Crank-angle-resolved laser-induced fluorescence imaging of NO in a spark-ignition engine at 248 nm and correlations to flame front propagation and pressure release.

Inside the combustion chamber of a spark-ignition engine, NO fluorescence is excited with a narrow-band tunable KrF excimer laser. The fluorescence light is detected by an intensified CCD camera that yields images of the NO distributions. Rotational-vibrational transitions of NO are excited by the A(2)Σ+ ? X(2)Π (0, 2) band system around 248 nm. Single laser shot planar NO distributions are obtained with good signal-to-noise ratio at all crank angles and allow us to locate areas of NO formation during combustion. The pressure within the combustion chamber is measured simultaneously with the NO distributions, which allows the evaluation of correlations between indicated work and NO formation. The crank-angle-resolved sequences of two-dimensional NO distributions and averaged pressure traces are presented for different engine-operating conditions. In addition, laser-induced predissociation fluorescence of OH excited by the same laser source is measured in order to visualize the corresponding flame front propagation and to compare the time of formation of NO relative to that of OH.