Extremely fuel-efficient new engine designs could considerably lower the environmental impact of vehicles, particularly if the engines function on renewable nonpetroleum-based fuels. The focus of a new computational study on fuel ignition behavior at KAUST was ensuring that these alternative fuels match next-generation engines.
The team, led by Hong Im at the KAUST Clean Combustion Center, explored the ignition of methanol-based fuel formulations. “Methanol is considered a promising fuel from both economic and environmental standpoints,” says Wonsik Song, a Ph.D. student in Im’s team. Methanol can be created renewably as a biofuel or by a solar-driven electrochemical reaction that forms methanol from carbon dioxide. However, pure methanol fuel is not suitable for the newest engine designs.
Conventional gasoline engines use a spark to burn the fuel. Some contemporary gasoline engines can change to compression ignition mode, working like a diesel engine under particular conditions to maximize fuel efficiency. But methanol is not sufficiently reactive for compression ignition, says Song. “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.”
The team applied computational analysis to examine methanol-DME combustion chemistry. Since combustion is highly complex to efficiently mimic in full, the scientists first produced a skeletal model of the process in which peripheral reactions have been removed.
“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,” explains Efstathios Tingas, a postdoctoral member of Im’s team.
The scientists demonstrated that DME dominated reaction pathways during the preliminary phase of ignition was a very effective ignition promoter. They also studied the effect of increasing the preliminary air temperature to mimic the hot spots that might form 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,” Tingas says. However, at increased temperatures the methanol itself becomes very reactive. They also considered DME’s effects on ignition timing.
“This study serves as a basic guideline to study the ignition of methanol and DME blends in combustion engines with compression ignition modes,” says Song. The following step will be to conduct more complex simulations that integrate the effects of turbulence on fuel ignition, he states.