The Effect of Blending with Octane Boosters on Aerosol Formation in Low-Temperature Combustion of n-Heptane
OMAR EL HAJJ, Brandon Rotavera, Rawad Saleh,
University of Georgia Abstract Number: 597
Working Group: Combustion
AbstractWe have previously shown that fuels with two-stage ignition also exhibit two-stage aerosol formation. First-stage ignition is undesirable in spark-ignition engines and is minimized by blending gasoline with high research octane number (RON) blendstocks. Previous studies have reported that some blendstocks blend synergistically or antagonistically for RON, meaning that they produce higher or lower RON, respectively, than predicted by a linear-mixing model.
Here, we investigated whether the low-temperature combustion chemistry governing blending behavior for RON affects first-stage aerosol formation. We used
n-heptane, a two-stage ignition fuel, as a primary fuel and blended it with three different blendstocks: ethanol, toluene, and 2,5-dimethylfuran (DMF).
n-heptane is known to blend synergistically with ethanol and DMF, whilst it blends antagonistically with toluene. We conducted combustion experiments in a reactor controlled at constant equivalence ratio (ϕ = 1) and O2/N2 = 0.1, and at temperatures between 250 °C and 600 °C to capture the first-stage ignition region. We measured the aerosol size distributions to calculate aerosol emission factors (EF) for each blendstock at different blending ratios.
The peak EF of pure
n-heptane was 600 mg/kg-fuel, while the pure blendstocks exhibited no first-stage aerosol formation (EF = 0). At 50% mole fraction, blending with toluene reduced EF to 330 mg/kg-fuel, slightly higher than predicted by a linear-mixing model (300 mg/kg-fuel). On the other hand, blending at 50% with ethanol and DMF reduced EF to 60 and 18 mg/kg-fuel, respectively. These results indicate that synergistic blending for RON leads to more efficient suppression of first-stage aerosol compared to antagonistic blending. Blending with DMF led to stronger reduction in first-stage aerosol compared to ethanol, which is in-line with previous reports on DMF being a superior RON booster and indicates that certain biofuel additives can simultaneously enhance ignition characteristics and reduce aerosol formation.