Coordinated Fuel-Air Control Strategy for Enhancing Transient Performance of Two-stage Turbocharged Diesel Engines at High Altitude

Abstract Diesel-engine transient performance deteriorates at high altitude because reduced air density intensifies air-supply deficiency and turbo lag, degrading fuel economy and emissions. This study improves plateau transients via coordinated fuel–air control on a two-stage turbocharged diesel engine. A two-stage test platform was developed from a baseline single-stage engine by adding a high-pressure variable nozzle turbocharger (VNT). Experiments at 0 m and 2400 m quantified the coupled effects of stepwise fuel injection and VNT nozzle-opening control on air-path dynamics, combustion, torque response, and emissions. Results show that stepwise fueling exploits residual in-cylinder oxygen at low load, rapidly increasing exhaust energy in the early transient stage. When combined with prompt VNT closure, turbocharger spool-up is accelerated, turbo lag is mitigated, intake air flow rises faster, and transient peaks of the fuel–oxygen equivalence ratio are reduced. At 2400 m, a larger fuel step and a more aggressive VNT schedule are required to compensate for reduced air density and sustain transient air delivery. A World Harmonized Transient Cycle (WHTC) test at 2400 m verified the optimized calibration: brake-specific fuel consumption decreased by 5.4 g/kWh, while carbon monoxide (CO), total hydrocarbons (THC), and particle number for particles > 23 nm (PN23) decreased by 692.7 mg/kWh, 64.1 mg/kWh, and 1.04 × 1011 #/kWh, respectively; however, nitrogen oxides (NOx) increased by 78.2 mg/kWh. Overall, the proposed strategy improves high-altitude transient performance with a NOx trade-off.