预混比和喷油定时对异丁醇/柴油RCCI燃烧与排放特性的影响

通过在一台六缸增压柴油机上进行了异丁醇/柴油双燃料活性控制压燃(reactivity controlled compression ignition,RCCI)的试验,探究了异丁醇作为低活性燃料在不同的预混比(premixing ratio,R_p)和喷油定时(start of injection,SOI)下对发动机燃烧和排放特性的影响。结果表明,随着SOI的提前,点火延迟(ignition delay,ID)延长,放热率峰值(maximum heat release rate,HRRm)先增大后减小;在排放方面,THC和CO的排放水平降低,NOx呈现先升高后降低的趋势,而在R_p=30%～50%时,颗粒物(particle matter,PM)的排放呈现先降低后升高的变化,当R_p=60%时则只表现出了降低的趋势。此外,随着异丁醇R_p的增加,ID和燃烧持续期(combustionduration,CD)都增长,HRR_m降低;在排放方面,THC和CO排放升高,NOx排放降低,而PM排放在R_p=30%下具有最高的水平,R_p=40%和R_p=50%时则都显示出较低的水平,而在Rp=60%下整体有所升高。此外,粒径分布显示3～30 nm的核态颗粒物(nuclear particle matter,PMN)排放数量最高,但质量占比非常小。而在PM排放质量分布中,主导的PM是粒径为30～500 nm的积聚态颗粒物(accumulated particle matter,PMA)。

Effect of premixing ratio and start of injection on RCCI combustion and emission characteristics fueled with iso-butanol/diesel

Reactivity control compression ignition (RCCI) has been shown to be effective in reducing nitrogen oxides (NOx) and particle matter (PM). A significant feature of RCCI is the ability to control the combustion process by creating reactivity stratification in the cylinder, which could reduce emissions. Since iso-butanol has a higher octane number, it can enhance the gradient of reactivity stratification, which will facilitate the control of combustion. The effect of iso-butanol on engine combustion and emission characteristics was investigated on a six-cylinder turbocharged diesel engine under RCCI combustion mode. In this study, iso-butanol (low reactivity fuel) was provided through the port injection (PI), and the diesel (high reactivity fuel) was injected directly into the cylinder. In the test, the combustion and emission characteristics of RCCI fueled with iso-butanol and diesel were investigated with different fuel supply strategies by changing the premix ratio (Rp) of iso-butanol and the start of injection (SOI) of diesel under low engine load. Four different premix ratios were selected, including 30%, 40%, 50% and 60%, and five different SOIs were designed, including -8°, -12°, -18°, -24° and -30° crank angle crank angle after top dead center(°CA ATDC). In this study, the engine was operated at a constant speed of 1 500 r/min, maintaining the brake mean effective pressure (BMEP) and the total cycle energy at 0.3 MPa and 1 280 J, respectively. The results showed that, with the advancement of SOI, the ignition delay (ID) was prolonged and the maximum heat release rate (HRRm) was increased first and then decreased. In terms of emission characteristics, with the advancement of SOI, the emission level of THC and CO decreased, and NOx emission increased first and then decreased. As SOI was further advanced to -30 °CA ATDC, it can be seen that the emission of CO increased when Rp was 30% or 40%. While the emission of PM at Rp from 30% to 50% showed a trend of decreasing first and then increasing, when Rp was 60%, it only showed a decreasing trend. In addition, the Rp of iso-butanol also had a large effect on RCCI combustion and emission characteristics. As the Rp increased, both the ID and CD (combustion duration) increased, while the HRRm decreased. In terms of emission characteristics, THC and CO increased and NOx decreased with the increase of Rp. When Rp was 30%, the total PM mass achieved the highest level under the four different Rp, and when Rp was 60%, it had a lower level of the total PM mass than that when Rp was 40% or 50% and SOI was -30 °CA ATDC. The size distribution of PM showed that the quantity of nuclear particle matter (PMN) was dominant, of which the diameter size was in the range of 3-30 nm. However, the proportion of PMN was very small for the total PM mass. In the mass concentration, the PM mass was dominated by the accumulated particle matter (PMA), of which the diameter size was in the range of 30-500 nm. The increase in the Rp reduced the whole PM quantity, but with the SOI advanced, the PMN quantity increased first and then decreased, while the PMA quantity decreased first and then increased, except when Rp was 60%. When Rp was 60%, PMA quantity only showed a decreasing trend with the advancement of SOI. The Rp had a complex effect on the mass distribution of PM. The most interesting part was that, when Rp was 60%, the total PM mass was higher than that when Rp was 40% or 50% at SOIs near the top dead center. The reason was that, when Rp was 60%, the peak value of PMA quantity was lower than that when Rp was 40% or 50% in the range of 30-500nm, whereas in the range 100-200 nm, more PMA quantity was exhibited, which lead to higher total PM mass. In addition, as the SOI advanced, the mass of PM generally showed a decreasing trend.