Conclusions
The effect on the spark-ignition combustion process of n-butanol blended in volume with pure gasoline was investigated through cycle-resolved visualization applied in a single-cylinder PFI SI engine working at low speed, medium boosting and wide-open throttle. Two injection timings were fixed in order to inject the fuel at closed intake valve and open intake valve, respectively. The spark timing was changed to identify the maximum brake torque and the knocking limit. Blends of butanol up to 40% allowed working in more advanced spark timing without negative effects on performance. To work with a stoichiometric mixture for both fuels, the duration of injection was slightly increased for the blend. DOI in CV resulted longer than in OV for both fuels because, in CV injection, part of the injected spray is deposited on the intake manifold surfaces, forming a layer of liquid film. If these fuel layers are not well atomized, they enter the cylinder as drops and ligaments. During the normal combustion process, only part of the fuel deposits was completely burned. Thus, more fuel should be injected to reach the selected air-fuel ratio measured at the exhaust. When the normal flame front reached the fuel deposits, abnormal combustion was incepted. This was characterized by intense diffusion-controlled flames. Their contribution to the combustion pressure was negligible. The different levels of intensity were related to different carbonaceous structures and soot precursor concentrations. CV condition was characterized by higher fuel deposition amount and thus more intense diffusion-controlled flames than OV. Gasoline in CV condition showed the highest luminosity, and BU40 in OV condition, the lowest one. This demonstrated that BU40_OV allowed the reduction of emission of ultrafine carbonaceous particles at the exhaust and the optimization of fuel consumption at fixed performance. Moreover, medium-low percentage of butanol in the gasoline allowed the reduction of NOx and unburned hydrocarbon emission. Finally, even if an increase in the injected fuel amount should be considered to obtain the same air-fuel ratio for butanol-gasoline blend, if compared to pure gasoline, the better efficiency of fuel deposit burning allowed the reduction of that amount.