however, NO increase [11]. The adaptability of engine on plateau
environment can also be improved by increasing oxygen content in
intake air [12]. Therefore, it was found in those studies that, intake
oxygen-enriched combustion could not only improve combustion
characteristics, but also significantly reduce particulate emissions.
The smoke capacity can be significantly reduced by 35%e73% by the
increase of intake oxygen concentration [5]. Researchers also found
that intake oxygen-enriched combustion could improve combustion
efficiency under some loads, and the fuel consumption could
be reduced by 2%e10% [13]. Engine power density can also be
improved by applying intake oxygen-enriched combustion, which
means engine can be downsized with the same power output. If the
intake oxygen concentration can be increased up to 32%, the
displacement of the engine can be reduced to 73% with the same
power output [6]. Increasing intake oxygen concentration can also
shorten ignition delay time and allow the use of low-quality fuel
[13]. However, the increase of NOx emissions is a key issue for
further development and application of oxygen-enriched combustion
in internal combustion engine. Many studies [14] have
shown that oxygen-enriched combustion can greatly reduce particulate
matter and soot significantly; however, NOx emissions will
significantly increase if no special measure was applied. The
harmful NOx emissions and PM purification methods of diesel engine
contradict with each other, which makes it difficult to fully
meet future stringent emission regulations