Study Of Intake Air Conditions On Natural Gas Dual-fuel Combustion

Natural gas/diesel dual-fuel combustion has the potential to achieve high efficiency and low emissions,but there are still several problems such as high carbon monoxide(CO)and total hydrocarbons(THC)emissions,low combustion efficiency,poor fuel economy at low load,and poor emission performance at high load.In order to improve the above problems,this paper used three-dimensional computational fluid dynamics(CFD)software Converge coupled with a reduced chemical reaction mechanism to study the effect of intake air conditions on dual-fuel combustion and emissions at low and high loads under different combustion modes and pilot fuel properties based on fixed combustion phase(CA50).The results of this paper include:(1)At low load,the effect of intake air pressure on the dual-fuel combustion and emission performance in conventional dual-fuel(CDF)combustion mode with different n-butanol blending ratios was studied.As the intake air pressure decreases,the total in-cylinder fuel-air equivalent ratio increases,the peak of pressure decreases,and the ignition delay is prolonged.When the pilot fuel is pure diesel(B0),as the intake air pressure decreases,the nitrogen oxides(NO_x)emissions increase firstly,and then obviously decrease.When the intake air pressure is0.6 bar,the NO_x emissions reach the lowest level.Compared with the intake air pressure of 1.0bar,the emissions of CO and THC are reduced by 43%and 93%respectively,and the combustion efficiency increases from 73%to 97%,indicating that throttling can significantly improve the combustion and emission performance at low load.At the intake air pressure of0.7 bar,with the increase of the n-butanol blending ratio,the combustion efficiency is further improved,and NO_x and THC emissions are reduced with slightly increased CO emission,indicating that the addition of n-butanol is beneficial to further improve the combustion and partial emissions at a lower intake air pressure.(2)At low load,the effect of intake air pressure on the combustion and emission performance in reactivity controlled compression ignition(RCCI)combustion mode with different n-butanol blending ratios was studied.The combustion efficiency increases with the decrease of intake air pressure.The addition of n-butanol prolongs the ignition delay and shortens the combustion duration,which further improves the combustion efficiency.The NO_x emissions are maintained at a low level at each intake air pressure.Compared with the intake air pressure of 1.0 bar,when the intake air pressure is 0.7 bar,the emissions of CO and THC are reduced by 59%and 80%,respectively.When the blending ratio of n-butanol is 10%(B10)and the intake air pressure is 0.7 bar,the CO and THC emissions are reduced by 77%and 83%respectively compared with that at the intake air pressure of 1.0 bar,and higher combustion efficiency and indicated thermal efficiency can also be obtained.Compared with the CDF combustion mode under this condition,the NO_x,THC,and CO emissions in the RCCI combustion mode were reduced by 60%,44%,and 38%,respectively.It shows that using RCCI combustion mode can improve emission performance more effectively.(3)At high load,the effect of intake air boost on the combustion and emissions performance in RCCI combustion mode with different n-butanol blending ratios was studied.When the intake air pressure is increased from 1.1 bar to 1.3 bar,the combustion duration is increased.At this time,adding n-butanol can accelerate the combustion rate in the cylinder and shorten the combustion duration.When the intake air pressure increases,the peak value of cylinder pressure increases and the peak value of heat release rate decreases.When the intake air pressure is 1.1 bar and 1.4 bar,B10 obtains the highest thermal efficiency.As the intake air pressure increases,NO_x emissions increase first and then decrease.When the pilot fuel is B0,increasing the intake air pressure can significantly reduce CO emission.With the increase of intake air pressure,THC emission increases,but the overall level is low.In addition,the addition of n-butanol to the pilot fuel can reduce the THC emission at the same intake air pressure.