Autoignition enhancing additives have been used for years to enhance the ignition quality of diesel fuel, with 2-ethylhexyl nitrate (EHN) being the most common additive. EHN also enhances the autoignition reactivity of gasoline, which has advantages for some low-temperature combustion techniques, such as Sandia’s Low-Temperature Gasoline Combustion (LTGC) with Additive-Mixing Fuel Injection (AMFI). LTGC-AMFI is a new high-efficiency and low-emissions engine combustion process based on supplying a small, variable amount of EHN into the fuel for better engine operation and control. However, the mechanism by which EHN interacts with the fuel remains unclear. In this work, a chemical-kinetic mechanism for EHN was developed and implemented in a detailed mechanism for gasoline fuels. The combined mechanism was validated against shock-tube experiments with EHN-doped n-heptane and HCCI engine data for EHN-doped regular E10 gasoline. Simulations showed a very good match with experiments.
EHN chemistry fundamentals were also studied. Under LTGC-AMFI engine conditions, EHN generates NO2, formaldehyde and a combination of ~85% 3-heptyl and ~15% 1-butyl radical and butoxy diradical. Results show that the 3-heptyl and 1-butyl radicals are responsible for the autoignition-enhancing effect of EHN. Each mole of these radicals rapidly generates 2 moles of OH, which accelerate the low-temperature chemistry of the fuel, increasing its reactivity. The effects of the operating conditions on the effectiveness of EHN to increase the autoignition reactivity of the fuel were also studied. EHN’s effectiveness for increasing the autoignition reactivity is highest in the low-temperature regime, and it decreases as the temperature increases. EHN’s effectiveness to increase autoignition reactivity decreases with the combination of intake-pressure boost and EGR for typical engine operation. The effect of EHN on autoignition reactivity increases as equivalence ratio increases, enhancing the fuel’s φ-sensitivity. Therefore, with fuel stratification, EHN’s larger enhancement of autoignition reactivity for richer regions makes stratification techniques more effective.
