Study on Hydrogen Direct Injection in RNG Combustion under Various Ignition Timings for Power Generation in a Retrofitted Gas Engine.

Renewable natural gas (RNG) is attractive for energy policy goals in the world. Therefore, a regional system is designed to explore RNG combustion for power generation in localities. This study investigates a direct injection (DI) engine fueled with hydrogen (H₂) blended into the simulated renewable natural gas, which consists of 50% methane (CH₄) and 50% carbon dioxide (CO₂) in volume. In order to obtain higher efficiency, comparisons between DI and port fuel injection (PFI) of H₂ addition were made. Then, the volume percentage of H₂ was changed from 20% to 100% by keeping the volume ratio of CH₄ and CO₂ at 1:1. Finally, results of power output, brake mean effective pressure (BMEP), brake thermal efficiency (BTE) and brake specific fuel consumption (BSFC) were discussed. Results showed that in contrast to PFI, H₂ DI injection could increase efficiency by 4%. Additionally, H₂ DI could retard the MBT ignition timing at 5 °CA. Compared with CH₄/CH₄ + CO₂ combustion, under stoichiometric combustion, BMEP increases with H₂ addition but BTE decreases significantly. However, by enlarging the excess air ratio (λ) to 1.24, both BMEP and BTE increase obviously with H₂ addition. Moreover, when λ < 1.3, the MBT ignition timing should be advanced from −10 to 15 °CA top dead center (TDC). But the MBT ignition timing is fixed at −25 °CA TDC when λ is larger than 1.3. Furthermore, if efficiency is the priority, 30% H₂ addition with λ at 1.24 (−15 °CA TDC) should be selected. If higher BMEP is preferred, 20% H₂ addition with λ at 0.99 (−10 °CA TDC) should be selected. [ABSTRACT FROM AUTHOR]