Energy & Environment

The Diesel Dilemma

The pollution-belching engines that power government buses and trucks have to go. What will replace them?
by | October 2001

With 4,500 buses on the Big Apple streets, New York's Metropolitan Transportation Authority operates the country's largest transit fleet. Most of those buses still run on sulfur-laden diesel fuel, and those noisy, soot-spewing engines create a significant threat to the health of drivers, mechanics, passengers and the millions of pedestrians who walk the city's sidewalks. "Diesel is a 19th-century technology that has limped its way into the 21st century," says Gina Solomon, a Natural Resources Defense Council scientist.

Like many other places, New York depends on durable but dirty diesel engines to power not only the buses that transport commuters but also the trucks that collect trash and haul other heavy loads. But now the pressure is growing to scrap those diesel workhorses that intensify smog and blast toxic petroleum residues into the urban air.

New York already operates 221 buses that run on cleaner compressed natural gas and has ordered another 255 for its fleet. It's also operating 11 "hybrid" electric buses that run on batteries working in tandem with small supplemental diesel motors, and another 325 are on order. But natural-gas and hybrid buses are more expensive than diesel-powered buses, and the New York transit agency concluded that it will have to continue relying on diesels as the mainstays serving most routes.

Transit officials figure they still can meet air-quality goals, however, by retrofitting old diesel engines to run cleaner or buying new buses with cleaned-up diesel engines. The clean-diesel buses not only cost less to purchase, they also save transit agencies hundreds of thousands of dollars by eliminating the need to install the expensive fueling stations that natural-gas vehicles require. "In the last 10 years, diesels have been getting a whole lot cleaner," says Dana Lowell, the New York City Transit's assistant chief maintenance officer. As a consequence, "you have to ask yourself whether it makes sense to invest in CNG infrastructure at this point."

A basic diesel transit bus costs $250,000 or so. A CNG bus costs roughly $280,000, not counting fueling-system expenses, and a diesel- electric hybrid goes for around $350,000. Lowell sees hybrid buses "as an extreme version of clean-diesel technology" that, once prices come down, can serve until buses powered by ultra-clean fuel cells become available.

Widespread use of fuel cells is far in the future, however, and governments around the country must make difficult choices now as they overhaul their fleets: Can they clean up the air most efficiently by converting entire fleets to cleaner--but more expensive--engine technology, or does it make sense to spend the same amount to buy better-performing diesels and retire old soot-spewing buses more rapidly?

As concerns about air pollution have mounted, big-city residents have grown used to seeing buses and trucks emblazoned with signs proclaiming that they're running on clean-burning natural gas. More than 100,000 natural-gas vehicles are now in use across the country, many operated by government fleets. Since 1995, Peachtree, Georgia, police have been driving 12 natural-gas patrol cars. The Los Angeles International Airport police drive nine CNG vehicles and have five more on order. Santa Monica, California, runs garbage collection trucks on compressed gas. In Washington, Pennsylvania, Waste Management Inc. successfully experimented with seven refuse trucks that operate on liquefied natural gas. According to the Natural Gas Vehicle Coalition, an industry-backed group, 65 transit agencies now run natural-gas buses; New York City, Los Angeles, Houston, Atlanta, and six other cities each now operate more than 100 CNG buses on their public transit systems. The Washington, D.C., regional transit authority this summer ordered 100 CNG buses for delivery by next January.

Although CNG and hybrid buses are generally regarded as cleaner than diesels, each has its own drawbacks beyond the higher price tags. Compared to conventional diesel engines, the most advanced natural-gas buses emit 97 percent fewer particles and from 15 to 70 percent less smog-forming nitrogen oxide. But natural-gas engines give off considerable amounts of methane, a greenhouse gas that contributes to global warming, as well as some formaldehyde and other potential carcinogens. The newest CNG buses now come equipped with methane controls, but CNG engines get 20 percent lower fuel mileage than diesels.

Meanwhile, diesel manufacturers have been redesigning their much- maligned engines to deal with the toxic unburned carbon, sulfates, ash and lubricating oil that conventional diesels pump out of their tailpipes. Competing engine makers are now offering new "clean diesel" engines that burn low-sulfur fuel and trap noxious particles with state-of-the-art catalytic emission controls. Old-line diesel-making companies got into trouble with the U.S. Justice Department a few years back by installing "defeat devices" to shut off emission controls once trucks got on the highway. As part of the legal settlement, Cummins Engine Co. is now financing a New York City Sanitation Department experiment with clean-diesel garbage trucks.

Cleaner diesels still give off twice as much NOx per mile as CNG buses, but they can virtually match them on particulates when burning low-sulfur fuel. The U.S. Environmental Protection Agency is moving ahead with new clean-diesel rules that will require engine makers to cut pollutants by 95 percent, starting with 2007 models. To make emission controls work most effectively, the agency also directed petroleum refiners to remove 97 percent of the sulfur from diesel fuel by 2006.

Hybrids, which combine small clean-diesel engines with batteries that are recharged by recovering kinetic energy from the braking system, don't quite match CNG in NOx emission control since they still rely on burning diesel fuel. But they come close, and they also do just as well on particulate emissions because the diesels don't run full-time, burn low-sulfur fuel, and employ sophisticated emission controls. Hybrids are the hands-down winner in fuel economy, moreover, yielding a 30 to 40 percent advantage over diesels, and unlike CNG-powered vehicles, they can be fueled from existing tanks and pumps.

