This application claims the priority of German Patent Document 100 38 744.6, filed in Germany, Aug. 9, 2000, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method and to an arrangement for reducing warm-up emissions of a direct-injection internal-combustion engine.
The pollutant emission of Otto cycle engines can be effectively reduced by a catalytic aftertreatment. This essentially involves the complete burning of the not yet completely burnt fuel. A catalyst promotes the afterburning of reactive CO (carbon monoxide) and HC (hydrocarbons) to harmless carbon dioxide (CO2) and water (H2O) and simultaneously reduces nitrogen oxides (NOx) occurring in the exhaust gas to neutral nitrogen (N2).
A three-way catalyst, for example, is customary which decomposes all three pollutants CO, HC and NOx simultaneously. It has a tube structure made of ceramics which are coated with precious metals, preferably platinum and rhodium, the latter metals accelerating the chemical decomposition of the pollutants.
The catalytic three-way method requires the mixture to have a stoichiometric composition. A stoichiometric mixture composition is characterized by an air ratio of xcex=1.00. At this mixture composition, the catalyst operates with a very high efficiency. A deviation of only one percent already considerably impairs the efficiency of the pollutant conversion.
For a use in the mixture control, the known xcex-probe supplies a signal concerning the momentary mixture composition to the control unit. The xcex-probe is installed in the exhaust pipe of the engine at a point at which the exhaust gas homogeneity required for the operation of the system exists over the entire operating range of the engine.
In order to completely utilize the consumption potential, particularly of a direct-injection Otto engine, it is necessary to operate the engine with an excess of air, thus, in a lean fashion (xcex greater than 1). However, in this lean operating mode, the known three-way catalysts are ineffective because they can no longer convert the nitrogen oxides (NOx).
As another possibility for removing NOx from the lean exhaust gas, the NOx storage catalyst known per se represents a promising alternative. In the new condition of such an NOx storage catalyst, exhaust gas limit values can be maintained which are known and valid today. However, NOx storage catalysts require a regular detoxification (desulfatization) in order to maintain their efficiency.
Known precatalysts or starting catalysts (VK), as they are required for meeting strict exhaust gas limit values in the warm-up phase, represent a serious impairment of the operation of such an NOx storage catalyst. As a result of their system-caused oxygen storage capacity, they impair the regeneration phases of the NOx storage catalyst and cause high additional costs for the also required desulfatization processes while the construction expenditures are high.
As an alternative, electrically heatable catalysts are known which permit the elimination of precatalysts. As a result of the high required electric power and the type of construction, these electrically heated catalysts also cause high indirect and direct costs.
It is an object of the present invention to provide a method and an arrangement for reducing warm-up emissions of a direct-injection internal-combustion engine, whereby the starting temperature of a catalyst device can be reached in a rapid and effective manner.
This object is achieved according to preferred embodiments of the invention by providing a method for reducing warm-up emissions in the exhaust system of a direct-injection internal-combustion engine, characterized by adjusting a lean air/fuel mixture for the engine-related combustion, carrying out a late injection into the exhaust gas during the exhaust cycle of the internal-combustion engine, so that the exhaust gas contains an essentially stoichiometric air/fuel mixture, and igniting the stoichiometric air/fuel mixture contained in the exhaust gas in the proximity of a catalyst device for heating the catalyst device.
This object is also achieved according to preferred embodiments of the invention by providing an arrangement for reducing warm-up emissions in the exhaust system of a direct-injection internal combustion engine, characterized by an adjusting device for adjusting a lean air/fuel mixture for the engine-related combustion, an injection device for implementing a late injection into the exhaust gas during the exhaust cycle of the internal-combustion engine, so that the exhaust gas contains an essentially stoichiometric air/fuel mixture, and an ignition device for igniting the stoichiometric air/fuel mixture contained in the exhaust gas in the proximity of the catalyst device for heating the catalyst device.
The method according to the invention for reducing warm-up emissions having the above-mentioned features and the corresponding arrangement have the advantage that, while the efficiency is the same as that of known systems, the precatalyst as well as the secondary air pump can be eliminated. Furthermore, the heating device absorbs only a fraction of the electric energy of the electrically heated catalysts.
The present invention utilizes the possibility limited to direction-injection engines of adding fuel again to the burnt exhaust gas during the exhaust phase or discharge phase, thus when the outlet valve is open, which is also called a late injection. Another advantage of such a direction injection is the fact that the composition of the air-fuel mixture, thus the xcex-value, can be freely selected for generating the engine power within system-dependent limits. It is differentiated in this case between the xcex-value in the engine (xcexM), which is the mixture composition during the combustion taking place for generating the engine power, and the xcex-value of the catalyst (xcexK), which is the mixture composition effective in the catalyst.
The invention suggests that, immediately after the cold start, the xcexM be selected to be so lean that, after the engine-related combustion, a significant O2 concentration remains in the exhaust gas; that is, xcexM is much greater than 1. As a result of the late injection after the opening of the outlet valve, an amount of fuel is added to the exhaust gas which corresponds to the O2 concentration, so that an essentially stoichiometric exhaust gas composition (xcexK=1) is present in the catalyst, so that, at this point in time, the exhaust gas, in addition to the oxygen, also carries along a corresponding amount of fuel.
The heating device is preheated to such high temperatures that it is suitable for igniting the stoichiometric air/fuel mixture fed in the exhaust gas again in the direct proximity of the catalyst device.
Advantageous features and further developments and improvements of the invention are described herein and in the claims.
According to a further development of preferred embodiments of the invention, the catalyst device has a first catalyst close to the engine and a second catalyst away from the engine which are arranged behind one another. The ignition takes place between the two catalysts. As a result of the placing of the ignition arrangement behind the first monolith, the heating device is protected from excess fluctuations of pressure and temperature, and the use of the second monolith as the starting catalyst takes into account its thermal stress which is low anyhow. The heating element is used for igniting the combustible air/fuel mixture, which is supplied by the engine, in the direct proximity of the second monolith, so that it is used for the heating of the second monolith and heats the latter, in the case of a corresponding coordination, within a short time, to a light-off temperature of approximately 250xc2x0 C.
According to another feature of preferred embodiments of the invention, the first catalyst is an NOx storage catalyst.
According to another feature of preferred embodiments of the invention, the second catalyst is an NOx storage catalyst or a 3-way catalyst.
According to another feature of preferred embodiments of the invention, the ignition is carried out by an ignition arrangement arranged in the exhaust gas flow, which ignition arrangement expediently has a frame, which is wound with a catalytically coated heating wire and can be installed into an exhaust gas train of the exhaust system transversely to the flow axis of the exhaust gas, and a power supply device for supplying the heating wire with electric current for generating a predetermined heating wire temperature. It may include, for example, a resistance wire which is arranged in a grid shape and is coated with platinum as the catalyst material. As a result of the lower heat capacity resulting from the low mass, the electric power required for the heating, even at a high temperature of the heating element, is considerably lower than would be required for heating an entire monolith.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.