Large-eddy simulation of diesel pilot spray ignition in lean methane-air and methanol-air mixtures at different ambient temperatures

In dual-fuel compression-ignition engines, replacing common fuels such as methane with renewable and widely available fuels such as methanol is desirable. However, a fine-grained understanding of diesel/methanol ignition compared to diesel/methane is lacking. Here, large-eddy simulation (LES) coupled with finite rate chemistry is utilized to study diesel spray-assisted ignition of methane and methanol. A diesel surrogate fuel (n-dodecane) spray is injected into ambient methane-air or methanol-air mixtures at a fixed lean equivalence ratio phi(LRF) = 0.5 at various ambient temperatures (T-amb = 900, 950, 1000 K). The main objectives are to (I) compare the ignition characteristics of diesel/methanol with diesel/methane at different T-amb, (2) explore the relative importance of low-temperature chemistry (LTC) to high-temperature chemistry (HTC), and (3) identify the key differences between oxidation reactions of n-dodecane with methane or methanol. Results from homogeneous reactor calculations as well as 3 + 3 LES are reported. For both DF configurations, increasing T-amb leads to earlier first- and second-stage ignition. Methanol/n-dodecane mixture is observed to have a longer ignition delay time (IDT) compared to methane/n-dodecane, for example approximate to three times longer IDT at T-amb = 950 K. While the ignition response of methane to T-amb is systematic and robust, the T-amb window for n-dodecane/methanol ignition is very narrow and for the investigated conditions, only at 950 K robust ignition is observed. For methanol at T-amb = 1000 K, the lean ambient mixture autoignites before spray ignition while at T-amb = 900 K full ignition is not observed after 3 ms, although the first-stage ignition is reported. For methanol, LTC is considerably weaker than for methane and in fully igniting cases, heat release map analysis demonstrates the dominant contribution of HTC to total heat release rate for methanol. Reaction sensitivity analysis shows that stronger consumption of OH radicals by methanol compared to methane leads to the further delay in the spray ignition of n-dodecane/methanol. Finally, a simple and novel approach is developed to estimate IDT in reacting LES using zero-dimensional IDT calculations weighted by residence time from non-reacting LES data.