Exhaust emissions regulations for internal combustion engines have become more stringent over recent years. The regulated emissions of NOx and particulates from internal combustion engines are low enough that in many cases the emissions levels cannot be met with improved combustion technologies. Therefore, the use of aftertreatment systems on engines to reduce harmful exhaust emissions is increasing. For reducing NOx emissions, NOx reduction systems, including selective catalytic reduction (SCR) systems, are utilized to convert NOx (NO and NO2 in some fraction) to N2 and other compounds. SCR systems utilize a reductant (e.g., diesel exhaust fluid (DEF), typically ammonia, and an SCR catalyst to convert the NOx. Currently available SCR systems can produce high NOx conversion rates allowing the combustion technologies to focus on power and efficiency. However, currently available SCR systems also suffer from several drawbacks.
Most SCR systems generate ammonia to reduce NOx present in the exhaust gas generated by the internal combustion engine. When just the proper amount of ammonia is available at the SCR catalyst under the proper conditions, the ammonia is utilized to reduce NOx. Due to the undesirability of handling pure ammonia, many systems utilize an alternate compound such as urea, that vaporizes and decomposes to ammonia before entering the SCR catalyst. Many SCR systems that utilize urea dosing to generate ammonia depend upon the real-time delivery of urea to the SCR catalyst as engine NOx emissions emerge. Urea dosers have relatively slow physical dynamics compared to engine transients, such as mass flow, temperature, and emissions. Therefore, urea doser dynamics can substantially affect an SCR controls system, particularly during transient operating conditions. For example, based on the operating conditions, the urea dynamics may result in an excess of ammonia causing ammonia slip out of the SCR catalyst or a deficiency of ammonia causing excess amounts of NOx entering the atmosphere.
Some currently available SCR systems account for the dynamics of the urea dosing and the generally fast transient nature of internal combustion engines by utilizing the inherent ammonia storage capacity of many SCR catalyst formulations. Certain currently available systems determine whether the SCR catalyst is at an ammonia storing (adsorption) or ammonia ejecting (desorption) temperature. When the SCR catalyst is storing ammonia, the system injects urea until the catalyst is full. When the SCR catalyst is ejecting ammonia, the system halts injection and allows stored ammonia to release and reduce NOx. Presently available systems tracking the SCR catalyst temperature in this manner suffer from a few drawbacks. For example, the amount of ammonia stored on the SCR catalyst varies with temperature. However, presently available systems assume a storage amount below a specified temperature, and zero storage above the specified temperature. Therefore, the controls may toggle significantly around the specified temperature, significantly overestimate ammonia storage capacity just below the specified temperature, and significantly underestimate ammonia storage capacity just above the specified temperature.