Christof Schulz

Innovative Ultra-low NOx Controlled Auto-Ignition Combustion Process for Gasoline Engines: the 4-SPACE Project

The purpose of the 4-SPACE (4-Stroke Powered gasoline Auto-ignition Controlled combustion Engine) industrial research project is to research and develop an innovative controlled auto-ignition combustion process for lean burn automotive gasoline 4-stroke engines application. The engine concepts to be developed could have the potential to replace the existing stoichiometric / 3-way catalyst automotive spark ignition 4-stroke engines by offering the potential to meet the most stringent EURO 4 emissions limits in the year 2005 without requiring DeNOx catalyst technology. A reduction of fuel consumption and therefore of corresponding CO2 emissions of 15 to 20% in average urban conditions of use, is expected for the « 4-SPACE » lean burn 4-stroke engine with additional reduction of CO emissions. This paper describes the first set of results of different experimental and numerical studies aiming to get such new combustion process in 4-stroke engines within the framework of this European consortium. One of the target of this consortium driven by IFP, is to develop a 4-stroke gasoline engine running conventionally at high load (with a normal compression ratio and without any intake air heating) and able to achieve Controlled Auto-Ignition (CAI) process at part load by reproducing the 2-stroke internal conditions (internal EGR rate and fluid dynamic control, temperature level...) favorable to this particular combustion process. For this purpose and as a starting point of the work program, a production 2-stroke engine known for its part load auto-ignition behavior is fully studied. Such work is focused on the analysis of in-cylinder conditions prior to auto-ignition using combined experimental testing, 3D CFD computations and optical diagnostics. From this analysis, 1D CFD computations have been extensively performed to evaluate the possible 4-stroke concepts able to reproduce internal conditions favorable to CAI. Then, the most “promising” configurations have been experimentally investigated. Encouraging preliminary results have already shown that NOx emissions are reduced by 10 to 40 times and the fuel economy is improved by 8 to 10% when compared with stoichiometric reference conditions. Other ways of getting auto-ignition of the lean fresh mixture are also explored by the project partners. The effects of several parameters, such as the fuel composition, the engine compression ratio, the intake air temperature level, etc... are also included in the research program. Thus, to analyze better analyze intrinsic autoignition process, specific tools as for example Rapid Compression Machine have been developed. Different fuels at various initial conditions (e.g. temperature, excess air) have been tested and compared, for example in terms for example of combustion rate and auto-ignition delay. Results obtained contribute to the better understanding of the auto-ignition process. Preliminary visualization results from specially designed single cylinder engines (2-stroke and 4-stroke) have been obtained for controlled auto-ignition combustion. The effect of charge stratification is briefly discussed.

Absorption and fluorescence of toluene vapor at elevated temperatures

Absorption and fluorescence of the S0 → S1 (π,π*) transition in toluene are studied in the temperature range 300 K to 1130 K and 300 K to 930 K, respectively. Experiments are conducted in a shock-tube and in a heated flow-cell. Fluorescence spectra are investigated after excitation at 248 nm and 266 nm using a nitrogen diluent at a total pressure of 1 bar. Over the temperature range studied the fluorescence quantum yield decreases exponentially by three orders of magnitude for 266 nm excitation and double exponentially by three orders of magnitude for 248 nm excitation. The fluorescence spectrum shifts to the red with increasing temperature. The vibrational structure of the absorption spectrum found at ambient conditions vanishes above 600 K. The absorption feature broadens and the maximum shifts to the red. Taking advantage of the distinctive temperature dependence of the fluorescence, we suggest potential techniques using toluene as a sensitive tracer molecule for temperature imaging in both homogeneously and inhomogeneously mixed flow-fields.

Simultaneous single-shot laser-based imaging of formaldehyde, OH, and temperature in turbulent flames

Single-shot formaldehyde (CH2O) laser-induced fluorescence (LIF) imaging measurements in turbulent flames were performed using XeF excimer laser excitation in the 410 transition of the A1A2-1A1 electronic system at 353 nm. Background contributions to the CH2O LIF detection were assessed in spectroscopic measurements. Simultaneous two-dimensional mapping of OH LIF, CH2O LIF, and temperature fields was carried out in a standard Bunsen flame. The zones of peak heat release rate localized via the product of OH and CH2O LiF intensities correlated with areas of intermediate temperatures. In addition, single-shot imaging of the transient formaldehyde distribution was performed in a 150 kW natural gas swirl burner. Formaldehyde distributions in strongly turbulent swirl, flames differed significantly from thin layers found in laminar and weak turbulent flames, indicating the presence of low-temperature chemistry in preheated gas pockets. The CH2O distribution measured in the swirl flame was compared with averaged fields of temperature, OH, and NO concentrations.

