Development of urea-SCR dosing control strategies for a diesel electric hybrid car
With every new revision of government emission regulations, the limit on the tailpipe NOx is getting lower, forcing engine manufacturers to innovate in order to meet new targets. NOx emission reduction can be achieved through engine management strategies, however, there is a limit to the level of NOx reduction and, often, a penalty is associated with completely relying on engine strategies. Urea-SCR system, a type of diesel aftertreatment, provides a means of reduction of NOx downstream of the engine so as to provide the engine more flexibility to operate at higher efficiencies, which can be associated with high engine-out NOx emissions. The aim of this research is to explore various urea-SCR dosing control strategies and evaluate the benefits and tradeoffs associated with change in operating parameters as well as amongst different strategies for a light-duty diesel application. Various components such as supply module, urea injector, sensors, etc., were calibrated and integrated to form the urea-SCR system, centrally controlled by Woodward SECM112 controller. The system integration included communication channel setup and internal diagnostics for fault detection. Two principal dosing control strategies were developed, namely stoichiometric dosing strategy and NH3 feedback control strategy. Stoichiometric dosing strategy doses DEF to produce NH3 which is in stoichiometric equivalence to the engine-out NOx. NH3 feedback control uses the NH3 sensor to track a certain NH3 slip set-point. Protocols were developed for dynamometer testing in order to ensure repeatability of tests and to maintain the system at conditions that would ensure all measurement equipment was in safe operating region. Testing of the current platform was done on a chassis dynamometer and measurements were taken from on-board sensors. Steady-state and transient tests were performed by multiple parameters such as catalyst temperature and ANR (Ammonia to NOx Ratio). Drive cycles such as the UDDS and the HWFET were run to perform transient testing. The lowest measured tailpipe emission levels were 0.47 g/mile for a UDDS cycle with a constant ANR of 2. The performance of the NH3 feedback control was evaluated in terms of ability to track an NH3 slip set-point for steady state and in the presence of disturbances such as fluctuating NOX concentrations upstream and fluctuating space velocities at the catalyst. The performance of the feedback control system was found to be robust. This thesis concludes with recommendations for improvement in the performance of the current urea-SCR system based on the results obtained from testing.
MECKL, Purdue University.
Automotive engineering|Mechanical engineering
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