China team propose method to reduce soot and CO emissions without NOx and thermal efficiency penalties in direct-injection natural gas engines
Researchers from Hebei University of Technology, CATARC (China Automotive Technology & Research Center) qnd Shandong University report that soot and CO emissions could be substantially reduced in direct-injected natural gas engines without penalties in NOx and thermal efficiency via the coordination of PNPI (proportion of natural gas pre-injection), NPSOI (injection timing of natural gas pre-injection) and EGR.
In a paper in the journal Fuel, Li et al. applied partially premixed combustion mode to a direct-injection natural gas engine based on a 3D model coupled with a dual-fuel chemical kinetic mechanism (n-heptane/natural gas).
They analyzed the effects of the proportion of the natural gas pre-injection and the injection timing of the natural gas pre-injection based on the combustion and emission results. To optimize further the partially premixed combustion mode, they also assessed the combined use of partially premixed combustion mode and EGR (exhaust gas recirculation).
It is widely accepted that internal combustion engines are one of the most predominant contributors to atmospheric pollution. Owing to the advantages in emission reduction and reserves, natural gas engines are more and more concerned. Currently, the more stringent emission standards call for more advanced technologies of emission control in natural gas engines. In high pressure direct injection natural gas engines, the combustion mode can be flexibly changed by adjusting injection parameters to achieve optimum generally performance, making it a favorable choice for the development of future natural gas engine.
… It can be concluded from the previous studies that partially premixed combustion mode is a promising way to reduce PM and CO emissions of direct injection natural gas engines owing to its low knocking tendency and high control flexibility. Nevertheless, the CFD model used in the previous study was based on very detailed mechanisms, which means the calculations have high demand for computational resources; moreover, they didn’t provide detailed analysis of the effects of the injection parameters and how the partially premixed combustion mode can be optimized.
The object of this paper is to assess the potential of partially pre- mixed combustion mode in achieving higher thermal efficiency and lower emissions.—Li et al.
Among the findings:
When partially premixed combustion mode is applied, pressure rise rate is higher and ITE is improved at most cases, implying that the appropriate use of partially premixed combustion mode can obtain better fuel economy, however, higher combustion noise may be accompanied. The variation of ITE with NPSOI is uncertain at different PNPI while ITE is relatively higher at higher PNPI.
The application of partially premixed combustion mode will lead to more sufficient mixing of the natural gas and air; consequently, the regions with low equivalence ratio, high OH concentration and high temperature will be enlarged with the increase of PNPI; meanwhile, during the combustion process, the peak values of the CO and soot fraction traces will be reduced along with increases in the peak values of NOx mole fraction at higher PNPI, resulting in reduced CO and soot emissions with sacrifices in NOx emissions. NPSOI has different optimized values for the control of CO, soot and NOx emissions at different PNPI owing to the coordination between natural gas injection and in-cylinder mixing.
The combined use of partially premixed combustion mode and EGR addition eliminates the higher NOx emissions of partially premixed combustion mode.
It can be summarized from the results that NOx emissions can be effectively controlled with inevitable increases in CO emissions and soot emissions when EGR is added. Besides, ITE may become lower than that of NGSI combustion mode if EGR is excessively added at PNPI of 30%, 90% and 100%. Generally, best compromise between fuel economy and emissions can be achieve at 50% or 70% PNPI with EGR fraction of 25% and 90% PNPI with EGR fraction of 15%.—Li et al.
Li, M., Zheng, X., Zhang, Q., Li, Z., Shen, B., & Liu, X. (2019). The effects of partially premixed combustion mode on the performance and emissions of a direct injection natural gas engine. Fuel, 250, 218-234 doi: 10.1016/j.fuel.2019.04.009