NOX purification system, and method for control of nox purification system

Abstract

In an NOx purification system and a method for control of the NOx purification system, two oxidation catalysts 4, 5 are provided upstream of an ammonia solution supply unit 6, and the in-cylinder fuelinjection is so controlled that hydrocarbon in exhaust gas is mainly oxidized by the upstream oxidation catalyst 4 and nitric oxide in the exhaust gas is mainly oxidized by the downstream oxidation catalyst 5. In the purification of NOx in the exhaust gas by feeding an ammonia solution for NOx reduction to a selective reduction NOx catalyst 6, this technique enables the system to adjust the concentrations of HC, NO and NO 2 in the exhaust gas flowing into the selective reduction NOx catalyst 6 to proper values so as to purify the NOx efficiently by the selective reduction NOx catalyst 6. TheNOx purification performance in a lower temperature range can be thus improved.

Description

technical field [0001] The present invention relates to a NOx purification system including an oxidation catalyst on the upstream side of an exhaust gas passage and a selective reduction NOx catalyst (SCR catalyst) on the downstream side, and a control method for the NOx purification system. Background technique [0002] Regulations on particulate matter (PM), nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HC) and the like discharged from diesel engines are being strengthened year by year. With the strengthening of this limitation, it is becoming more and more difficult to cope with the improvement of the engine alone. Therefore, a technique of installing an exhaust gas post-processing device using a catalyst in the exhaust gas passage to reduce these substances emitted from the engine is being developed and adopted. [0003] Such as Figure 4 As shown, a conventional NOx purification system 1X including a selective reduction NOx catalyst (SCR catalyst) 7 for re...

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Problems solved by technology

[0010] However, in this internal combustion engine exhaust purification system, it is necessary to provide an oxidation catalyst bypass or a parallel passage with a weak oxidation catalyst, and a passage switching valve, and there is a problem that the NOx purification system tends to become larger.

In addition, the heat of oxidation reaction in the oxidation catalyst cannot raise the temperature of the exhaust gas below the specified temperature at which the exhaust gas does not flow through the oxidation catalyst bypass

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