Laser Spontaneous Raman Scattering Line Imaging For Mole Fraction And Temperature Measurements In Internal Combustion Engines

The key issue of modern internal combustion engine is to solve the problems ofcombustible mixture preparation process and the effect of the concentration andtemperature distribution of the in-cylinder charge flow state and the in-cylinder component（fuel, air and burnt exhaust gas） on the ignition and combustion in the internal combustionengines. By using the combustion diagnostic technology, the inhomogeneous degree ofcharge and the concentration and temperature distribution of the in-cylinder component canbe measured on the microcosmic in an internal combustion engine. This will surelypromote the development of engine combustion theory and the industrialization process ofa new generation of internal combustion engine.The technique is based on laser spontaneous Raman scattering which can realize thein-cylinder major species measurement, such as the concentration and temperaturedistribution of N₂、O₂、H₂O、CO₂、unburnt HC etc, and has the distinguish ability to thetemporal in nanosecond level and to spatial in millimeter level and to a single engineworking cycle.In this paper, an optical diagnostic system based on line imaging of spontaneousRaman scattered light has been developed. It mainly consists of Nd: YAG lasers, imagingspectrometer, ICCD, laser pulse stretcher, a laser beam expander, and a laser beamcontracting, the gases sample cell, the collector of the line light source, Notch filters,energy meter. In this study, the laser output energy is400mJ, the wavelength is532nm,pulse width of FWHM is7ns, the divergence angle is0.8mrad, the spot diameter is8mmand the frequency is10Hz. Although the gas spontaneous Raman scattering intensity isproportional to the intensity of the laser energy, using directly the higher energy outputfrom a pulsed laser which may damage to the quartz glass window and cause gasbreakdown in the measurement volume, even ignite the combustible gases, can notmeasure the weak Raman signal successfully.In this study, in order to avoid effectively the occurrence of these phenomena, a laserpulse stretcher including two ring cavities which were composed mainly by the beamsplitter mirrors and reflecting mirrors has been developed, it can extend the value of thelaser FWHM from7ns to more than35ns. The basic principle of the laser pulse stretcher isthat when the pulsed laser entered each ring cavity, it produced a part of reflecting outputpulse laser and a part of the transmission pulse laser, the part of the transmission pulselaser remain cycled in the ring cavity, and each loop will continue to produced a portion ofthe reflecting output pulse laser and part of transmission pulse laser which remain stayed inthe ring cavity. Many such ring cavities are needed to merge these small pulse lasers whichwas formed by single ring cavity into an extended and peak power was reduced ideal apulsed laser. The beam splitter ratio of50%and the ring cavity of the delay time ratio of 1:2were selected by calculating through simulation and optimization of the optical path,thus, the absolute distance of each mirrors in each ring cavity were determined of2.1mand4.2m. According to the actual experimental platform structures, the distance betweenthe laser and stimulate area can be measured, in this study, in order to measure the enginein-cylinder components, so the distance is about9.5meters.Due to the existence of the laser divergence angle, a laser beam expander which cantransform the external light path into a parallel light was mounted to the outlet of the laser,the parallel light turned into the desired diameter size laser （such as1.5mm） by passing thebeam contracting and the focusing mirror.As the maximum image height of CCD is6.6mm, a line source collector which wascomposed by a group of achromatic lens was designed, the length of66mm light source inthe linear excitation region turned into a real image of the length6.6mm through the linesource collector, then output to the slit of the spectrometer, realized the line imagingmeasurement.By using the DDGTMdigital delay generator in the ICCD and a signal generator orTDC signal from the experiment engine, the timing synchronization between the laser andthe ICCD was realized.The Raman spectroscopy of the mixed gases in the gases sample cell was measured indifferent pressures by using the optical diagnostic system, it can concluded that obtainedthe O₂and CO₂response factor relative to nitrogen N₂is1.47and1.29and the spontaneousRaman spectroscopy of10small areas （6.6mm） species on a66mm length×1.5mmdiameter stimulate line were acquired simultaneously. The concentration information andmole fraction were acquired with85%of the detection accuracy from the species spectralpeak intensity under the condition of the absolute gas pressure is0.5MPa, CO₂molefraction is0.05, having an average of the actual ratio of0.07mole fraction O₂was90%.In order to complete the measurement of the charge concentration and temperature inthe engine cylinder, a single cylinder optical engine and optical diagnostic experimentplatform which can realize the spark ignition （SI） combustion mode and controlled autoignition （CAI） combustion mode was developed, It mainly consists a single cylinderoptical engine with a fully variable valve train by electrohydraulic control, its ECUelectronic control system, laser spontaneous Raman spectroscopy optical diagnosticplatform.The single-cylinder optical engine was converted through refitting a single metalgasoline cylinder. The refitting includes a elongated cylinder with cooling water jacket, alengthened cylinder liner, a lengthened piston which has a diameter of70mm quartzwindow, a45°mirror which was mounted on the original cylinder, and a laser ring gasketwith the laser entrance windows in it.By mounting the electro-hydraulic control fully variable valve mechanism on theoptical engine cylinder head, the flexible adjustment of the intake and exhaust valve lift and phase can be achieved.The optical engine electronic control system mainly consist of the microcontroller ofIntel80c196kc, valves control circuit, fuel injection circuit and ignition circuit,optical-electricity encoder, valve lift sensor, cylinder pressure sensor, speed sensor, oxygensensor, and λ （excess air coefficient） analyzer.The signal which was given by the microcontroller was used to trigger the laser andICCD, so it can realize that the engine operation synchronize with the Raman measuringoptical system and the concentration and temperature information of components which inthe laser excitation line in the cylinder can be obtained at any fixed crank angle.Under the SI ignition and CAI combustion mode with60°of negative valve overlapangle, the laser spontaneous Raman spectroscopy of the species which in a length of66mm,a diameter of about1.5mm laser excitation line below7mm of the spark plug was acquiredbefore7°of the compression top dead center. All species in cylinder were measured by theoptical diagnosed system, such as, the position H2O（660nm）、CH（628nm，fuel）、N₂（Stokes，607.2nm）、O₂（580nm）、CH3（576.5nm，fuel）、CO₂（v1peak，574.4nm）、CO₂（2v2peak，571nm）、CC（557.7nm，fuel）and N₂（anti-Stokes，457nm）. The twopeaks of CO₂were significantly well because the effect of the EGR. The mole fraction ofthe measurement areas which were calculated by the Raman light intensity was uneven.The one-dimensional（1-D）distribution result of temperature calculated by the N₂andmole fraction from Raman spectrum data have a better consistency with thethree-dimensional （3-D）numerical simulations from STAR-CD software.The study shown that the optical diagnostic system based on laser spontaneous Ramanline imaging technique has a semi-quantitative detection capability to measure theconcentration and temperature of the cylinder species. It is essential to improve and refinesome aspects of the system for the more precise measurements.