Sugar-derived levulinic esters and cyclic ether show superior anti-knock quality to Euro95 reference gasoline
A team from The Netherlands and the US reports that the sugar-derived levulinic esters methyl levulinate (ML) and ethyl levulinate (EL) and the sugar-derived cyclic ether (furfuryl ethyl ether (FEE) demonstrate superior anti-knock quality (in 10% blends) to a reference Euro95 gasoline.
The sugar-derived ethyl tetrahydrofurfuryl ether (ETE), another cyclic ether, conversely, performed markedly worse than the reference fuel on both setups. ETE this may be a more appropriate fuel additive for compression ignition engines, the authors suggest in an open-access paper published in the journal Fuel.
All the selected bio-compounds are either side products or derivatives formed in the production of 2,5-furandicarboxylic acid (FDCA)—a building block for renewable polyesters from sugar. When FDCA is successfully used, the potential of using levulinate as well as furan derivatives for fuel applications is highly relevant, the authors suggest.
Furfuryl ethyl ether (FEE) and ethyl tetrahydrofurfuryl ether (ETE) have reportedly the potential to curb soot emissions in compression ignition engines. Similar tests conducted for EL showed an equally promising soot reduction potential. ML and EL have a very low derived cetane number (DCN) (<10). Considering the inversely linear relationship between cetane number (CN) and research octane number (RON), both levulinic fuels are expected to have a high RON and might therefore be attractive candidates for use as bio-octane boosters in spark-ignition (SI) engines.
In order to test the potential of aforementioned compounds as octane boosters, the anti-knock quality of the neat oxygenates blended with gasoline was evaluated in an SI engine, and ignition delay times were measured over a range of temperature in a modified ignition quality tester (IQT).—Tian et al.
Engine experiments used a Volvo T5 turbocharged port fuel injected 5-cylinder SI engine. Because SI engines tend to be more knock prone at low speeds, the researchers selected 1500 rpm as the reference point. They chose to use only the full load or wide open throttle (WOT), as other work has shown that the anti-knock quality of various fuels is quite similar at part-load and full-load.
They used the signal energy of pressure oscillation (SEPO)—the signal energy of the band pass filtered pressure over a certain knock window—to determine the knock intensity (KI), whereby the pressure signal is filtered by a 6–25 kHz band pass filter from 10 to 40° CA after top dead center (aTDC). The KI threshold is defined by the sharp increase of SEPO that occurs at a different crank angle for different fuels as the spark timing is advanced.
The main reason for the distinctions in anti-knock quality can be found in the molecular structure of the neat biofuels. ML and EL are levulinic esters, with a carbonyl group and an ester group on the carbon chain. They can readily produce stable intermediates during the auto-ignition process, thereby slowing down the overall reaction rate.
The furanic cyclic ether (FEE) has very strong ring C–H bonds. However, the saturated cyclic ether (ETE) has weak ring C–H bonds, which facilitate more readily ring opening reactions. Long side chains on the cyclic ethers further accelerate the reaction rate.—Tian et al.
Miao Tian, Robert L. McCormick, Jon Luecke, Ed de Jong, Jan C. van der Waal, Gerard P.M. van Klink, Michael D. Boot (2017) “Anti-knock quality of sugar derived levulinic esters and cyclic ethers,” Fuel, Volume 202, Pages 414-425 doi: 10.1016/j.fuel.2017.04.027