Potential of 2-ethylhexyl nitrate (EHN) and di-tert-butyl peroxide (DTBP) to enhance the cetane number of ethanol, a detailed chemical kinetic study

Ethanol is one of the most promising renewable liquid fuels to decarbonize the internal combustion engine due to its high production capacity, reasonable cost and low carbon intensity. At the same time, diesel engines are preferred over other powertrain technologies in most hard-to-electrify applications, such as heavy-duty, off-road, marine and rail. However, ethanol's properties are not suitable for modern diesel engines with ignitability being the main technical barrier for ethanol compression-ignition combustion. Cetane improvers, such as 2-ethylhexyl nitrate (EHN) or di-tert-butyl peroxide (DTBP) may be a solution to enable diesel-like ethanol combustion, but the potential of those additives to increase the cetane number of ethanol is still unclear. In this investigation, detailed chemical kinetic simulations were performed to better understand the mechanisms by which EHN and DTBP affect the ignition reactivity of ethanol. Sub-models of EHN and DTBP chemistry were merged with a chemical kinetic mechanism for ethanol and validated against experimental data. Rate of production analyses and sensitivity analyses were performed to better understand the reactions and species that control the autoignition of ethanol additized with EHN or DTBP. The formation and accumulation of acetaldehyde from ethanol-derived radicals is the bottleneck for ignition, since the remaining ethanol acts as a sink of the OH radicals required for the decomposition of acetaldehyde. A simple model to predict the derived cetane number of fuel blends is proposed and used to estimate the effectiveness of EHN and DTBP in improving the reactivity of ethanol. EHN works better as an additive as compared to DTBP, as it speeds up the main decomposition reactions of the fuel by providing the necessary OH radicals as compared to CH3 provided by DTBP.