Highly fuel-efficient new engine designs could significantly reduce the environmental impact of combustion-engined vehicles, especially if the engines run on renewable non-petroleum-based fuels. Ensuring these unconventional fuels are compatible with next-generation engines was the aim of a new computational study on fuel ignition behavior at KAUST. A paper on their work is published in the journal Combustion and Flame.
The team, led by Hong Im at the KAUST Clean Combustion Center, investigated the ignition of methanol-based fuel formulations. Methanol can be produced renewably as a biofuel or by a solar-driven electrochemical reaction that makes methanol from carbon dioxide. However, pure methanol fuel is ill-suited to the latest compression ignition (CI) engine designs.
Our approach is to blend a more reactive fuel, dimethyl ether (DME), with methanol to make a fuel blend usable in compression ignition engines that provide better combustion efficiency than the spark-ignition counterpart.—Wonsik Song, a Ph.D. student in Im’s team and lead author
The overarching practical question of the study was the extent to which the addition of DME (CN = 55) improves the ignitability of the methanol (CN = 5) at the temperature and pressure conditions that are relevant to CI engine applications.
The team used computational analysis to investigate methanol-DME combustion chemistry. Because combustion is too complex to efficiently simulate in full, the researchers first generated a skeletal model of the process in which peripheral reactions have been stripped away.
Starting from the detailed model, including 253 chemical species and 1542 reactions, we generated a skeletal model comprising 43 species and 168 reactions that accurately describe the ignition and combustion characteristics of methanol and DME.—Efstathios Tingas, a postdoctoral member of Im’s team
The researchers showed that DME dominated reaction pathways during the initial phase of ignition and was a highly effective ignition promoter. They also examined the effect of increasing the initial air temperature to simulate the hot spots that might develop inside the engine.
At high temperatures, DME actually retards ignition slightly, because DME chemistry relies on the formation of some highly oxygenated molecules, which are inherently unstable at higher temperatures.—Efstathios Tingas
However, at high temperatures the methanol itself becomes highly reactive. They also studied DME’s effects on ignition timing.
As a baseline analysis to assess the chemical aspects, the uniform temperature condition at 850 K for the fuel versus air mixing layer was investigated. At this condition, the low temperature chemistry associated with the DME fuel was found to be highly effective in promoting autoignition of the fuel mixture. Detailed CSP analysis of the chemical pathways associated with the explosive mode for the DME0 and DME50 clearly revealed the distinct reaction pathways for methanol and DME ignition, the latter of which involves various oxygenated intermediates and favors the fuel-lean conditions and dominates the initiation of the ignition process. The methanol reaction pathways were subsequently activated during the second stage ignition of DME.
With the understanding of chemical characteristics of the two fuels, the effect of the oxidizer side temperature was examined next, as a practical relevance to fuel injection into hotter air in the combustion chamber of the IC engines. As the overall reactivity increases and the dominant chemical pathways become shifted towards the high temperature reactions, the benefit of the DME addition in enhancing ignition was found to diminish as the oxidizer temperature was raised. At the oxidizer temperature of 1300 K, even a slight adverse effect in ignition was observed as DME was added to the fuel stream, although at this condition the methanol itself becomes highly reactive and does not need much ignition boost by DME blending.
Finally, the strain rate effect on the ignition delay time was investigated. While the pure methanol case showed a significant sensitivity to the strain rate variations, the sensitivity was found to diminish as highly reactive DME addition was increased.—Song et al.
The next step will be to perform more complex simulations that incorporate the effects of turbulence on fuel ignition.
Wonsik Song, Efstathios-Al. Tingas, Hong G. Im (2018) “A computational analysis of methanol autoignition enhancement by dimethyl ether addition in a counterflow mixing layer,” Combustion and Flame, Volume 195, Pages 84-98 doi: 10.1016/j.combustflame.2018.03.037