Power plants – Internal combustion engine with treatment or handling of... – Methods
Reexamination Certificate
2000-10-23
2003-02-11
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
Methods
C060S286000, C060S303000, C060S295000
Reexamination Certificate
active
06516607
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a method and a device for cleaning exhaust gas containing nitrogen oxides (NO
x
) from an internal combustion engine which includes at least one combustion space with an exhaust gas outlet, to which is connected an exhaust tract with an exhaust line and with at least one catalytic converter.
The combustion of hydrocarbons, such as occur, for example, in gasoline, with air in an internal combustion engine, in particular a diesel or gasoline engine, gives rise not only to carbon dioxide and steam as the main combustion products, but also to secondary products and pollutants. These are essentially hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxide (NO
x
). The amount of these pollutants which the exhaust gases contain depends, in particular, on the air/fuel ratio which prevails in the internal combustion engine. Low air/fuel ratios are referred to as “rich” mixture composition (air deficiency). A “lean” mixture composition (air excess) is due high air/fuel ratios.
It is known that, with an air deficiency, the exhaust gas contains a relatively large amount of CO and HC, whereas, in the case of an air excess, CO and HC may be oxidized almost completely. The NO
x
content passes through a maximum in the range of a slightly lean mixture composition. In this range, however, the specific consumption is also at an optimum for internal combustion engines, in particular for gasoline engines. If, therefore, for example gasoline engines are set at an optimally low consumption, there are high NO
x
concentrations, along with moderate CO and HC concentrations in the exhaust gas.
In particular, developments in traffic density in conurbations and metropoles have lead to the emission of the pollutants CO, HC and NO
x
being limited by law, first in the USA and later in Europe. The limits applicable at the present time, in particular for NO
x
, cannot be adhered to through the use of so-called primary measures, that is to say by optimizing the combustion process in order to avoid the formation of pollutants, in particular of thermal NO
x
. So-called secondary measures, in particular catalytic exhaust gas cleaning, are therefore employed in order to remain within these limits. Depending on the catalytic converter used, high conversion rates for all three of the above-mentioned pollutant components can be achieved in the case of a stoichiometric air/fuel ratio, but, in gasoline engines with lean mixtures and, in general, in diesel engines, the NO
x
fraction cannot be converted without additional measures.
For this purpose, it is known, for example from German Published, Non-Prosecuted Patent Application DE 40 03 515 A1, corresponding to U.S. Pat. No. 5,085,840, that, in order to reduce the NO
x
to N
2
, a reducing agent is introduced from a reducing agent reservoir through a reducing agent line into the exhaust tract and is intermixed with the exhaust gas flowing in the latter. The exhaust-gas/reducing-agent mixture is then subsequently supplied to a catalytic converter for the catalytic conversion of the NO
x
.
In this case, however, a stoichiometric ratio between NO
x
and reducing agent can be achieved only with difficulty because of fluctuating NO
x
concentrations in the exhaust gas: On one hand, continuous quantity metering of reducing agent at the level of NO
x
maxima which occur leads, below the NO
x
maxima, to an undesirable emission of reducing agent in the exhaust gas; on the other hand, a continuous quantity metering of reducing agent, for example at the level of an NO
x
average, leads, at the NO
x
concentrations in the exhaust gas which occur above this, to an undesirable emission of NO
x
.
In order to solve this problem, in particular to minimize the losses of reducing agent, catalytic converters have been developed, the coating of which can absorb to saturation, that is to say has a storage capacity for reducing agent and/or NO
x
. Using such a catalytic converter, the reducing agent can be introduced continuously in an average quantity into the exhaust tract. For example, overdoses of the reducing agent which occur are then stored in the catalytic converter and are discharged when NO
x
maxima occur later, and vice versa. However, for sufficient storage, this procedure necessitates a catalytic converter with a large storage volume.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method and a device for cleaning exhaust gas containing nitrogen oxide (NO
x
) from an internal combustion engine, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type and which are effective, in particular, even when a catalytic converter has a small storage volume.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of cleaning exhaust gas containing nitrogen oxide from an internal combustion engine having at least one combustion space with an exhaust gas outlet, and an exhaust tract with an exhaust line connected to the exhaust gas outlet and with at least one catalytic converter, the method which comprises:
introducing reducing agent from a reducing agent reservoir through a reducing agent line into the exhaust tract and intermixing the reducing agent with the exhaust gas flowing in the exhaust tract to form an exhaust-gas/reducing-agent mixture, whereby the reducing agent is metered more than once to the exhaust gas in dependence on an amount of NO
x
generated in the internal combustion engine, for each emission of exhaust gas from each combustion space, by at least one metering device and a further control device and/or regulating device, and thereby adapting portions of the reducing agent to the amounts of NO
x
respectively generated; and
supplying the exhaust-gas/reducing-agent mixture to a catalytic converter for catalytically converting the NO
x
.
With the above and other objects in view there is also provided, in accordance with the invention, a device for cleaning exhaust gas of an internal combustion engine containing nitrogen oxide, wherein the engine has at least one combustion space with an exhaust gas outlet, and an exhaust line of an exhaust tract is connected to the exhaust gas outlet, comprising:
a reducing agent reservoir and a reducing agent line connecting the reducing agent reservoir to the exhaust tract;
at least one metering device for introducing a reducing agent from the reducing agent reservoir through the reducing agent line into the exhaust tract and for intermixing the reducing agent with an exhaust gas flowing in the exhaust tract;
a catalytic converter connected in the exhaust tract for receiving an exhaust-gas/reducing-agent mixture for a catalytic conversion of nitrogen oxide produced in the internal combustion engine; and
a control device connected to the metering device and configured to meter the reducing agent to the exhaust gas more than once by portion during each working stroke in dependence on a NO
x
content generated by the internal combustion engine.
In other words, the objects of the invention are satisfied in that the reducing agent is metered to the exhaust gas flowing in an exhaust tract, as a function of the generation of NO
x
by the internal combustion engine, through the use of a metering device and a further control device in synchronism with and/or out of phase with the respective emission of exhaust gas from a combustion space and in portions adapted to the NO
x
quantities generated, in particular in synchronism with and/or out of phase with the working stroke of the individual combustion spaces or in synchronism with and/or out of phase with a complete working stroke of the internal combustion engine. Such metering almost completely avoids overdoses of the reducing agent in the exhaust gas, as a result of which, for example, a stoichiometric ratio between NO
x
and reducing agent is achieved and cleaning exhaust gas containing NO
x
in an internal combustion engine is advantageously improved.
Mor
Brück Rolf
Maus Wolfgang
Denion Thomas
EMITEC Gesellschaft fuer Emissionstechnologies mbH
Greenberg Laurence A.
Locher Ralph E.
Stemer Werner H.
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