| Low temperature combustion(LTC) has become a competitive and attractive technology to achieve clean and high efficiency combustion for diesel engines. The mixture of gasoline and diesel is found to be more suitable for diesel LTC compared to single gasoline or diesel fuel. In order to gain deeper understanding on the effects of fuel properties on the LTC combustion process, in this study, a reduced PRF-PAH kinetic mechanism was coupled with the KIVA-3V CFD code to investigate the effects of fuel properties on the mixing, combustion, emissions and important intermediate species evolution processes under LTC condition.Numerical study was firstly conducted to study the effects of fuel volatility on the mixing, combustion processes and soot emission under low load condition in a low temperature combustion diesel engine. The results show that due to the higher volatility and weber number, the evaporation rate of n-heptane is faster than that of diesel, which significantly promotes the in-cylinder mixing process, thus the concentration gradient with n-heptane fuel is smaller than diesel prior to the ignition and combustion. As a result, the soot emission of n-heptane is lower than that of diesel under the same operating condition. The soot emission can also be reduced at lower injection pressure condition with n-heptane compared to diesel. It is found that fuel with higher volatility can overcome the adverse effects of low injection pressure on the mixing process. Therefore, fuel with higher volatility is helpful to reduce the high injection pressure requirement in low temperature combustion. Although the effects of fuel volatility on the mixing and combustion processes and soot emission is reduced under medium load operating condition, improvements in soot emission can still be obtained through fuel properties optimization.Then the effect of fuel properties with regard to three different diesel/gasoline mixtures on LTC is analyzed through numerical simulations. The result shows that under conventional combustion mode(no EGR), the cetane number can be effectively reduced by increasing the gasoline blending ratio in the mixtures to prolong the ignition delay; the higher volatility of the blended fuels is also helpful to enhance the mixing process, thus both the production and consumption rates of the most important OH radical can be accelerated, results in more premixed combustion and shorter combustion duration. Therefore the combustion rate of conventional combustion mode can be increased by applying gasoline/diesel mixture fuels. The combustion processes(ignition and combustion/reaction rate) is more sensitive to the fuel reactivity under LTC mode(high EGR). Since the fuel reactivity can be effectively reduced by applying high EGR and high gasoline blending ratio, thus the overall combustion reaction rate depicts a firstly increase and then decrease trend as gasoline blending ratio increases. Much retarded combustion phasing is observed with DG40 fuel due to the higher gasoline component in DG40 fuel; while for DG20 fuel, the reaction rate can be increased and the combustion duration can be shortened due to the fact that more OH radical is available with DG20 fuel. These results demonstrate that improvement in combustion process can still be obtained by blending gasoline to diesel even under high EGR LTC condition. It is observed that the mixing processes and equivalence ratio distributions prior to the ignition and combustion can be improved due to the higher volatility and longer ignition delay brought by the gasoline/diesel mixture, which is helpful for soot formation and emission reduction. However, it is also found that high gasoline blending ratio greatly retards the combustion phasing, especially under high EGR conditions, results in deteriorated combustion efficiency and emissions. Therefore, fuel properties optimization should be collaborated with combustion strategy simultaneously to improve the combustion and emissions of diesel engines. |
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