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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