Experimental and numerical study on non-monotonic effect of higher alcohols on ignition delay of diesel/alcohol blends under low-temperature conditions

The addition of alcohols is generally considered to reduce diesel emissions at the cost of power performance. However, the increasing low-temperature reactivity of higher alcohols has the potential to promote the diesel/alcohols low-temperature ignition (LTI) and improve the cold-start performance of diesel engine. The purpose of this work is to investigate the effect of higher alcohols (including n-propanol, n-butanol, n-pentanol and n-hexanol) on the spray and ignition performances of diesel under low temperature conditions. The experiments were conducted by constant-volume chamber system under ambient temperature of 800 K with blended fuels of 20 vol%. It was found that with longer alcohol carbon chain, the ignition delay time (IDT) of blended fuel decreases and the IDT of diesel/hexanol is shorter than that of diesel. The 3D simulation showed that the addition of alcohols mainly affects the LTI. At the spray edge region where LTI occurs, the distributions of temperature and equivalence ratio are the same. For fuel reactivity, it was found by flux analysis of alcohols that the longer carbon chain favors the low-temperature branching reaction pathways, which increases the radical pool and promotes the diesel low-temperature reactions. Compared to other higher alcohols, n-hexanol can promote ignition by increasing free radicals.