A Fuel-ish Inconsistency

By Derek Green

It’s all about the fuel for the vehicles we drive or ride in: How efficient is it? How clean can it burn? How little can it cost? Can new fuels replace fossil fuels? Answering these questions is as great a challenge as were the quests for the New World, antibiotics, atomic energy or space travel.

It’s a global race, and U-M is competing in it, just as it has since the dawn of the Automotive Age. In a quiet, windowless laboratory in the College of Engineering on North Campus, researchers test how well various precious metals can scrub sulfur from gasoline byproducts in prototype fuel cells. At a lab just a street over, a huge diesel engine roars on and on while computers snap high-speed photos of the fuel burning within its cylinders.

Thompson, the champion of hydrogen fuel cells

The quest follows two main paths: toward improving existing fuel technologies or toward finding alternatives to fossil fuels. We’ll follow both of them in the labs of two U-M fuel experts. Though their approaches differ, they complement, rather than rival, one another. Both see a future—one not all that far off—in which we fundamentally transform the way we power our cars and just about everything else, from wristwatches and tools to home appliances and even whole cities.

‘The hydrogen economy is coming.’

An internal combustion (IC) engine powers almost every car and truck on the road today. IC engines burn fuel—usually a hydrocarbon compound like gasoline or diesel—and rely on the expansion of hot gases to do mechanical work. IC engines, however, come with well-known drawbacks.

First off, they’re not very efficient at converting the energy derived from gasoline or diesel into the mechanical energy needed to turn the wheels. (Most reports indicate that only 20 percent of the energy produced in an IC engine reaches the road, though that number varies, depending on whom you talk to.) Secondly, IC engines emit pollutants that cause acid rain and global warming.

Hydrogen fuel cells at first glance seem to offer a way around those problems. Though the technology has a futuristic sound to it, the first fuel cell was devised in 1839, meaning it predates the earliest gasoline engines by a generation. Modern fuel cells use catalysts to facilitate electrochemical reactions between hydrogen and oxygen. The reactions generate direct current useful for all sorts of applications, including powering an automobile, and the byproduct of the basic reaction is water.

So why not pull out your car’s conventional engine and replace it with an efficient and environmentally friendly fuel cell? For one thing, although hydrogen is abundant in free form outside Earth’s atmosphere, in the biosphere it’s almost always locked up in water, hydrocarbons (like gasoline), alcohol compounds (methanol and ethanol) and even vegetable oils. Hydrogen can be extracted from these compounds in many ways, including the electrolysis of water, the burning of fossil fuels and even using heat generated by nuclear reactors.

But extracting hydrogen on a large scale requires energy, and once extracted other challenges remain. Hydrogen is a low-density gas, meaning it’s hard to store. Using current technology, a car with a 250-mile range would require a hydrogen tank 15 yards wide. Another problem: hydrogen is highly explosive—meaning a risk of a mini-Hindenburg blast at every fender bender.

But say you’ve managed to store hydrogen in a secure tank. You’ll find next that there are no filling stations with hydrogen pumps. Some experts say it will cost trillions of dollars to build a national system of hydrogen filling stations. (Currently, only two prototype hydrogen fuel stations exist, one in Sacramento and the other in Dearborn—which makes for a long haul between refills.)

All this means that the most practical form of fuel cell in the near future would be one that could use more readily available forms of fuel. Devices known as fuel processors, or reformers, can convert hydrocarbonslike gasoline, diesel and alcohol fuels into hydrogen.

Prof. Levi Thompson, a chemical engineer and associate dean of engineering, directs a U-M research team that hopes to come up with a fuel processor that can remove sulfur from hydrocarbons (sulfur spoils the chemical catalysts needed to convert hydrocarbons into pure hydrogen) and convert the hydrocarbons efficiently and inexpensively into hydrogen. Existing processors that carry out these complicated chemical conversions are too big to fit on an automobile.

