Exhaust gas purification system of an internal combustion engine and method for purifying the exhaust gases thereof

Abstract

An exhaust gas purification system for purifying the exhaust gas of an internal combustion engine, especially an internal combustion engine featuring auto-ignition and / or direct fuel injection, is provided. The system includes at least one oxidation catalytic converter disposed in an exhaust gas duct of the internal combustion engine, at least one device for selective catalytic reduction of the exhaust gases which is installed downstream of the oxidation catalytic converter, and a feed device for feeding reducing agent into the exhaust gas stream upstream of, or in, the device for selective catalytic reduction. A switch-over device is provided for selectively feeding reducing agent into the exhaust gas stream upstream of, or inside, the at least one oxidation catalytic converter. A corresponding method for purifying the exhaust gas of an internal combustion engine is also provided.

Description

FIELD OF THE INVENTION [0001] The present invention relates to an exhaust gas purification system of an internal combustion engine having a device for selective catalytic reduction, and relates particularly to a method for purifying exhaust gases of an internal combustion engine, in which an exhaust gas stream is passed through a device for selective catalytic reduction. BACKGROUND INFORMATION [0002] To lower the nitrogen oxide content of oxygen-rich exhaust gas, such as that emitted by diesel internal combustion engines and by internal combustion engines featuring direct gasoline injection, it is known to introduce a reducing agent into an exhaust tract. A suitable reducing agent is, for example, NH3, which may be introduced as a gas into the exhaust gas stream. In such so-called selective catalytic reduction (SCR), the ammonia is selectively reacted with the nitrogen oxides present in the exhaust gas to form molecular nitrogen and water. [0003] The insufficient activity of the kno...

AI Technical Summary

Benefits of technology

[0007] If a configuration having a switch-over device instead of a separate feed device for the oxidation catalytic converter is chosen, this may reduce assembly costs. The present invention provides, however, an example embodiment having separate and separately controllable feed devices for reducing agent.

[0009] One embodiment of the invention provides for the switch-over device to be in the form of a mixing valve. In that manner, it is possible for reducing agent to be admitted to the oxidation catalytic converter and the SCR catalytic converter simultaneously during a transition period. Using such a mixing valve, it is possible to avoid an abrupt switch-over, so that, depending upon the operating temperatures reached by the catalytic converters, an optimum purifying effect may be obtained.

[0012] At least one oxidation catalytic converter is disposed in the immediate vicinity of an exhaust gas outlet of the internal combustion engine, with the result that it reaches relatively high temperatures and thus achieves a high purifying effect after only a short time.

[0017] After such a period of time, the SCR catalytic converter will normally also have reached its operating temperature and injection of reducing agent is switched to the SCR catalytic converter. That may be done at temperatures of approximately from 150° C. to 200° C. in the SCR catalytic converter. Metering of reducing agent onto the oxidation catalyst is possible in principle at operating points with a low exhaust gas temperature—that is to say, not only in the case of cold starting—and provides a very effective NOx-lowering potential in cases where only insufficient activity is achieved with the SCR catalytic converter. With injection upstream of the oxidation catalytic converter up to a time of about 600 s, therefore, a marked increase in conversion by the exhaust gas purification system may be achieved. A suitable, sensible switch-over point of the temperature-controlled switch-over valve may lie at 100 to 200° C., e.g., at 130 to 180° C.

Problems solved by technology

The insufficient activity of the known SCR system at exhaust gas temperatures below approximately 250° C. is a problem.

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