O. El Hajj, K. Atwi, Z.Cheng, A. L. Koritzke, M. G. Christianson, N. S. Dewey, B. Rotavera, R. Saleh
Fuel, Vol. 304, 121322
Publication year: 2021

Low-temperature combustion is a promising strategy for reducing pollutant formation in internal combustion engines. However, there is a lack of understanding of how the chemistry governing the differences in ignition between low-temperature and conventional combustion affects the emission rates and physicochemical properties of particulate matter (aerosols). Here, we conducted combustion experiments in an atmospheric-pressure reactor controlled at constant equivalence ratio (ϕ = 2.3) and O2/N2 = 0.06, and at temperatures varied between 250 °C and 1035 °C. We used two fuels: toluene, which has high sooting propensity, and n-heptane, which has a comparatively lower sooting propensity but exhibits two-stage ignition that is not present in toluene combustion. We performed real-time measurements of aerosol size distributions, volatility, and light-absorption properties. We also performed offline molecular-size characterization. Aerosols emitted from both fuels were comprised of light-absorbing organics that are categorized as brown carbon. At the highest combustion temperature (1035 °C), the aerosol emissions from toluene combustion were a factor of 20 larger than n-heptane. The aerosol emissions from toluene combustion had more abundance of large molecular-size species, were less volatile, and were more light-absorbing than n-heptane. For both fuels, aerosol emission factors exhibited a steep drop with decreasing temperatures. However, there was a resurgence in aerosol emissions at lower temperatures with a peak at 290 °C for n-heptane combustion that was not observed for toluene. This is consistent with chemical kinetics simulations that show prominent two-stage ignition behavior for n-heptane, but not for toluene.