Experimental Investigation of Butanol Isomer Combustion in Spark Ignition Engines

Butanol has been proposed as a biologically derived fuel that has significant advantages over ethanol in terms of energy density and miscibility with diesel. This has generated the need to study separately and evaluate comparatively the in-engine combustion of the four isomers of butanol. Previous studies on the combustion of butanol isomers in laminar premixed flames have shown that while the isomers exhibit several similar combustion characteristics, including adiabatic flame temperature and flame speed, pollutant formation is highly dependent on the precise chemical structure of each isomer. The objective of this study is to build on these findings by investigating the effect of three of the four butanol isomers (n-butanol, isobutanol, and sec-butanol) on engine performance and emissions. The three isomers were blended as 30% butanol and 70% gasoline on a mass basis. These fuel blends were tested in a single-cylinder port-injection spark-ignition engine. In order to characterize the three fuels, engine performance and emissions were measured at three different engine loads while maintaining consistent air-fuel ratios and spark timing. Engine torque and cylinder pressure as well as emissions of oxides of nitrogen (NO_x), unburned hydrocarbons (UHC), carbon dioxide (CO₂) and carbon monoxide (CO) were measured and compared between the different isomer blends. It was established that while n-butanol, isobutanol, and secbutanol behaved similarly with regard to brake torque and peak in-cylinder pressure, they behaved quite differently with regard to emissions. While sec-butanol exhibited the highest average UHC emissions, isobutanol exhibited the highest average CO emissions, and n-butanol exhibited the highest average NO_x emissions.