Legals ¦ KIT


Engler-Bunte-Ring 1
76131 Karlsruhe 

Building 40.13.II 


Phone: +49(0)721 608 7078
Fax: +49(0)721 661501


E-mail: Sekretariat

Events of SFB 606

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TP - C5
Instationäre Ruß- und Speziesverteilung bei Einspritzvorgängen flussiger und gasförmiger Brennstoffe unter motorähnlicher Bedingungen (externes Projekt)
project finished

PD Dr. Thomas Dreier
CH - 5232 Villigen
Telefon: 0041 (0) 563104471
Telefax: 0041 (0) 563102199

Ergebnisse des Teilprojekts Veröffentlichungen


Due to environmental concerns the reduction of toxic exhaust gas (i.e., NOx, CO, etc.) and particulate (i.e., soot) emissions from car engines still remains a demanding task. Our basic understanding of in-cylinder combustion processes in car engines is a prerequisite for improving combustion efficiency and understanding the sources of these emissions. The proposed research work will focus on three aspects of Diesel combustion for engine- like conditions:

-     Spatially and temporally resolved quantitative planar soot imaging during high pressure spray combustion.

-     Effect of oxygen content and fuel chemical structure on soot formation tendency using various Diesel fuel blends

-     Detection of intermediate species and temperature in the pre-ignition period of lean fuel/air mixtures at high pressures.

In the proposed research work we focus on multiscalar simultaneous imaging using combinations of two double-framing cameras and laser techniques.

Measurements will be performed in a high- pressure high-temperature cell (HTDZ) with optical access, valve controlled gas in- and outlet and electronic fuel injection, which can be operated at pressures and temperature up to 8 MPa and 800 K, respectively. Instantaneous in-situ, quantitative soot diagnostics during spray combustion will be accomplished using RAYLIX – the simultaneous application of laser-induced incandescence (LII), Rayleigh scattering and extinction. The technique requires the coplanar alignment of two laser light sheets through the probe volume, thus allowing the planar imaging of soot volume fraction, number density and mean soot particle radius with high spatial and temporal resolution. The spatial discrimination of soot residing in regions of high and low temperature, respectively, will be achieved through simultaneous imaging of LII and soot blackbody emission using two intensified CCD cameras.

To study the influence of fuel structure and fuel oxygen content on the soot formation tendency measurements will be performed with commercial Diesel fuel and fuel blends containing oxygen-enriched components, such as isobutanol, DME, butylal etc..

The fate and spatial distribution of reactive intermediates during the pre-ignition stage of high pressure, high temperature fuel/air mixtures is important for a better understanding of engine combustion chemistry during the compression stroke. In the project laserinduced fluorescence (LIF) will be applied for the detection of minor concentrations of formaldehyde (H2 CO) and methoxy (CH3 O) radicals, which are formed as important intermediate species in the cool flame combustion stage after liquid or gaseous fuel injection into high pressure, high temperature air.

As an associated project to SFB 606, the current work should provide valuable knowledge and data for sub-projects A3 ( Selbstzündungsprozesse bei instationären Freistrahlen), B1 (Rußbildung in nicht-adiabaten, instationären Flammen in laminarer und turbulenter Strömung unter Berücksichtigung der Flamme-Wand-Interaktion) and C4 (Rußbildung in DI- Motoren). The combined knowledge gained in each of these subtasks will advance knowledge in engine combustion in a large parameter range and accelerate possible improvements made to engine design.


3.3 Stand der Forschung

The most important technologies for energy conversion until now and in the near future still rely on the combustion of fossil fuels in engines, gas turbines and technical combustors. In most applications the thermal energy is generated in high temperature and pressure environments, which makes detailed investigations more difficult.

Main concerns in the massive use of combustion as an energy conversion process are environmental pollution (soot, UHC emissions, NOx, SOx) and global climate changes introduced through the exhaust of greenhouse gases. Optimising engines/combustors with respect to efficiency, fuel savings and toxic emissions are the main objectives in combustion engineering. This aim, however, only can be reached through a better understanding of basic physical and chemical phenomena in the fuel conversion process from mixing until exhaust gas emission.


