The abnormal combustion did not contribute to the engine work, and it did not influence the pressure signal [35]: it induced surface diffusion flames that warmed up the nearby in-cylinder gas by thermal diffusion. This phenomenon increased the pressure much slower than the reduction of pressure produced by the movement of the piston during the expansion stroke. A comparison between the pressure-related measurements and processed optical data was performed.
Figure 7 shows the evolution of the combustion pressure signal and the integral luminosity measured during the engine cycles of Figures 3 and 4. The combustion pressure signal was calculated by the subtraction of the motored in-cylinder pressure from the fired one. From the spark ignition to the maximum values, the luminous and pressure signals showed similar trends for all the fuels and injection conditions. The sharp increase was due to the chemical reactions occurring in the first moments of the combustion process that are exothermic and radiative in the wavelength range of the CMOS camera. Figure 8 reports typical spectra detected in the center of the combustion chamber in the early stage of the combustion process for gasoline fuel [37]. In Figure 8a, the results obtained at 7 CAD BTDC are shown. For both signals, the spectral features of OH and CH were detected [38–40]. In particular, the highest heads at 306 to 309 nm of the OH band system (250 to 320 nm) were well resolved. Excited OH radical was formed in the primary combustion zone by the chemiluminescent reaction: CH + O2 → CO + OH. Moreover, the CH systems were observed near 431, 390 and 314 nm. The 431-nm band is the brightest; the 390-nm band system is very weak with closely packed heads. The 310-nm band is usually obscured by OH. In addition to the OH and CH features, a continuum on which two groups of diffuse bands were superimposed was detected. The first group was due to the Emeleus' bands of formaldehyde molecule CH2O, and it had the highest emission in the range of 350 to 460 nm. The second band system identified the Vaidya's bands of HCO with the highest heads from 290 to 360 nm. Thus, the longer wavelength bands were overlapped by the CH2O and continuum emission [39–41]. When the flame front overcame the spectroscopic measurement region, CH disappeared and high OH emission was measured, as shown in Figure 8b. Moreover, the burned gas is characterized by a broadband emission from UV to visible that is related to the CO2 chemiluminescence [42]. Even if, in the flames, there is no sufficient energy to excite stable atoms or molecules to high electronic states, electronic states of CO2 can be excited during the combustion by consecutive transitions from the ground state level to intermediate vibrationally activated levels [43, 44]. The emission of CO-O appears as a continuum, which extends from 300 to 600 nm with a broad maximum around 375 nm.
การสันดาปปกติไม่ได้ไม่ร่วมงานเครื่องยนต์ และมันไม่ได้มีผลกระทบต่อสัญญาณความดัน [35]: จะทำให้เกิดเปลวไฟแพร่ผิวที่ warmed ขึ้นก๊าซในถังใกล้เคียง โดยการแพร่ความร้อน ปรากฏการณ์นี้เพิ่มความดันช้ากว่าการลดลงของความดัน โดยการเคลื่อนที่ของลูกสูบที่ผลิตในระหว่างการอัดขยาย การเปรียบเทียบระหว่างการวัดความดันที่เกี่ยวข้องและข้อมูลประมวลผลแสงที่ดำเนินการFigure 7 shows the evolution of the combustion pressure signal and the integral luminosity measured during the engine cycles of Figures 3 and 4. The combustion pressure signal was calculated by the subtraction of the motored in-cylinder pressure from the fired one. From the spark ignition to the maximum values, the luminous and pressure signals showed similar trends for all the fuels and injection conditions. The sharp increase was due to the chemical reactions occurring in the first moments of the combustion process that are exothermic and radiative in the wavelength range of the CMOS camera. Figure 8 reports typical spectra detected in the center of the combustion chamber in the early stage of the combustion process for gasoline fuel [37]. In Figure 8a, the results obtained at 7 CAD BTDC are shown. For both signals, the spectral features of OH and CH were detected [38–40]. In particular, the highest heads at 306 to 309 nm of the OH band system (250 to 320 nm) were well resolved. Excited OH radical was formed in the primary combustion zone by the chemiluminescent reaction: CH + O2 → CO + OH. Moreover, the CH systems were observed near 431, 390 and 314 nm. The 431-nm band is the brightest; the 390-nm band system is very weak with closely packed heads. The 310-nm band is usually obscured by OH. In addition to the OH and CH features, a continuum on which two groups of diffuse bands were superimposed was detected. The first group was due to the Emeleus' bands of formaldehyde molecule CH2O, and it had the highest emission in the range of 350 to 460 nm. The second band system identified the Vaidya's bands of HCO with the highest heads from 290 to 360 nm. Thus, the longer wavelength bands were overlapped by the CH2O and continuum emission [39–41]. When the flame front overcame the spectroscopic measurement region, CH disappeared and high OH emission was measured, as shown in Figure 8b. Moreover, the burned gas is characterized by a broadband emission from UV to visible that is related to the CO2 chemiluminescence [42]. Even if, in the flames, there is no sufficient energy to excite stable atoms or molecules to high electronic states, electronic states of CO2 can be excited during the combustion by consecutive transitions from the ground state level to intermediate vibrationally activated levels [43, 44]. The emission of CO-O appears as a continuum, which extends from 300 to 600 nm with a broad maximum around 375 nm.
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