Optical Study of Soot Characteristics of Biofuel Spray Combustion

Liquid biofuels for internal combustion engines will likely be part of the future solution for mitigating CO2 emissions to the atmosphere. Several types of biofuels for compression ignition (CI) engines are being developed at the moment, made from sustainable feedstock such as waste animal fat, forest residue and algae. These fuels will only reach the consumer market if they become economically viable and meet the requirements set by authorities for fuel quality and hardware compatibility. Additionally, issues related to pollutant emissions of particulate matter and nitric oxides from CI engines must be considered, where advanced aftertreatment systems have been developed in order to comply to regulations. Particle matter emissions from CI engines poses a considerable challenge since expensive aftertreatment systems are needed and can over time reduce engine performance. By introducing biofuels, an opportunity to reduce the particulate emissions is offered. Biofuels can be designed to have low sooting tendencies and favorable combustion properties, such that clean combustion can be achieved. Biofuels can also be blended to diesel fuel gradually, where both well-to-wheel CO2 emissions and pollutant emissions can be reduced. In order to achieve this, research needs to be conducted on combustion and emission characteristics of existing and novel biofuels. An experimental suite has been developed, enabling detailed investigations on combustion and soot processes in CI engines fueled with biofuels. A redesigned engine with optical access to the combustion chamber has been commissioned, i.e. the optically accessible compression ignition chamber (OACIC), which enables direct optical measurements of a single CI spray. The OACIC is a reciprocating rapid compression machine designed to perform fuel comparison studies, allowing for fast fuel switching and high speed data acquisition. The intake air is heated and compressed, offering a range of engine-like thermodynamic conditions. For the current work, the optical technique diffuse back-illuminated extinction imaging (DBIEI) of soot has been applied to the OACIC. The technique measures the optical density of in-flame soot, which can be related to the soot mass and concentration. Further development of the technique has been performed, including an improved method for correcting the flame luminosity interference on the measurement. Uncertainties related to beam steering have also been assessed in detail, which led to