| Single-photon NO PLIF imaging was applied to study combustion of a sooting diesel reference fuel under low-NOx operating conditions in the Sandia compression-ignition optical research engine (SCORE). The SCORE, which is located in the Sandia CRF, is a single-cylinder version of a Caterpillar heavy-duty truck engine. A transition in the A2Sigma+--X 2pi (0,0) band of NO was chosen for excitation. An optical filtering technique employing multiple dielectric mirrors provided a three-fold improvement in detection sensitivity over previously used bandpass filters. The single-photon NO PLIF results show qualitative spatial and temporal distributions of NO throughout the combustion, expansion, and exhaust portions of the engine cycle.; Second, a two-photon NO LIF technique was developed and applied to study NO concentrations within the cylinder of the SCORE. This represents the first known application of two-photon NO LIF in an internal-combustion engine. The technique avoids many of the problems associated with in cylinder, single-photon NO PLIF measurements including: (1) fluorescence interference from the Schumann-Runge bands of hot O2, (2) broadband absorption of a UV excitation beam by in cylinder gases, and (3) difficulty rejecting scattered laser light while simultaneously maximizing fluorescence signal collection. A group of twelve closely spaced rotational lines in the A2Sigma +--X2pi (0,0) band of NO, consisting of P1 + O21(7.5--10.5), P2 + Q 12(22.5), O1(3.5), and S2(6.5) at 452.724 nm vac, coincident with the P1 bandhead was chosen for excitation. Two-photon NO LIF images were acquired throughout the combustion, expansion, and exhaust portions of the engine cycle providing useful NO fluorescence signal levels from 60° after top-dead-center through the end of the exhaust stroke. Verification that the signal resulted from NO was provided by tuning the laser to a vibrational offline wavelength. This resulted in near-zero signal levels from fluorescence and interference at in cylinder pressures greater than 20 bar. The in-cylinder two-photon NO LIF measurements were compared with chemiluminescence NO/NOx exhaust-gas measurements and showed good agreement over a wide range of engine loads from 1.4 to 16 bar gIMEP. |
