Internal-combustion engines – Charge forming device – Combustible mixture ionization – ozonation – or electrolysis
Reexamination Certificate
2001-05-15
2002-10-22
McMahon, Marguerite (Department: 3747)
Internal-combustion engines
Charge forming device
Combustible mixture ionization, ozonation, or electrolysis
Reexamination Certificate
active
06467467
ABSTRACT:
The invention relates to the plasma-chemical reduction of gaseous and/or solid pollutants in exhaust gases of internal combustion engines by using dielectric-barrier discharges, and to a device intended for this purpose.
In the scope of tightening the standards for the limitations on pollutant emissions during the operation of internal combustion engines, in particular in motor vehicles, new technologies have also been proposed in addition to the improvement of conventional, in particular catalytic, methods.
Dielectric-barrier discharge (hereafter abbreviated to DBD) in a plasma, the physical principles of which have already been known for a long time, is highly promising in this regard. The plasma-is generated by applying an alternating voltage or pulsed voltage between two electrodes; if a dielectric is arranged between the electrodes, however, it is not possible to form a steady-state discharge but only discharges which become quenched again after a short time, so-called silent transient discharges.
From Rosocha, Louis A. et al.: “Innovative Technologies for Removing Toxic Compounds from Groundwater and Air”; New Mexico Conference on the Environment, Sep. 13—15, 1992, Albuquerque, N. Mex., it is known that hydrocarbons can be cleaned from mixtures with air or argon/oxygen in the mass ratio 80:20. Reduction of the concentration of trichloroethene, trichloroethane, CCl
3
CF
3
, CCl
4
and aliphatic hydrocarbons is described as an example. Using only a few watts of electrical power at a flow rate of 10l/m (corresponding to a few tens of J/l), with a saturated water content in an argon/oxygen mixture (argon/oxygen mass ratio =80:20) it was possible, for example, to clean trichloroethene from 650 ppm to values below 1 ppm, and with powers of 1 k
j
/l to below 1 ppb. Using a power input of a few tens kWh/kg, trichloroethane at initial concentrations in the percent range could be reduced to ppm ranges, and aliphatic hydrocarbons with initial concentrations of from 1000 ppm to 3000 ppm and CCl
3
CF
3
with an initial concentration of 200 ppm could be reduced by 80% to 90%.
Dhali, S. K. and Sardja, I.: “Dielectric-barrier Discharge for Processing of SO
2
, NO
x
”; J. Appl. Phys. p69 (9), May 1, 1992 describes studies of the treatment of conventional inorganic pollutants, in particular nitrogen oxides and sulfur dioxide. According to this article, nitrogen monoxide could be fully oxidized and, depending on the conditions used, sulfur dioxide could be cleaned by 40%-70%.
The term gaseous pollutants is here intended to mean undesired substances, in particular those defined in exhaust gas standards, primarily hydrocarbons, nitrogen oxides and sulfur dioxide. An example of a solid pollutant, especially in emissions from diesel engines, is soot.
DE-A-42 31 581 describes a device and a method for the plasma-chemical decomposition and/or elimination of pollutants using a DBD electrode arrangement to which the pollutants are fed, wherein all the operating parameters of the plasma-chemical reactions, for example temperature, nature and mass flow rate of impurities, electrical power and dielectric constant of the dielectric, are intended to be controlled in such a way that the desired decomposition of the pollutants is maximized. Said document relates, however, only to exhaust-gas post-treatment by means of DBD.
In relation to this, it is an object of the invention to ensure further reduction of pollutants by means of DBD, with the intention that the pollutants are reduced not only by exhaust-gas post-treatment but, primarily, by avoiding the formation of pollutants. It is also an object to provide a method and a device which ensure balancing of the combustion process in internal combustion engines and reduction of the fuel consumption.
The invention is based on a method for the plasma-chemical reduction of gaseous and/or solid pollutants in exhaust gases of internal combustion engines by using dielectric-barrier discharges.
The solution in accordance with the method according to the invention is then characterized in that the dielectric-barrier discharges are carried out in the combustion space of the internal combustion engine.
It has been found that DBDs can economically be performed directly in the combustion space of the internal combustion engine.
When a strong electric field is applied to electrodes that have a gas between them, free electrons are accelerated and, when a threshold field strength is exceeded, they can excite and ionize heavy particles (atoms, molecules). The primary electrons originate from the natural background electron density in the gas, which is due to natural radioactivity and cosmic radiation.
The released electrons are in turn accelerated, and avalanche-type growth of the electron density occurs, together with gas breakdown (Raether breakdown). By overlapping successive avalanches, a high-conductivity discharge channel (streamer) is created, which leads to the formation of a spark or arc. When the discharge is formed in this way, if metallic electrodes are present the gas is strongly heated as the flow of current continues, and a plasma in local thermal equilibrium is created.
If at least one dielectric barrier (whence dielectric-barrier discharge, DBD) is present in the gas space, then the internal field is reduced as the discharge continues, by local build-up of charges on the surfaces of the dielectrics, to such an extent that the discharge becomes quenched again, i.e. it does not change into a thermally relaxing plasma with hot sparks and arcs. By applying a pulsed voltage, the energy input into the polluting gas can be dictated externally by means of the voltage amplitude, frequency, pressure etc.
Dielectric-barrier discharges, or silent discharges, are hence transient glow discharges that can be operated even at standard pressure. The electrodes are in this case separated from the filler gas by suitable dielectrics, and a large number of different reaction-chamber geometries (plate, tube, coaxial and multi-chamber geometries) can readily be implemented, which permits greater flexibility in terms of the installation position.
For use in order to reduce pollutants in exhaust gases, it is important for the DBD to be self-terminating, i.e. the energy input into the exhaust-gas plasma takes place during the period of the active current pulse (<1 &mgr;s).
In this case, although high electron temperatures in the range of from 10,000 K to 100,000 K are reached, depending e.g. on the pressure, reactor geometry, voltage and frequency the heavy-particle temperature nevertheless reaches substantially lower values owing to the low ion mobility and the short pulse period.
Thermal non-equilibrium hence exists, which is very important for the treatment of pollutants in exhaust gases, since (with suitable cooling) treatment only slightly above room temperature is possible.
The pollutants are accordingly disassociated not thermally, but primarily by collisions with electrons (average electron energy from 1 to 10 eV). The effectiveness of the method relies on the high electron densities (10
18
cm
31 3
) that can be produced in this case. Furthermore, the high-energy photons (in the vacuum-UV and UV range) that are emitted during the discharge additionally contribute to the disassociation or excitation by means of secondary processes.
The application of a high voltage causes pollutant particles in the gas flow to be ionized, with the released electrons in turn contributing to the ionization or excitation. This leads to avalanche-type growth of the electron density and gas breakdown, in conjunction with the emission of photons due to the transition of excited particles to their ground state. Therefore, both electrons (primarily) and photons (secondarily) contribute to the exhaust-gas treatment by means of DBD. In this context, it should be noted that higher energies can in principle be transferred during collisions with electrons than in the case of photons, and that new species, such as super-excited neutral or charged particles can be created.
The precise mechanism
Foley & Lardner
McMahon Marguerite
Mitsubishi Aluminum Co., LTD
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