An Experimental Investigation of Alcohol/Diesel Fuel Blends on Combustion and Emissions in a Single-Cylinder Compression Ignition Engine

UV-visible digital imaging and 2D chemiluminescence were applied on a single cylinder optically accessible compression ignition engine to investigate the effect of different alcohol/diesel fuel blends on the combustion mechanism. The growing request for greenhouse gas emission reduction imposes to consider the use of alternative fuels with the aim of both partially replacing the diesel fuel and reducing the fossil fuel consumption. To this purpose, the use of ABE (Acetone-Butanol-Ethanol) fermentation could represent an effective solution. Even if the different properties of alcohols compared to Diesel fuel limit the maximum blend concentration, low blend volume fractions can be used for improving combustion efficiency and exhaust emissions. The main objective of this study was to investigate the effects of the different fuel properties on the combustion evolution within the combustion chamber of a prototype optically accessible compression ignition engine. N-butanol was investigated as the predominant product of ABE fermentation (60%) and for its properties closer to diesel if compared with acetone and ethanol. Ethanol was examined for its highest oxygen content (34.8 % wt) that may contribute to the post-oxidation with the potential to make local leaner mixtures because of the longer ignition delay. 2D chemiluminescent emission measurements during the whole combustion process were carried out using an Intensified Charge-Coupled Device (ICCD) camera coupled with two different bandpass filters: the first one centered at 310 nm, corresponding to the (0,0) transition of OH, the latter centered at 690 nm, representative of soot emission. Tests were carried out comparing combustion and emissions of the reference commercial diesel fuel to a blend of 60%v diesel and 40%v n-butanol (BU40) and 60%v diesel, 20%v n-butanol and 20%v ethanol (BU20E20). A single injection strategy was set, with a sweep on the start of injection (SOI). For the diesel case, the whole amount of injected fuel and injection pressure were set at 22 mg/str and 80 MPa, respectively, corresponding to a medium load regime for an automotive light duty diesel engine. For any investigated blend and operating point, engine tests were carried out changing the injection interval to achieve the same chemical energy as the reference diesel (935 J/str). Both investigated alcohol blends provided a significant increase in induction time, enhancing the air/fuel mixing with a simultaneous reduction of both Particulate Matter (PM) and Nitrogen Oxides (NOx). The UV-visible digital imaging and 2D chemiluminescence highlighted a change in combustion mechanism as pointed out by integral OH signal evolution: two OH peaks are more evident when switching from diesel to alcohol blends suggesting a double stage combustion process with a wide premixed combustion before the diffusion controlled stage. The simultaneous use of ethanol and n-butanol further prolonged the ignition delay with a slight decrease in energy content compared to n-butanol diesel blend. Moreover the higher oxygen content of BU20E20 improved the soot post-oxidation with a consequent benefit on PM-NOx trade off.