Investigation of the Knocking Intensity Mitigation Mechanism by Dielectric Barrier Discharge

Abstract

In our previous study, the mitigating effect of dielectric barrier discharge (DBD) on the intensity of end-gas auto-ignition was observed. In this paper, the mechanism of the effect was investigated through chemical analysis and combustion experiments using a rapid compression and expansion machine (RCEM). Comprehensive GC×GC with time of flight mass spectroscopy (GCxGC-TOFMS) was performed, and the generation of alkyl-hydroperoxide (ROOH) was successfully confirmed for the first time, based on accurate mass analysis. To study the mechanism of the mitigation effect, the influence of ozone was assessed using different fuel-air mixtures, such as primary reference fuel (PRF90) and surrogate gasoline (S5R). The addition of ozone showed the same mitigation effect in the case of PRF90, but a lesser effect in the case of S5R. A characteristic blue light was also observed when ozone was mixed in the end gas prior to auto-ignition. Since ozone is known to promote low temperature oxidation (LTO) reactions, the effect of DBD application likely involves the same mechanism. The difference in effect with the different fuels may be explained in terms of an ozonolysis reaction, because S5R contains olefins and PRF90 does not. Since applying DBD to the fuel-air mixture did not show a difference in effect, between S5R and PRF90, the DBD mitigation phenomena is not induced by ozone, but a plausible candidate is the ROOH. To investigate the precursor phenomena to the blue light emission, planer laser induced fluorescence measurement (PLIF) for formaldehyde (HCHO) was employed in the combustion experiment. Without DBD application, the HCHO distribution in the end gas exhibited gradual homogenization before auto-ignition; whereas, with applied DBD, the characteristic blue flame appeared in the inhomogeneous distribution of HCHO in the end-gas region. This result may support the hypothesis that the mitigating effect is caused by the promotion, by DBD-induced ROOH, of inhomogeneous progress in the end-gas chemical reaction.