In-cylinder Optical Measurement Setup

3.2.1

Figure 3.14 shows the Mie scattering layout for liquid spray penetration length measurement.

A 532 nm laser beam is generated from the frequency-doubled output of a pulsed Nd:YAG laser (SpectraPhysics Quanta-Ray INDI-40-10). With the second-harmonic generator optimized for the best conversion efficiency, the output is approximately 55.9 mJ per pulse measured at the Nd:YAG laser output. Characterized by a Gaussian profile, the 8 mm diameter laser beam is expanded into a horizontal laser beam of 20 mm diameter using a spherical concave (f = − 400 mm, Lattice Electro Optics, UF-PC-25.4-400) and spherical convex (f = 1000 mm, Lattice Electro Optics, UF-PX-50.8-1000) lens pair. An iris is adjusted to isolate the middle section of the

Gaussian beam profile, creating a relatively uniform light beam. Truncation of the laser beam edges with the iris serves to minimize reflections within the side window mount cavities. This laser beam is aligned parallel to the fire deck and is delivered through the combustion chamber via a series of flat fused silica windows mounted both in the adapter plate separating the cylinder head from the crank-case, and the piston crown. This configuration results in laser beam access to the piston bowl around firing TDC, from −27° aTDC to 27° aTDC. The elastic laser signal is acquired by an intensified CCD camera (Princeton Instruments PI-MAX2, GEN II intensifier, 512×512 pixels, 16-bit image depth) once every engine cycle. The ICCD camera is aligned with the axis of the extended piston through a stationary 45° first surface mirror and sapphire window in the bowl. The aperture ratio of the ICCD lens is set to f/4.0, and the gain is set to an intermediate value to avoid saturation while maintaining a high signal to noise ratio.

A field-programmable gate array microprocessor (National Instruments cRIO/DRIVVEN Inc.) is used to trigger a delay generator (Stanford Research DG535). The delay generator then provides triggers for the Q-switch, the laser lamp and ICCD camera gate. The width of the detection gate is kept at 65 sec, overlapping the laser elastic signal to overcome the inductive electrical interference present during firing of the piezo injector. In such a configuration, the camera’s field of view covers the 40 mm diameter combustion bowl.

Figure 3.14. A schematic of engine optical access and Mie-Scattering experimental setup

3.2.2 Natural chemiluminescence imaging

CH2O* natural chemiluminescence is acquired by an ICCD camera (PI-MAX2) once every engine cycle. To maximize the signal-to-noise ratio, the camera gain was set to the maximum value of 250, and 65 µsec exposure time was used with no filtering. A 3-element, ultraviolet lens (Electrophysics) of 78mm focal length and maximal aperture f/3.8 was used in the OH* and CH2O* chemiluminescence measurement. Although this filter scheme does not block emission from species such as CH*, OH*, C2*, HCO*, PAH or soot incandescence [198, 199], the chemiluminescence signal prior to high temperature combustion, the region of interest here, has been observed in this and other laboratories to be dominated by CH2O* [200-202]. Kinetic simulation of n-heptane oxidation over a range of equivalence ratio and temperature confirms

the mole fraction of excited state HCO* is at least an order of magnitude lower than the mole fraction of excited state CH2O*.

OH* natural chemiluminescence is acquired with the same ICCD camera (PI-MAX2) and setup.

An interference band pass filter of 10nm FWHM having center wavelength of 307.1nm was used to isolate OH* chemiluminescence from other emission bands. The aperture ratio of the lens was again set to maximum opening of f/3.8, and intensifier gain set to the maximum value of 250. Triggered by the FPGA system (cRIO/DRIVVEN), frames were synchronized with engine operation at a pre-defined crank angle position with 65 µsec exposure time.

CO2* natural chemiluminescence was acquired by a high-speed visible-range digital CMOS color camera (Vision Research Phantom v7.3, 800 x 600 pixels, 14-bit image depth, and 6688 fps full resolution up to 950,000 fps at the resolution of 32 x 32 pixels). For this experiment, the camera resolution was set to 400 x 416 pixels and the frame rate of 14440 fps. To maximize the signal to noise ratio, no filters other than the red, green and blue Bayer filter of the color camera are used. The high-speed CMOS frames were triggered at 0.5 CAD intervals. The aperture ratio is set to f/2.8 and the maximize signal-to-noise ratio at lower luminosity. The time interval within a cycle over which imaging is conducted, the “memory gate”, is set at 12.5 msec, which under the 1200 RPM engine speed covers 90 CAD. Triggered by the FPGA system, the CMOS camera recorded frames covering the active combustion period with 65 µsec exposure times every 0.5 CAD from -10°aTDC (trigger starting point) through 80°aTDC. Figure 3.15 shows the quantum efficiency of each color channel of the CMOS camera.

Figure 3.15. Quantum efficiency of each color channel of the CMOS camera used in CO2*

chemiluminescence measurement.

3.2.3 Natural soot luminosity imaging

Combustion images were captured by a visible range digital color camera (Vision Research Phantom v7.3, 800×600 pixels, 14-bit image depth, and 6688 fps full resolution up to 950,000 fps at the minimum resolution of 32×32 pixels). In this camera, the square pixel grid of the CMOS sensor is covered by a broadband color filter array arranged in a particular (Bayer) pattern, which spans the range from 400 to 700 nm. Frames were triggered at 0.5 CAD intervals and synchronized to the engine crankshaft position through a cRIO field-programmable gate array (National Instruments) and optical encoder-compatible input module (DRIVVEN). Exposure time is set to 40 µsec as an optimal trade-off maximizing signal-to-noise ratio at lower source intensities, while minimizing saturation from stronger signals. The aperture ratio is set to f/11.0 to have greater depth of field and maintain focus over a wider range of crank-angles. Operating parameters of the high-speed digital color camera are shown in Table 3.4. The memory gate width is set to 10 msec, which is the time interval within a cycle over which imaging is conducted, and at the 1200 RPM, the engine speed is equivalent to 72

CAD. Images thus cover the combustion duration from SOI through late-cycle burning.

Throughout this interval, the CMOS camera is storing each frame with 40 exposure time every 0.5 CAD.

Table 3.4. High-speed digital color camera operating parameters

Model Phantom v7.3

Resolution (pixels) 416×400

Operating Mode Sync with engine speed at 0.5CAD interval

The spectrum wavelength measurement was performed using an ICCD camera (PI-MAX2) with a Czerny-Turner type spectrograph employing a ruled diffraction grating used for spectral measurements. This spectrometer (Acton Research SpectraPro-2150i) has a 115 mm focal length, aperture ratio of f/4, and is equipped with a 300 grove/mm grating. The corresponding level of dispersion is 0.48nm/pixel. The grating blaze wavelength is 500 nm and the experiments are performed with a central wavelength of 400nm. The exposure time is 65 us, and 30 on CCD accumulations were used to gain enough signal from the low temperature cool flame process. The spectrometer was calibrated for wavelength with a mercury light source whose spectrum is known. In order to correct the spectral response from the optical system used in the experimental measurement (sapphire window, 45 degree mirror, spectrometer grating, lens and camera intensifier), a correction factor η is obtained by taking the ratio of the CCD ouput signal to the irradiance observed from a deuterium calibration light source. This

correction factor shown in Figure 3.15 is used to correct the measured spectrum, and the resulting spectrum plots are shown in the results.

Figure 3.16. Spectral response of spectrograph to calibration light source and corresponding correction factor