| 摘要 |
<P>PROBLEM TO BE SOLVED: To appropriately purify exhaust gas by performing injection control of urea water while estimating the concentration of urea based on the temperature of the urea water. Ž<P>SOLUTION: A relationship between concentration and freezing point temperature of urea water is preliminarily stored as a map in a ROM or the like, and the concentration is estimated from the temperature of urea water, and injection quantity is controlled according to the estimated concentration. In the estimation of the concentration from the temperature of urea water, since two concentration values exist relative to the freezing point temperature, NOx value NL detected by an NOx sensor while performing the injection control on the assumption of the lower concentration of urea water is compared with NOx value NH detected by the NOx sensor while performing the injection control on the assumption of the higher concentration of urea water (202-212), and which of the values is appropriate is determined, whereby the concentration value is estimated (214 and 216). Ž<P>COPYRIGHT: (C)2010,JPO&INPIT Ž
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