Nitric oxide in a diesel engine : laser-based detection and interpretation

Contains fulltext : 18774.pdf (publisher's version ) (Open Access)Nitric oxide (NO) is one of the most polluting components in the exhaust gases of a diesel engines. Therefore, knowledge of the time and place where it is produced during the combustion process is of interest to find a way to reduce diesel engine emissions. Non-intrusive optical diagnostics, based on Laser Induced Fluorescence (LIF), are used to study the NO content and distribution within the combustion chamber of a one-cylinder, two-stroke, direct injection diesel engine. The engine has been made optically accessible by mounting quartz windows in the cylinder wall as well as centrally in the cylinder head. It is operated steadily running on standard commercial diesel fuel. A tuneable ArF excimer laser beam is used to excite the NO molecules in the D(v'=0)-X(v''=1) band at 193 nm.The ensuing fluorescence is either dispersed in its different wavelength components in an Optical Multichannel Analyser (OMA) or imaged by a gated CCD camera through a narrow-band transmission filter. One of the goals of our present research is to quantify the LIF data as much as possible. Although the NO fluorescence is proportional to the NO density, the proportionality constant depends on the local circumstances at a specific position. Therefore, in order to compare the fluorescence yields throughout the combustion process, they have to be interpreted as an in-cylinder NO density taking into account the changes in laser intensity, pressure and temperature during the stroke. Evaluation of the data results in the conclusion that, in this engine, the bulk of NO formation takes place relatively late in the stroke, indicating that the premixed combustion and the early mixing-controlled combustion contribute only little. Towards larger crank angles the NO content decreases, possibly as a result of oxidation to NO_2 in the colder part of the stroke.XIII, 154 p