Optimization of a Diesel Engine with Variable Exhaust Valve Phasing for Fast SCR System Warm-Up

Early exhaust valve opening (eEVO) increases the exhaust gas temperature by faster termination of the power stroke and is considered as a potential warm up strategy for diesel engines aftertreatment thermal management. In this study, first, it is shown that when eEVO is applied, the engine main variables such as the boost pressure, exhaust gas recirculation (EGR) and injection (timing and quantity) must be re-calibrated to develop the required torque, avoid exceeding the exhaust temperature limits and keep the air fuel ratio sufficiently high. Then, a two-step procedure is presented to optimize the engine operation after the eEVO system is introduced, using a validated diesel engine model. In the first step, the engine variables are optimized at a constant eEVO shift. In the second step, optimal eEVO trajectories are calculated using Dynamic Programming (DP) for a transient test cycle. The optimized results indicate that with early EVO, the boost pressure should be increased to provide enough cylinder air charge and to maintain the engine torque. External EGR can be reduced due to increased internal EGR while maintaining the same engine out NOx. An optimal zone to maximize temperature benefit with least impact to BSFC has been observed. The study also shows some of the penalties related to eEVO including increased flow pulsation at the air flow sensor location. Finally, with optimal eEVO, a 6.5% - 11% reduction is observed in the light-off time of the selective catalytic reduction (SCR) catalyst and 45% reduction in tailpipe NOx compared to the baseline operation without eEVO.