In-Cylinder Measurements of NO Formation in a Diesel Engine

The formation of Nitric oxide (NO) in a Diesel engine has been studied as a function of crank angle through-out the whole combustion cycle, using the Laser Induced Fluorescence (LIF) technique. Measurements were performed in an optically accessible one-cylinder, two-stroke, direct injection Diesel engine. The engine was operated in steady state at different loads and compression ratios. A tunable ArF excimer laser beam was used to excite the NO molecules in the D²∑⁺(v′=0) ← X²Π(v″=1) band at 193 nm. Dispersed fluorescence spectra allowed to discriminate between NO and interfering oxygen fluorescence. From the spectra, a relative measure for the NO density present in the probed volume of the cylinder was obtained. This density was transformed into an in-cylinder NO content, taking into account the changes in laser intensity, pressure, temperature and volume during the stroke. The resulting NO content curves show a slow start of the NO formation at the beginning of the combustion, gradually rising to a broad maximum around 50° aTDC. It is concluded that, in this engine, the bulk of NO formation takes place relatively late in the stroke. This suggests that the diffusion burning phase of combustion makes an important contribution to the NO formation, contrary to the model which assumes that NO is formed mainly during the initial premixed burn.