Kinetic and Fluid-Dynamic Modeling of a CFR Engine Fueled by Oxygenated Gasoline

Abstract

ABSTRACT: In view of the constantly increasing energy use and unavoidable reliance of modern society on internal combustion engines, it is crucial to find ways to prevent and mitigate their environmental impact. The search for short- to medium-term fuel alternatives compatible with the available infrastructure while reducing CO2 life-cycle has led the attention to renewable fuels. In particular, addition of oxygenated compounds to conventional fuels has been found to increase octane rating and enhance knocking resistance, as well as reduce particular matter (PM), CO and unburned hydrocarbon emissions, making them promising candidates in terms of engine performance and pollutant emissions, but knowledge of their combustion behavior and possible emission of harmful by-products is still limited. Thus, comprehensive computational models that consider reaction kinetics and fluid dynamics could improve the understanding of their behavior under realistic combustion conditions, which would otherwise involve complex and costly testing. A Computational Fluid Dynamics (CFD) plus reaction chemistry model of an ASTM Cooperative Fuel Research (CFR) engine was implemented with the aim of assessing the potential of oxygenated gasoline. In particular, high-octane gasoline and blends (5 and 10 vol.%) with ethanol, dimethyl carbonate (DMC) and diethyl carbonate (DEC) were simulated in this spark-ignition engine model. Ethanol has been widely used as oxygenated fuel, while DMC and DEC have recently gained attention as fuel components. In fact, a previous joint research between Environmental Catalysis Research Group and GIMEL studied the role of these three fuels as gasoline additives and obtained promising experimental data for in-cylinder pressure and pollutant emissions. Thus, this work aims at providing further insight into the effect of these fuels on in-cylinder conditions during combustion, as well as on engine performance and pollutant emissions, allowing observations beyond the experimental capabilities. To our knowledge, no other CFD models considering DMC or DEC combustion have been reported. Good agreement with experimental data was observed with the model, both for cold-flow and combustion simulations. Furthermore, simulation yielded in-cylinder pressure, temperature and species temporal/spatial profiles, and Indicated Mean Effective Pressure (IMEP) values, facilitating the comparison between fuels. In addition, phenomena that would require using challenging and costly experimental techniques, such as flame evolution and formation/ consumption of chemical species, were easily assessed. Emission of species not measured experimentally was also estimated. In general, ethanol and DMC blends showed higher and earlier peaks of pressure and temperature, as well as faster flame evolution, when compared to neat gasoline, while DEC blends had lower and delayed peaks, and slower flame evolution. All regulated pollutant emissions were reduced with the use of oxygenated compounds, 10 vol.% blends showing greater reduction in most cases; in addition, the estimated emissions of non-regulated compounds would be very low to be a matter of concern. Differences in IMEP were not significant, but a consistent increase was observed at all blends with an earlier spark, confirming the slightly improved performance of the engine. Finally, results of this work corroborate that ethanol continues to be a good option as oxygenating agent for gasoline; however, once the proper economical and production conditions are given DMC could replace ethanol in the fuels market, being also a safe and environmentally friendly option.