Soot formation

The microscopic mechanisms governing soot formation from the growth of small hydrocarbon chains to larger PAHs (the building blocks for particulates in flames), their subsequent agglomeration and condensation, until destruction by oxidation, still is not fully understood [1, 2, 3]. Soot formation routes involving radicals as well as ionic species are discussed in the literature [4]. The importance of initial conditions on the amount of soot being formed, i.e., mixture fraction [5, 6], temperature [7], fuel chemical structure and composition [8, 9], radical pool [10] and oxygen content in the fuel [11, 12,13], has early been recognized. For the validation of chemical kinetic mechanisms of soot formation/destruction in Diesel engine combustion optical measurements in laminar and turbulent flames at high pressure are of special interest.

Systematic in-situ experimental investigations of soot formation and burnout during fuel injection and combustion for Diesel engine-like conditions are scarce. Partly, this is due to difficulties in adapting optical measurement techniques to real engines without significant limitations in the quality of the obtained data. In addition, operating parameters often can not be varied independently thus inhibiting the investigation of correlations between different reactive and non-reactive scalars. Hampson et al. [14] applied time resolved two-color pyrometry as a line-of-sight imaging technique of soot concentration and temperature in an optical accessible Diesel engine. Pioneering work on Diesel spray combustion diagnostics has been performed by Dec et al. using laser-induced incandescence (LII) imaging of spatial soot distributions [15], and OH laser-induced fluorescence (LIF) for flame front location in an optically accessible Diesel engine [16]. The authors developed a constistent model describing the physical chemical mechanisms of fuel evaporation and combustion during a typical Diesel engine cycle [17]. Recently, LII measurements were conducted in a rapid compression facility with Diesel fuel to investigate the effect of in-cylinder pressure (< 9 MPa) and fuel injection pressure (< 160 MPa) on the spatial location and structure of soot formation centers [18]. Similar experiments were conducted by Kosaka et al. in a high pressure spray chamber [19]. Radiation effects on soot emission were investigated by Liu et al. [20] by comparing results from model calculations and experimental data obtained in atmospheric pressure co-flowing diffusion flames.

The simultaneous and quantitative mapping of total soot and soot residing in hot flame regions within a high pressure liquid fuel ignition process has not been performed yet for combustion processes close to Diesel engine conditions. Therefore, one of the tasks in the present proposal is aimed at the measurement of both soot classes via LII [21, 22] and luminescence imaging in a high pressure fuel injection combustion vessel. The combined results of these individual studies as well as work performed in other sub tasks of SFB 606 will provide a better understanding of the temporal and spatial evolution/ destruction of soot during Diesel combustion.


Alternative fuels in engine applications:

As already discussed, the modification of fuel composition by additives is one approach to significantly reduce particulate matter emission from Diesel engine exhausts. Experimentally, this combustion strategy mostly was investigated by performing exhaust analysis [23, 24], in-cylinder optical transmission measurements, or pulsed gas sampling. The compound mixtures studied contained low molecular weight alcohols, dimethyl and diethyl carbonates, dimethyl ether (DME), dimethoxy methane (DMM) and others. Within the community, however, opinion is divided as to the role of the oxygenated species chemical nature in the reduction of soot levels. In several studies it was shown that the admixture of acetals to Diesel reduces the particulate emission up to 90% when used with conventional car engines [25, 26]. Recently, a modeling study has investigated the effects of the chemical nature of oxygenated fuel additives on the peak mole fraction of aromatic species (up to pyrene), considered to be responsible for final soot production and growth [27]. Since soot levels in the exhaust streams will also be strongly influenced by fuel oxygen levels and burnout within the cylinder, the variation of fuel composition will give further insights into the chemical nature of soot formation and destruction mechanisms at high pressures. The in-situ quantitative analysis of soot spatial distribution during Diesel spray combustion for these conditions has not been done yet in great detail, which motivates this topic as a sub-task in the present proposal.


