Vehicle standards for lower emissions and better fuel efficiency have driven the development of smaller engines that deliver more horsepower in a lighter package. To achieve higher horsepower, engines are designed with higher compression ratios, which raise the temperature of pre-combustion gases in the engine cylinders. Unfortunately, those higher temperatures promote engine “knock”—the sound that results when the fuel-air mixture ignites out of sequence with the spark timing and piston motion. This offset can cause engine damage, but engine control system changes to spark timing for reducing knock end up sacrificing engine power.

Higher octane fuels can reduce knocking but are expensive to produce. In addition, studies show that octane requirements for idling are much lower than for acceleration; a discrepancy that leads to inefficient use of the high-octane fuel components. To more efficiently utilize the high-octane fuel components, an on-board system can be used to separate ethanol or other oxygenates from a single fuel source into higher and lower octane components. The system can then meter the appropriate ratio of high- and low-octane fuels to the engine, as dictated by driving conditions. This metering can improve fuel efficiency by 17.5–30 percent.

A tested and proven technology to improve fuel efficiency

PNNL has developed and tested a separations system to extract both ethanol and butanol from commercial gasoline to deliver “octane-on-demand.” The system can use one of three chemical separation approaches: solid-supported amines with reusable filter media, ionic liquids, and switchable-polarity solvents. In proof-of-concept testing, all three approaches demonstrated 95 percent efficiency in separating ethanol from commercial gasoline. Test results also indicate promise for extracting oxygenated hydrocarbon additives, such as isomers of butanol.

PNNL’s liquid-solid and liquid-liquid extraction techniques provide advantages over the pervaporation (partial vaporization) membranes currently used, which lose efficiency as the oxygenate is removed. In addition, these membranes do not separate all the oxygenate, leading to losses of the valuable high-octane fuel component.

Additional test results showed the separation materials can preferentially remove aromatics from the fuel. Aromatics are crude-oil derived gasoline components that boost octane ratings but contribute to soot formation, especially during startup when the engine is cold. Reintroduction of the aromatics into the warmer, running engine would reduce harmful emissions and contribute to more efficient fuel use.

Applicability

For some vehicle types, such as passenger cars and light/medium duty trucks that use high-octane oxygenated fuels in a spark ignition combustion scheme, PNNL’s on-board chemical separations approaches could be used to improve fuel efficiency. Additionally, if developed, separation of aromatic fuel components for later reintroduction would reduce cold start emissions for many vehicles using spark ignition (gasoline) and compression ignition (diesel) engines.