Computational studies of hydrogen post-injection in direct-injection natural gas engines

Abstract

The impact of using a hydrogen (H2) post-injection on combustion emission characteristics in a glow plug (GP) assisted direct-injection natural gas (DING) engine was investigated in this paper. This study was conducted by using the KIVA-3V code integrated with a detailed kinetic chemical model and a modified phenomenological soot model. Previous studies have indicated two sources of unburned methane (CH4) and soot particles in such an engine, namely the injector sac volume and the glow plug shield. The emissions were reduced in previous work by improving the shield design. However, the contributions from the injector sac volume were not solved. In these simulations, a small amount of hydrogen was injected following the major injection of natural gas (NG), and the unburned CH4 was pushed out of the injector into the combustion chamber. The injected hydrogen then affects the chemical reactions to form more hydroxyl (OH) radical, and the unburned methane from the sac volume and that originally left in the combustion chamber were oxidized to reduce its final value, compared to the baseline NG only case. As well, the soot particles contributed from the glow plug shield and from the injector sac volume and thereby the total particle emissions were reduced, compared to the baseline case. The reduction is due to the additionally formed OH which reduces the soot precursor acetylene (C2H2) and enhances the soot oxidation. The hydrogen post-injection also highly affected the CO–CO2 reaction, leading to more carbon monoxide (CO) and less carbon dioxide (CO2) emissions in the DING engine. In addition, more simulations were conducted by injecting different amounts of hydrogen fuels, and the emission values were generally changed as expected. The simulations also indicated that there is a minimum requirement for the hydrogen amount, below which the formed OH radical is not enough to oxidize the soot caused by the unburned CH4 from the injector sac volume, compared to the NG only case.