Optimization of a Diesel Engine with Variable Exhaust Valve Phasing for Fast SCR System Warm-Up 2019-01-0584
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, the impact of EVO timing on the exhaust temperature, emissions, performance and efficiency is analyzed using a validated GT-Power diesel engine model. 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 maintain the desired torque, avoid exceeding the engine safety limits such as exhaust temperature limit and keep the air fuel ratio high to avoid penalty on emissions and driveability.
An optimization procedure is described for the engine re-calibration at a constant EVO shift. The optimized results indicate that the boost pressure should be increased at high speed and loads to provide enough cylinder air charge and maintain the engine torque. Start of injection and EGR both being effective in NOx reduction come with penalty on fuel consumption. The optimization balances these parameters to maintain the NOx levels while minimizing the fuel consumption. An optimal zone is detected where highest temperature benefit with eEVO is achieved with the least impact on BSFC. The study also shows some of the penalties related to eEVO including increased flow pulsation at the air flow sensor location and reduced cylinder air charge. The latter increases turbolag during a torque transient and is mitigated by retarding EVO during a large tip-in. Finally, a simplified mode based engine model with the optimized engine maps is used to simulate the eEVO effect on the aftertreatment thermal dynamics. The optimized engine maps provide a fuel economy of ~30% when compared to the baseline maps with eEVO on the cold phase of FTP cycle. The simulation predicted a 7.5% - 18% reduction in the light-off time of the selective catalytic reduction catalyst compared to the baseline calibration, depending on the catalyst light-off temperature.