Researchers at the US Department of Energy’s Pacific Northwest National Laboratory (PNNL) have developed an onboard separation system that could support increased fuel economy and lower greenhouse gas emissions as part of an octane-on-demand fuel-delivery system. A paper on the team’s exploratory research, which was supported under the DOE’s Co-Optima initiative, was published in the ACS journal Energy & Fuels.

Designed to work with a car’s existing fuel, the onboard separation technology is the first to use chemistry—not a physical membrane—to separate ethanol-blended gasoline into high- and low-octane fuel components. An octane-on-demand system can then meter out the appropriate fuel mixture to the engine depending on the power required: lower octane for idling, higher octane for accelerating.

PNNL’s onboard separation technology enables octane-on-demand to improve performance and overall fuel economy for passenger vehicles and light duty trucks. | PNNL

Studies have shown that octane-on-demand approaches can improve fuel economy by up to 30% and could help reduce greenhouse gas emissions by 20%. But so far, the pervaporation membranes tested for octane on demand leave nearly 20% of the valuable high-octane fuel components in the gasoline.

In proof-of-concept testing with three different chemistries, PNNL’s patent-pending onboard separation technology separated 95% of the ethanol out of commercial gasoline. The materials are also effective for separating butanol, a promising high-octane renewable fuel component.

High-compression engines that squeeze the most work out of each drop of fuel are the engines of the future. Unfortunately, these engines exacerbate engine knock. Knock happens when an engine’s piston and combustion sequence are momentarily out of sync—usually during acceleration. Knock can rob vehicles of power and even cause expensive engine damage.

Higher-octane fuels can eliminate knock but are expensive to produce. Ethanol is an inexpensive fuel additive that increases the octane rating to combat knock. The additive modestly curbs greenhouse gas emissions, but also reduces vehicle performance and fuel economy. When a car burns gas while sitting at a stop light or idling at the curb, it’s wasting the valuable high-octane fuel better used for acceleration.

That’s where PNNL’s onboard separation technology comes in. As part of an octane-on-demand system, the technology optimizes the available fuel by staging the right fuel for the right time.

Allan Tuan, a commercialization manager at PNNL, said that federal requirements for both renewable fuels and increasing fuel economy make new fuel strategies like octane on demand more important and more relevant than ever.

With the increasing use of ethanol and, over time, other biofuels, a technology like PNNL’s onboard separation technology means we don’t need to choose between reduced greenhouse gas emissions and fuel economy.

—Allan Tuan

Tim Bays, a chemist in PNNL’s Energy Processes and Materials Division, led the research team. With colleagues John Linehan, David Heldebrant and Kat Grubel, the team developed three new approaches for onboard separations of ethanol from gasoline.

The objective of each approach was to tie up ethanol, then release it back when specified. All three chemistries worked effectively, but one approach has the fewest barriers to market: solid-supported amines with reusable silica sorbent media.

SAMMS is a sponge-like silica material that can latch onto certain molecules and hold them indefinitely or release them when specified. The image shows molecules attaching to the material in layers. | PNNL

The solid-supported amine approach is based on self-assembled monolayers on mesoporous supports, or SAMMS, technology pioneered at PNNL by scientists Glen Fryxell and Thomas Zemanian. SAMMS has expanded to many different commercial applications. Bays had previously worked with Fryxell, Zemanian and others on a carbon dioxide separations technology that the US Navy now uses to purify the air in its submarines. These and other PNNL advances in chemical separations and vehicle emissions research prompted the separation ideas the team pursued.

Based on early industry feedback, the team is focusing on the SAMMS approach. Bays said the biggest science question at this point is material stability or degradation over time. Other questions are more about engineering, such as time, cost and weight.

Make no mistake, anytime you increase the complexity of the auto, it’s a barrier. We just need to make the technology as simple and hands-off as possible, so it’s transparent to the consumer. It just happens.

—Tim Bays

A happy surprise along the way, said Bays, were test results that showed the separation materials could preferentially remove aromatics from the fuel. Aromatics are gasoline components derived from crude oil. These components boost octane ratings but also form soot particles, especially during start-up when the engine is cold. Reducing the amount of these particles is a priority for newer, cleaner-burning fuels.

The PNNL team believes that, with further development of the technology, the aromatics could be reintroduced into the warmer, running engine to reduce harmful cold-start emissions. This problem is common to both gasoline and diesel engines.

Companies interested in the technology can contact the PNNL Commercialization Office.

Resources

• Katarzyna Grubel, Wilaiwan Chouyyok, David J. Heldebrant, John C. Linehan, and J. Timothy Bays (2019) “Octane-On-Demand: Onboard Separation of Oxygenates from Gasoline” Energy & Fuels 33 (3), 1869-1881 doi: 10.1021/acs.energyfuels.8b03781