Comparison of Long-Chain Alcohol Blends, HVO and Diesel on Spray Characteristics, Ignition and Soot Formation

Spray characteristics of fossil Diesel fuel, hydrotreated vegetable oil (HVO) and two oxygenated fuel blends were studied to elucidate the combustion process. The fuels were studied in an optically accessible high-pressure/ high-temperature chamber under non-combusting (623 K, 4.59 MPa) and combusting (823 K, 6.04 MPa) conditions. The fuel blends contained the long-chain alcohol 2-ethylhexanol (EH), HVO and either 20 vol.% Diesel or 7 vol.% rapeseed methyl ester (RME) and were designed to have a Diesel-like cetane number (CN). Injection pressures were set to 120 MPa and 180 MPa and the gas density was held constant at 26 kg/m3. Under non-combusting conditions, shadow imaging revealed the penetration length of the liquid and vapor phase of the spray. Under combusting conditions, the lift-off length and soot volume fraction were measured by simultaneously recording time-resolved two-dimensional laser extinction, flame luminosity and OH* chemiluminescence images. The ignition delay and start of soot formation were also recorded. Under non-combusting conditions at both injection pressures, the liquid penetration length was higher for the blends and HVO compared to Diesel, whereas the vapor penetration length was similar for all fuels. Under combusting conditions, the liquid penetration length of all the tested fuels was similar. Despite different CNs, the ignition delay was similar for Diesel and HVO. The EH blends had an increased ignition delay compared to Diesel, despite having the same CN. The lift-off length was found to be highest for the blend containing the highest share of EH. In agreement with previously published scaling relations, the lift-off length increased with increasing injection pressure. The soot volume fraction was found to be lower for the blends, in agreement with engine studies.