Throttling effect study in the CDF/RCCI combustion with CNG ignited by diesel and diesel/biofuel blends

Abstract

Compressed natural gas (CNG) enrichment ignited by the diesel fuel in the compression ignition engine generally reveals low thermal efficiency and high carbon monoxide (CO) and total hydrocarbons (THC) emissions at low load due to the over-lean mixture. This paper aims to numerically study the improvement effectiveness of throttling on the efficiency and emissions in the conventional dual-fuel (CDF)/reactivity controlled compression ignition (RCCI) combustion modes with CNG mixture ignited by diesel and diesel/n-butanol blends at low load. Two pilot fuels and two combustion modes were compared by sweeping the intake air pressure (Pin) from 1.0 to 0.7 bar based on a fixed CA50 of 2 °CA ATDC, indicated mean effective pressure (IMEP) of 2.5 bar approximately, and CNG substitution rate of 50%. The numerical results showed that reducing Pin can significantly improve the combustion efficiency and THC emission due to the increased fuel/air concentration. In comparison with CDF combustion, RCCI combustion obtains higher combustion efficiency and lower THC emission. CO is reduced gradually with the decreasing Pin in the RCCI combustion, and having the similar level as that in the CDF combustion at Pin of 0.7 bar. N-butanol addition can further reduce the CO and THC emissions in the RCCI combustion with the reducing Pin. Overall, it reveals that diesel blended with n-butanol as the pilot fuel in the RCCI combustion at low intake air pressure is more favorable for the improvement of efficiency and emissions at low load.