Minor species, lean combustion:

The fate and amount of toxic emissions (i.e., NOx) is mainly determined by the local temperature, pressure and state of mixing in the preheating zone. To cope with these problems new and promising engine designs have emerged such as direct fuel injection systems with spark ignition (DI, DISI engines), as well as Diesel concepts utilizing stratified or homogeneous cylinder charging (DISC, HCCI). In HCCI operation the well mixed lean fuel/air charge leads to lower temperatures with accompanying low NOx and soot emission. However, this also leads to more unpredictable ignition behaviour with increased knock tendency as well as higher concentration levels of unburned HC and CO – factors not desirable for driving comfort, prolonged engine lifetime and fuel economy [28, 29]. The determination of ignition delay times for engine-like conditions of temperature and pressure for representative Diesel fuel compounds, such as longchain hydrocarbons performed, e.g. in rapid compression facilities, is one measure to gain information on the low temperature chemistry [30, 31, 32]. Recently, model calculations were conducted to assess the effects of temperature, dilution and the amount of exhaust gas recirculation on the flammability limits in HCCI operation [33]. Additionally, fuel chemical structure and composition also affect the knocking behaviour, i.e., the two-stage ignition processes of the diluted charge, and it was shown that ignition delay and burn rate can be independently controlled [34]. At low temperature two-stage ignition takes place and ignition delay increases with increasing temperature owing to the negative temperature dependence of chain branching and chain propagation reactions [35]. Chemiluminescence emissions spectra from the “cool flame” and the final combustion regime in such engines have recently been obtained from a HCCI engine [36]. Measurements of minor species in engine combustion and flames at high pressure are limited and mainly concentrate on formaldehyde (H2CO), OH, CN and CH [37, 38, 16]

In a further subtask in the proposal, we plan the spatially resolved detection via laserinduced fluorescence of H2CO or OH as important intermediate species in the preignition stage of gaseous air-fuel mixtures in a high pressure combustion vessel. This challenging task will provide a unique opportunity for a better understanding of the chemistry involved in these processes.



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[18]   C. Crua, D. A. Kennaird, M. R. Heikal: Combust. & Flame 135, 475 (2003)

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[20]   F. Liu, H. Guo, G. J. Smallwood, Ö. L. Gülder: Effects of gas and soot radiation on soot formation in a coflow laminar ethylene diffusion flame. J. Quant. Spectrosc. Rad. Transf. 73, 409 (2002)

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[27]   K. Hoon Song, P. Nag, A. Litzinger, D. C. Haworth: Effects of oxygenated additives on aromatic species in fuel-rich, premixed ethane combustion: a modeling study. Combust. & Flame 135, 341 (2003)

[28]   A. A. Pekalski, J. F. Zevenbergen, H. J. Pasman, S. M. Lemkowitz, A. E. Dahoe, B. Scarlet: J. Hazardous Mat. 93, 93 (2002)

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[32]   J. F. Griffiths, Q. Jiao, W. Kordylewski, M. Schreiber, J. Meyer, K. F. Knoche: Experimental and numerical studies of ditertiary butyl peroxide combustion at high pressures in a rapid compression machine. Comb. & Flame 93, 303 (1993)

[33]   Y. Huang, C.J. Sung, J.A. Eng: Dilution limits of n-butane/air mixtures under conditions relevant to HCCI combustion. Combustion & Flame 136, 457 (2004)

[34]   S. Tanaka, F. Ayala, J. C. Keck, J. B. Heywood: Two-stage ignition in HCCI combustion and HCCI control by fuels and additives. Comb. & Flame 132, 219 (2003)

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[37]   H. Kosaka, V. H. Drewes, L. Katalfamo, A. A. Aradi, N. Iida, T. Kamimoto: Two-dimensional imaging of formaldehyde formed during the ignition process of a Diesel fuel spray. SAE Technical Paper Series 2000-01-0236

[38]   T. D. Fansler, B. Stojkovic, M. C. Drake, M. E. Rosalik: Local fuel concentration measurements in internal combustion engines using sparkemission spectroscopy. Appl. Phys. B 75, 577 (2002)


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