Laser-induced incandescence for soot diagnostics at high pressures

The influence of pressure on laser-induced incandescence (LII) is investigated systematically in premixed, laminar sooting ethylene/air flames at 1-15 bar with wavelength-, laser fluence-, and time-resolved detection. In the investigated pressure range the LII signal decay rate is proportional to pressure. This observation is consistent with the prediction of heat-transfer models in the free-molecular regime. Pressure does not systematically affect the relationship between LII signal and laser fluence. With appropriate detection timing the pressure influence on LII signals proportionality to soot volume faction obtained by extinction measurements is only minor compared with the variation observed in different flames at fixed pressures. The implications for particle sizing and soot volume fraction measurements using LII techniques at elevated pressures are discussed.

Plasma synthesis of nanostructures for improved thermoelectric properties

The utilization of silicon-based materials for thermoelectrics is studied with respect to the synthesis and processing of doped silicon nanoparticles from gas phase plasma synthesis. It is found that plasma synthesis enables the formation of spherical, highly crystalline and soft-agglomerated materials. We discuss the requirements for the formation of dense sintered bodies, while keeping the crystallite size small. Small particles a few tens of nanometres and below that are easily achievable from plasma synthesis, and a weak surface oxidation, both lead to a pronounced sinter activity about 350 K below the temperature usually needed for the successful densification of silicon. The thermoelectric properties of our sintered materials are comparable to the best results found for nanocrystalline silicon prepared by methods other than plasma synthesis.

VCSEL-based, high-speed, in situ TDLAS for in-cylinder water vapor measurements in IC engines

We report the first application of a vertical-cavity surfaceemitting 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.

Strategies for laser-induced fluorescence detection of nitric oxide in high-pressure flames. II. A–X(0,1) excitation

A-X(0,1) excitation is a promising new approach for NO laser-induced fluorescence (LIF) diagnostics at elevated pressures and temperatures. We present what to our knowledge are the first detailed spectroscopic investigations within this excitation band using wavelength-resolved LIF measurements in premixed methane/air flames at pressures between 1 and 60 bar and a range of fuel/air ratios. Interference from O2 LIF is a significant problem in lean flames for NO LIF measurements, and pressure broadening and quenching lead to increased interference with increased pressure. Three different excitation schemes are identified that maximize NO/O2 LIF signal ratios, thereby minimizing the O2 interference. The NO LIF signal strength, interference by hot molecular oxygen, and temperature dependence of the three schemes are investigated.

Laser-induced-fluorescence detection of nitric oxide in high-pressure flames with A–X(0, 2) excitation

Laser-induced fluorescence techniques have been used successfully for quantitative two-dimensional measurements of nitric oxide. The commonly applied D-X(0, 1) or A-X(0, 0) schemes are restricted to atmospheric-pressure flames and engines driven with gaseous fuels because of strong attenuation of the exciting laser beam by combustion intermediates. The properties of a detection scheme for which excitation in the nitric oxide A-X(0, 2) band was used were investigated. We discuss the advantages of the A-X(0, 2) system (excited at 247.95 nm) based on measurements in laminar premixed methane/air flames at 1-40 bars.

Ultraviolet absorption spectra of shock-heated carbon dioxide and water between 900 and 3050 K

Abstract Spectrally resolved UV absorption cross-sections between 190 and 320 nm were measured in shock-heated CO 2 between 880 and 3050 K and H 2 O between 1230 and 2860 K. Absorption spectra were acquired with 10 μs time resolution using a unique kinetic spectrograph, thereby enabling comparisons with time-dependent chemical kinetic modeling of post-shock thermal decomposition and chemical reactions. Although room temperature CO 2 is transparent (σ −22 cm 2 ) at wavelengths longer than 200 nm, hot CO 2 has significant absorption (σ>10 −20 cm 2 ) extending to wavelengths longer than 300 nm. The temperature dependence of CO 2 absorption strongly suggests sharply increased transition probabilities from excited vibrational levels.

NO-flow tagging by photodissociation of NO2. A new approach for measuring small-scale flow structures

Abstract A new flow tagging technique was developed which visualizes small-scale flow structures by writing a spatial line of NO into an air flow homogeneously seeded with NO2. The NO-line was generated by photodissociation of NO2 at 308 nm using a XeCl excimer laser and was imaged by planar laser-induced fluorescence at various delays after its formation. With seeding levels of 600 ppm, signal-to-noise levels are shown to be sufficient for detecting the shifted structure at delays of up to 20 ms. The technique was used to resolve small scale turbulent flow structures within a quartz cell.