Improving the Performance of Natural Gas Engine at High Altitude Based on Response Surface Method and NSGA-II Optimization

Natural gas is an emerging alternative fuel for internal combustion engines in the transportation sector. However, the performance of natural gas engines can be significantly affected by changes in atmospheric pressure and temperature at high altitudes. To address this issue and enhance the performance of natural gas engines in plateau environments, a study focused on a two-stage turbocharged heavy-duty spark-ignition natural gas engine and its performance improvement is conducted targeting at operating altitude of 4000 m. A one-dimensional model of the engine is firstly developed and validated against experimental data at varying altitudes. The experimental and simulated data suggest engine power loss of 3% and 18% at 2500 m and 4000 m altitudes, respectively. Then, a response surface model of the engine is constructed employing the Box–Behnken experimental design method, considering optimization factors such as the compression ratio (CR), spark timing (ST), and bypass valve equivalent diameter (BVED). The objectives of the optimization are to enhance power, reduce brake specific fuel consumption (BSFC) and minimize nitrogen oxide (NOx) emissions. Finally, while adhering to engine durability constraints, the NSGA-II optimization algorithm is utilized for the multi-objective optimization. The optimization results demonstrate that at an altitude of 4000 m, the engine power recovers to approximately 86% of that at sea level, with a slight increase in BSFC and a decrease in NOx emissions. Therefore, this proposed engine optimization method effectively restores the performance of natural gas engines at high altitudes.