NOx/N ₂ O Formation and Coordinated Urea-SCR Control in an Ultra-Lean Hydrogen-Fueled Automotive SI Engine: Engine Mapping and WLTC Strategy Evaluation

To address the challenge of simultaneously achieving high efficiency and low NOx emissions for hydrogen-fueled automotive engines under the China 6b regulation, an integrated study is conducted on a four-cylinder direct-injection spark-ignition hydrogen engine, Covering in-cylinder NOx formation, SCR aftertreatment, and operating strategies. Engine and SCR bench tests are performed to obtain brake thermal efficiency (BTE) and raw/converted NOx characteristics across a wide range of speeds and loads. Based on these calibrated engine and SCR maps, a WLTC-based vehicle model is established to compare an economy-oriented strategy with a low-NOx-oriented strategy. Results show that under ultralean combustion with excess air ratios λ = 2.4-3.0, the engine achieves a maximum BTE of 43% with generally low raw NOx emissions at low and medium loads. The SCR system delivers nearly 100% NOx conversion at appropriate exhaust temperatures when the NH₃/NOx ratio is maintained at 1.1-1.3. For both strategies, WLTC tailpipe NOx remains below 35 mg/km, and the economy-oriented strategy yields the lowest hydrogen consumption. The study indicates that coordinated control of ultralean hydrogen combustion and a high-efficiency SCR system is a feasible pathway to high-efficiency, low-NOx operation for hydrogen-fueled automotive engines.