Optical investigations of plasma assisted ignition in a DISI engine fuelled with gasoline and butanol

A plasma ignition system was tested in a direct injection (DI) spark ignition (SI) engine with the target of combustion efficiency improvement without modifying engine configuration. The plasma was generated by spark discharge and successively sustained to enhance its duration up to 4 ms. The innovative ignition system was tested on a single-cylinder DISI optically accessible engine to investigate the effects of plasma on kernel stability and flame front propagation under lean mixture lambda value = 1.4. The engine was equipped with the head of a commercial turbocharged power unit with similar geometrical specifications (bore, stroke, compression ratio). All experiments were performed at 2000 rpm and 100 bar injection pressure with gasoline and n-butanol fueling. Optical access to the combustion chamber was performed through a piston crown that allowed the application of two techniques, UV-visible 2D chemiluminescence and natural emission spectroscopy. The first one gave information on the flame morphology and the second investigative method ensured insight into the specific processes related to the active chemical species. With the alternative ignition system, an increase in engine stability was obtained and the effects of butanol were linked to the different chemical properties compared to gasoline. The differences observed when analyzing the in-cylinder pressure data were correlated to the characteristics of flame shape and displacement, especially during the first stages of kernel formation and propagation