Effects of n-butanol on the combustion process in a direct injection spark ignition engine were investigated through flame visualization and spectroscopy. An optically accessible engine was equipped for the trials with a commercial cylinder head and wall guided injection system. Injection pressure (100 bar) and engine speed (2000 rpm) were fixed while injection timing and duration were changed to realise stoichiometric and lean fuelling in homogenous charge conditions.
Specifically, UV-visible digital imaging was applied in order to study the flame front inception and propagation with particular interest in the early combustion stages. UV-visible natural emission spectroscopy was applied to investigate the formation and the evolution of the main chemical compounds characterizing the spark ignition and combustion processes. Detailed image processing allowed to correlate the morphology and the local flame front curvature with thermodynamic data.
Overall engine operation was found to be roughly the same for the both fuels, with butanol providing a slight decrease of performance and a significant reduction of NOx and soot emissions compared to gasoline. Injection phasing resulted a key point to enjoy the alternative fuel benefits.
Regarding flame wrinkling, flame distortion and width of the probability density function (PDF) decreased at increasing peak pressure. Butanol flames were slightly more wrinkled than those for gasoline. The effect of fuel type on the PDF mean value was negligible.