Pilot Ignition of Ammonia Spray Using Dimethyl Ether Spray at Elevated Temperature: A Numerical Study

Ammonia (NH3) is a promising zero-carbon fuel to eliminate carbon footprint while the high autoignition temperature and low combustion rate of NH3 remain challenging for practical implementation. Using dimethyl ether (DME) as pilot ignition fuel can substantially promote the reactivity of NH3, thus paving the way for a widespread application of NH3. In this study, the ignition process and nitrogen oxides (NOx) emissions of the NH3 liquid spray ignited by liquid DME spray were numerically investigated using Converge software. The ambient temperatures (Tamb) ranging from 900 K to 1100 K were used to mimic the in-cylinder temperature typically encountered in turbocharger engines. The effect of ammonia energy ratio (AER) and fuel injection timing was examined as well. It is found that only half of NH3 is consumed at Tamb = 900 K while 97.4% of NH3 is burned at Tamb = 1100 K. Nitric oxide (NO) and nitrogen dioxide (NO2) formation also have strong correlation with Tamb and NO2 is usually formed around the periphery of NO through these two channels HO2 + NO = NO2 + OH and NO + O(+M) = NO2(+M). Extremely high nitrous oxide (N2O, formed by NH + NO = H + N2O) and carbon monoxide (CO) are produced with the presence of abundant unburned NH3 at Tamb = 900 K. Additionally, increasing AER from 60% to 90% results in slightly declined combustion efficiency of NH3 from 98.7% to 94%. NO emission has a non-monotonical relationship with AER owing to the ‘trade-off’ relationship between HNO concentration and radical pool at varying AERs. A higher AER of 95% leads to failed ignition of NH3. Advancing DME injection not only increases combustion efficiency, but also reduces NOx and CO emissions.