“We’re working to develop more practical, smaller fuel processors,” Thompson says. So far his project is progressing “slightly better than we expected,” but it still faces big hurdles, such as how to deal with the fact that gasoline-to-hydrogen conversion produces carbon monoxide.

Especially promising is the Thompson group’s development of catalyst materials to replace the strategically important and expensive noble metals—such as platinum—currently used in fuel processors. Because of patents pending on the work, Thompson won’t give details about the new catalysts. All he’ll say now is that “the new materials we’ve developed significantly out-perform the materials available right now.”

Thompson and his group expect to develop a prototype 10-kilowatt working gasoline processor within four years. A typical V8 gasoline engine generates 250 kW, meaning it will be a while before a hydrogen-powered car is on the market.

An amiable man with a visionary’s knack for infecting a listener with enthusiasm for his technology’s future applications, Thompson emphasizes that “an important part of the national energy policy focuses on energy security. Imagine if wherever we use gasoline we could replace it with water. Everywhere you see gasoline, take it out. Replace it with hydrogen fuel. Wherever you use a battery, take it out. Replace it with a fuel cell.”

In such a world, he says with a smile, “The Great Lakes region would be more important in terms of energy security than the Middle East.” That possibility accounts, in part, for the federal government’s willingness to fund his laboratory with a $6 million Department of Energy grant.

“The hydrogen economy is coming,” Thompson says. “The bottom line is, it’s still pretty early and there are problems to be solved. But it’s coming.”

‘Internal combustion is here to stay’

Dennis Assanis believes that reports of the internal combustion engine’s death are greatly exaggerated. Sure, hydrogen fuel cell technology will be vastly important down the road, he agrees, but he thinks it’s just as important now to make gasoline and diesel engines cleaner and more efficient.

“One obvious advantage of the internal combustion engine is that it already exists,” says Assanis, the John R. and Beverly S. Holt Professor of Engineering, who chairs the Department of Mechanical Engineering and directs the U-M’s Automotive Research Center (ARC). “We can now have advanced designs of up to 45 percent efficiency, about twice the current mark for typical passenger car engines in North America.” And that, he adds, is nearly as efficient as future hydrogen engines promise to be when measured in “well-to-wheel efficiency” (a measure that takes into account all steps, from the extraction of raw resources at the oil well to getting energy working “where the rubber meets the road”).

Assanis, the champion of internal combustion

One area of focus for Assanis and his team is the development of cleaner diesel engines and hybrid power trains. It might sound like a back-to-the-future approach to most Americans—diesel engines as a clean technology? But as Assanis points out, the diesel engine is already the power train of choice in Europe, where EU automakers, unlike their US counterparts, must meet Kyoto Accord emission requirements.

“Diesel engines offer excellent mileage; they’re robust and last forever,” Assanis says. He thinks it’s unfortunate that Americans associate the word diesel with big sooty buses. “Those engines are older designs,” Assanis continues, “not the latest engines, which treat exhaust byproducts and reduce emissions. Diesel engines go for a million miles, so people who frown on diesels are looking at technology that’s 30 to 40 years old.”

Although modern IC engines are “fuel tolerant”—meaning they can burn just about any combustible liquid or gas as fuel—they predominantly use gasoline or diesel fuels. Engines achieve ignition differently, however. Gasoline engines use a spark to ignite premixed fuel and air in the combustion chamber. In contrast, a diesel engine injects the fuel directly into the chamber and compresses the air-fuel mixture to a much higher pressure, resulting in the auto-ignition of fuel for a “sparkless” ignition.

By design, diesel engines operate with much higher compression ratios than gasoline engines, as they are not limited by the “knocking” caused by poor timing of the spark and therefore achieve much higher fuel economy. However, both gasoline and diesel engines produce harmful emissions, which include nitrogen oxides, unburned hydrocarbons and, for diesels, soot particulates and smoke, as well.

