1. Field of the Invention
The present invention pertains to an exhaust gas control system of internal combustion engine for purifying exhaust gas by adding urea water pumped out from a urea water tank to the exhaust gas.
2. Description of Related Art
Urea SCR (Selective Catalytic Reduction) apparatus is known as an exhaust gas control system of internal combustion engine. Urea SCR apparatus is arranged to add urea water to exhaust gas. The urea water is thermally decomposed and hydrolyzed with exhaust gas heat, leading to generation of ammonia. With use of generated ammonia serving as a reduction agent, nitrogen oxide (NOx) in exhaust gas is subjected to reduction reaction on a selective reduction catalyst. This leads to a decomposition of NOx into water and nitrogen. Such a urea SCR apparatus is provided with a urea water tank for storing urea water, and a urea water addition valve for adding urea water pumped out from the urea water tank to the exhaust gas.
Urea water gets freezing at −11° C. or lower. During cold period, urea water in the urea water tank is possibly frozen so as not to be allowed to be supplied to the urea water addition valve. In a urea water SCR apparatus in Japanese Patent Application Publication No. 2008-115784 (JP 2008-115784A), the urea water tank is provided with a heater for melting frozen urea water in the urea water tank by heating with the heater.
When an internal combustion engine is left for a long time while stopping under cold circumstance, urea water in the urea water tank entirely freezes. In this circumstance, the internal combustion engine may start operating. In this instance, after the internal combustion engine starts operating, the heating is initiated with a heater. The urea water is gradually melted from a periphery of the heater, and the melted urea water is pumped out with a pump, and supplied to the urea water addition valve. However, when the urea water is consumed at a speed larger than a melting speed of the heater, the melting urea water is exhausted, not allowing to be added to the exhaust gas. Besides, the urea water is entirely pumped out at the periphery of the heater, leaving a void space between residual frozen urea water and the periphery of the heater within the urea water tank. In this condition, the heat of the heater is hardly transmitted to the frozen urea water, causing delay in restart of the melting of urea water and addition of urea water.