Due to the stringent requirements to reduce the tail pipe emissions of NOx, Selective Catalytic Reduction (SCR) systems are used to remove NOx using ammonia. When a urea solution is injected into the exhaust system, urea will undergo hydrolysis and decomposition reaction that produces ammonia. At the catalyst surface, ammonia will react with the exhaust gases to convert NOx into nitrogen, N2 and water, H2O. One of the challenging problems is to make sure the urea solution is available for the SCR system at cold start conditions. At extreme cold temperatures, the urea solution will begin to freeze at −12°C. At the start up of a vehicle under such low ambient temperatures, a heating system is used to provide the heat required for melting the frozen urea.
Therefore, there will be a time lag between the vehicle start up and the availability of urea solution to the SCR system. The response time of the urea tank heating system to deliver adequate quantities of urea solution depends on the initial temperature, mass of the frozen urea, heating system design and dimensions. In this paper, a transient thermal analysis model describing the heating process is developed. Test data for the warm up time at different test conditions are also presented.
