-art optical techniques, as laser light-sheet, laser doppler anemometry and laser shadowgraphy, can help to support the understanding of the interaction of swirl flow development, spray formation, auto-ignition and combustion in near production-line direct-injection diesel engines and thus advances the development of engines with lower fuel consumption and emissions.