Auxiliary Power Units to Reduce Idling in Diesel Engines

Electron-microscope images showing graphite layers on nickel catalyst (above), and carbon nanotubes that form on surface of nickel catalyst during steam reforming of the graphite layers (below). (Photo courtesy of Johannes Schwank)

When the engines of heavy-duty trucks idle during loading and unloading and at freeway rest stops, they burn about $1 billion worth of diesel fuel every year and put considerable volumes of pollutants into the atmosphere. Idling also increases the wear and tear on engines, creating another $1 billion in annual expenditures for engine maintenance.

Johannes Schwank, a professor in the Department of Chemical Engineering and director of Michigan Engineering’s Transportation Energy Center (TEC), has put together a team that’s working on fuel processors and fuel cells as auxiliary power units (APUs) that will reduce the need to keep truck engines idling.

“Fuel cell auxiliary power units would decrease fuel consumption, reduce emissions of greenhouse gases and particulate matter and be much quieter than idling diesels,” Schwank said. “And that’s not all. They could also serve as generators, battery chargers and heat supplies.”

An APU is about the size of a small refrigerator, contains solid oxide fuel cells and can operate either with pure hydrogen or with a mixture of hydrogen and carbon monoxide gas. However, APUs need only a small amount of fuel – an amount too small to justify the development of a dedicated hydrogen-fueling infrastructure. So, to be efficient, an APU will have to run on diesel fuel and share the on-board fuel with the truck’s main engine. It would also have to be capable of reaching full power rapidly after start-up.

Schwank said that making hydrogen for the fuel cell from diesel fuel on board the vehicle is particularly challenging. “Industry has a lot of experience in making hydrogen from natural gas in large-scale steam reforming plants. But small-scale production is a huge challenge.”

Steam reforming is a method of producing hydrogen from hydrocarbons, such as natural gas and diesel fuel.

The ability to convert gasoline or diesel into hydrogen suitable for a portable solid oxide fuel cell on board a truck “will require major breakthroughs in catalytic materials and reactor technology,” Schwank said. “The catalyst must be active for both partial oxidation and steam reforming reactions, and it must withstand coking during operation.”

Coking occurs when carbonaceous materials are converted at high temperature into a residue of impure carbon. This residue can block the surface of the catalyst and eventually plug up the reactor.

“Some attempts have been made to understand how the chemical structure of fuels influences coking behavior, but a clear picture doesn’t currently exist. The research our team is doing in the TEC is focused on reforming large hydrocarbon molecules over a specially formulated nickel catalyst, where the nickel particles are supported on an oxygen-storage ceramic substrate.”

Strong, thin-walled ceramic substrates make it possible to increase the surface area of catalysts, which improves their efficiency and durability.

For a catalyst to be successful in an APU, it must have certain characteristics. It can’t sinter – that is, it can’t become a solid mass – at the high temperatures reached during fuel reforming. It has to be tolerant to some sulfur found in fuels. It must start up and shut down without complicated regeneration procedures. And, finally, the catalyst must work with different fuel compositions because there are seasonal and regional changes in diesel fuel formulations.

“These are tough, tough problems,” Schwank said, “But our goal is to develop the catalysts and compact on-board reactor systems needed to break down liquid fuels such as gasoline, jet fuel and diesel into hydrogen and carbon monoxide. These reactors will supply a stream of hydrogen and carbon monoxide to solid oxide fuel cell auxiliary power units, without the need to store hydrogen on board of the vehicle.”

Michigan Engineering researchers in the TEC are demonstrating that, in a world where every bit or energy is becoming more precious, and every contaminating particle is making life more precarious, even an idling diesel engine offers an opportunity to improve the quality of life.

Support for this program comes from the U.S. Army.