In engine test cells in the Walter E. Lay Automotive Laboratory, Assanis and his team are trying to figure out ways to maximize the efficiency of both types of engine and minimize their emissions so as to satisfy the most stringent regulatory standards. One example is variable valve timing. At relatively cooler temperatures just after start-up, when gasoline engines generate most of their toxic emissions, the exhaust valve can be opened and closed earlier. This strategy, already simulated and proven in the engine lab, allows hot combustion products to flow down the exhaust pipe to the catalyst faster, markedly reducing the emission of unburned hydrocarbons. By closing the valve earlier, a variable-timing engine can recycle a portion of the unburned hydrocarbons for the next combustion cycle.

Another innovation is a variable compression ratio system developed jointly by Assanis’s group and Ford Motor Company. It uses specially designed, spring-loaded piston heads capable of adjusting their shape in order to maximize fuel efficiency. His team is also studying hybrid power trains, such as those already available in the Toyota Prius and Honda Insight, which combine gasoline and electric systems.

Assanis is interested in more exotic hybrids as well. “We’re very keen here on hydraulic hybrids,” he says. When a vehicle brakes, it dissipates a lot of kinetic energy in the form of heat. Hydraulic hybrids use a hydraulic pump/motor, reservoir and accumulator to recover, store and reuse that energy to assist the engine at lower speeds. “These are robust and proven designs,” he says. “We’ve seen a fuel economy benefit in the area of 50 percent for a delivery truck in city driving.”

Fuel-cell powered vehicles and hybrids have been stealing headlines in recent years. But Assanis believes much of the enthusiasm for these unusual technologies—many of which are extremely expensive or still in the earliest phases of development, or in some cases even being abandoned by several companies—is in part the result of “hype and irrational exuberance.”

Assanis and his colleagues are working on a promising but lesser-known alternative to the traditional gasoline engine as the likely power train of the near future. It’s the homogenous-charge compression ignition (HCCI) engine, a high-tech design that essentially combines the traditional power of a spark-ignition gasoline engine with the more efficient compression-ignition design of a diesel.

HCCI engines use a premixed air-fuel mixture similar to spark-ignition gasoline engines’, but they rely on high compression spontaneous combustion like diesels, getting the best of both designs. The current problem with HCCI engines is trying to control the timing of ignition.

Whereas gasoline engines use a timed spark to ignite the fuel mixture and diesels inject fuel into highly compressed air to produce combustion, HCCI engines, Assanis explains, “do not have a trigger in close proximity to the start of heat-release; instead, this is more like spontaneous ignition, which can result in uncontrollable engine knocking.”

To solve the problem, the US Department of Energy and several automakers have provided $4 million to form the Multi-University Consortium on Homogenous Charge Compression Ignition Engine Research. Michigan heads the project in collaboration with MIT, Stanford and U-Cal Berkeley.

“We’re studying characteristics of fuel combustion in computer simulations as well as advanced laser diagnostics in actual engines,” Assanis says. The goal is to come up with chemical and mechanical methods for controlling the moment of combustion in the HCCI engine.

“HCCI is a clean combustion engine that can be commercially viable,” Assanis says. “We believe it could be in use within five years. It’s an alternative that’s much closer to being ready than fuel cells.”

Assanis believes that eventually hydrogen technology will advance to the point of being a cost effective and practical energy source. But even then he envisions a “wide spectrum of energy plants” in the future—with fuel cells, advanced diesel, spark ignition and HCCI internal combustion engines, and hybrid power trains, all remaining important throughout this century.

“We expect many things in an automobile,” Assanis says. “Practicality, mobility, excitement, entertainment, passion. Internal combustion engines satisfy these desires. They are relatively low-cost, and we are making them more environmentally friendly all the time. The infrastructure already exists to deliver their fuel. In my view they are going to remain part of the mix for many years to come. Internal combustion engines are here to stay.”

Derek Green is an Ann Arbor freelancer who frequently writes about the auto industry.