Analysis of the Effects of Certain Alcohol and Furan-Based Biofuels on Controlled Auto Ignition

For gasoline engines controlled autoignition provides the vision of enabling the fuel consumption benefit of stratified lean combustion systems without the drawback of additional NO_x aftertreatment.

In this study the potential of certain biofuels on this combustion system was assessed by single-cylinder engine investigations using the exhaust strategy "combustion chamber recirculation" (CCR). For the engine testing sweeps in the internal EGR rate with different injection strategies as well as load sweeps were performed. Of particular interest was to reveal fuel differences in the achievable maximal load as well as in the NO_x emission behavior. Additionally, experiments with a shock tube and a rapid compression machine were conducted in order to determine the ignition delay times of the tested biofuels concerning controlled autoignition-relevant conditions. The tested biofuels included the pure alcohols 1-Butanol and 2-Butanol as well as the furans Tetrahydro-2-Methylfuran (2-MTHF) and 2-Methylfuran. Conventional RON 95 pump fuel and Ethanol served as state-of-the-art non-bio and bio references.

The engine tests revealed Ethanol and 2-Butanol as the biofuels which needed the highest rate of trapped internal residuals and are least prone of autoignition. 1-Butanol, RON 95 pump fuel as well as 2-Methylfuran show similar autoignition behavior. 2-MTHF can be auto ignited with the lowest amount of internal EGR. It could be shown that for comparable center of combustion the fuel individual necessary internal EGR amount correlates almost linearly with its octane number, especially MON. Also the ignition delay measurements are mostly in line with the engine results. All investigated alcohol fuels reveal comparably low NO_x emissions due to their high enthalpy of vaporization and particularly lower adiabatic flame temperatures. E.g., Ethanol shows in contrast to the conventional RON 95 pump fuel up to 76% less NO_x emissions at comparable center of combustion. At sufficiently lean conditions the highest engine loads with controlled autoignition combustion are obtained with the high-knock-resistant fuels Ethanol and 2-Butanol. For both fuels the maximum engine load can be enhanced by approximately 10% compared to the RON 95 pump fuel at an engine speed of n = 1500 min^-₁, whereas 2-MTHF revealed an approximately 8% lower feasible maximal load.