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A urea injection control strategy for urea-selective catalytic reduction under a transient process is investigated on a heavy-duty diesel engine test bench in this study. The aim is to improve NO x conversion efficiency and reduce ammonia slip. With the selective catalytic reduction system as the research object, an open thermodynamic conservation system is established. The conservation relationship in the process of urea injection, NO x reduction reaction, ammonia storage, and ammonia slip is investigated. The ideal target ammonia storage area and the ammonia storage characteristics during the transient process are studied. The ammonia storage area and boundary, which change with the transient temperature, are established. Correction of real-time ammonia injection is further deduced from the boundary of the area. The world harmonized transient cycle test cycle result showed that compared to feed-forward control, the NO x conversion efficiency increased by 16% and the NH3 slip decreased by 75% when using the proposed real-time ammonia storage-management control method.

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To improve engine response and decrease exhaust smoke and NOx emission during transient operations, variations in cycle-to-cycle and cylinder-to-cylinder combustion were investigated in a two-stage turbocharged, exhaust gas recirculation heavy-duty diesel truck engine. Variations in combustion were confirmed to be caused by insufficient mixing of the exhaust gas recirculation with intake air. And steady operation, by increasing the length of the mixing path, coefficient of variation of the exhaust gas recirculation rate in cylinder 1 declined from 8.12% to 3.68%, while that for the other cylinders decreased from approximately 4.0% to 2.98%. These improvements also caused reductions in coefficient of variations for indicated mean effective pressure and crank angle of 50% fuel burned (CA50). By using an intake valve closing timing retarding actuator system, coefficient of variation of the exhaust gas recirculation rate for cylinder 1 was further decreased from 3.68% to 2.82%. It was interesting to find that EGR/air mixing length revealed a great effect on soot and NOx emissions during transient operations, especially those involving sudden increases in load. By using long mixing path instead of short mixing path combined with suitable fuel injection strategy and exhaust gas recirculation rate, transient response of load was reduced by 10% and transient smoke spike decreased from 0.8m-1 to 0.5m-1.