CFD analysis of combustion and emission characteristics of primary reference fuels: from transient Diesel spray to heavy-duty engine

The pre-blending of low- and high-reactivity fuels for a single direct injection system has been proven to be an effective way to control the reactivity of mixtures in compression ignition engines, having the potential to simultaneously reduce fuel consumption and pollutant emissions. However, there is not much knowledge about the complex physical-chemical phenomena in the turbulent sprays with fuels having widely different autoignition qualities, although this information is critical for the design and development of cleaner combustion systems based on this concept. For this reason, a computational analysis of ignition behavior, flame structure, and soot production for reacting sprays with five primary reference fuels (PRFs), from PRF0 (n-heptane) to PRF80 (20% n-heptane, 80% iso-octane) with 20% increment in iso-octane mass fraction, was first performed using the Tabulated Flamelet Progress Variable (TFPV) approach based on the tabulation of diffusion flamelets for different scalar dissipation rates. The temporal and spatial features of the flame structure and soot formation for different fuels were investigated with the so-called intensity-axial distance-time (IXT) plots. Then, ten PRFs, from RRF0 to PRF90 with 10% increment in iso-octane mass fraction, were investigated and compared in a heavy-duty Diesel engine operating at the conventional high-temperature, short-ignition delay (HTSID) condition. The injection timing was altered from -5 to -13 degrees ATDC to optimize the combustion phase and engine performance for different fuels. The results showed that PRF70 exhibited the best performance at the tested condition, which reduced the soot mass to 5% of the baseline value without sacrificing fuel efficiency.