Engineers continue to tinker with all three technologies, and comparative advantages may well shift. But transit fleet managers must start making decisions now that will determine what kind of engines they'll rely on for a decade or so. Nowhere is more at stake for alternative engine technologies than in California. The California Air Resources Board has begun issuing a series of diesel clean-up rules to get the heaviest polluters off the state's streets and highways.

Two early rules focused on the state's 8,500 public transit buses and 24,000 school buses. The board at first set out to require that local agencies buy only buses burning CNG or other alternative fuels. But regulators backed off when fleet operators objected to the expense of converting their entire fleets. As a result, state rules adopted last year leave it to fleet administrators to decide whether to switch to alternative fuels such as natural gas. They've now got the option of replacing old polluting buses with vehicles powered by diesel engines that can comply with the state's emission goals. To simplify compliance, the board required transit systems to choose one path or the other by last January. Nearly 50 stuck with diesels, while fewer than 30 chose alternative fuels.

The Air Resources Board took a different approach with school buses, offering California districts grants approved by the state legislature for converting to clean-burning engines. Adopted last December, the state program provides $25 million in grants to buy natural-gas buses and install infrastructure to fuel and maintain them. In addition, the agency will grant districts another $25 million to make diesel buses run cleaner, with $12.5 million available to buy new buses and $12.5 million to retrofit existing engines. To make clean diesels work as intended, California, like the EPA, also directed fuel refiners to start marketing low-sulfur diesel fuel.

But school districts and transit agencies in Southern California have fewer choices. The way regional air-quality regulators see things, "compressed natural gas is the predominant fuel of choice right now" to make vehicles run cleaner, says Henry Hogo, the deputy for planning and rules for the powerful South Coast Air Quality Management District, the agency that implements air pollution laws in the Los Angeles Basin.

Regulators have ordered local transit systems, including the huge Los Angeles County Metropolitan Transit Authority, to phase diesels out and convert completely to natural gas. The Los Angeles agency now runs more than a thousand natural-gas buses, Hogo points out, "and the engines are holding up pretty well."

As for school buses, the South Coast regulators aren't ordering school districts to get rid of old diesels all at once. But the new regulations require them to make buying natural-gas vehicles their first choice when they apply for state-funded grants to buy new buses. Only after CNG bus grants are exhausted can they seek funding for cleaner diesels. School buses are durable, and cash-strapped districts are still running diesels they bought a decade or more ago. Those heavy-duty engines are big polluters, "and we want to see them eliminated as early as possible," says Hogo.

But many of the region's school transportation managers say the regional agency's pro-CNG stance may actually force them to keep old diesels running longer, because they won't be able to afford to replace their fleets as rapidly. School buses cost roughly half what mass transit buses do, but a new diesel still runs more than $100,000. A CNG school bus sells for $30,000 more, not even counting the cost of installing tanks for fueling CNG vehicles. Sandra Fiddler, a transportation analyst for the Orange County education department, calculates that the air-quality decree will raise transportation costs for Los Angeles area schools by 36 percent a year by forcing districts to invest both in new buses and in CNG fueling stations. "For the same money, we could put more clean diesels on the road more quickly," she says.

The costs of expensive natural-gas infrastructure concern other transit managers around the country, and the past year's run-up in energy prices has highlighted another drawback of gas power: poor fuel mileage. Some California school districts took CNG buses off the road when gas prices skyrocketed. Around St. Louis, the Bi-State Development Agency's transit program canceled plans to convert its fleet to CNG buses. The agency now runs 36 natural-gas vehicles, but it's been fighting with the manufacturer over high maintenance costs and the limited range that keeps the buses from serving the agency's longest routes. The builder is covering the cost of additional CNG tanks to expand the buses' range, but "it would have been shooting ourselves in both feet" to buy any more natural-gas buses, says Delester Jefferson, an agency transit equipment analyst.

Although the St. Louis system is going back to diesels, it will run them on cleaner fuels. The agency plans to start using a fuel made by blending conventional petroleum with "biodiesel." While biodiesel fuel is typically derived from soybean oil, it can be made from any vegetable oil; in Las Vegas, local government fleets now run on diesel fuel made from kitchen grease at the MGM Grand Hotel. It is the environmental promise of biodiesel, though, that interests air regulators and transit managers. Biodiesel burns without releasing hydrocarbons or sulfur, and conventional diesels can run on it without requiring modifications. Lyle Howard, the Bi-State agency's product development manager, thinks shifting to the new fuel "will be more than adequate here" to address regional air-quality concerns without scrapping the system's diesel fleet. Cincinnati has already begun running buses on biodiesel, and the Cedar Rapids, Iowa, transit system has successfully tested the fuel.

But while biodiesel fuel may help governments meet air-quality goals- -and give soybean farmers a new market in the bargain--it is still the question of cost that is foremost in the minds of government officials making the choice among alternative engine technologies. Last spring, Cleveland's Regional Transit Authority decided to purchase 225 clean- diesel buses, with an option to buy 200 more. Although environmentalists had pushed for natural-gas vehicles, officials concluded they would have cost the agency another $21 million over their useful lives.

While NRDC and other environmental groups don't necessarily object to replacing old dirty diesels with cleaner versions, they insist that natural-gas buses deliver more certain improvements in air quality. They argue that CNG buses will be less likely to pollute more heavily as engines start wearing out. "Today's so-called clean diesels will become tomorrow's smoking monsters if their engines do not age well over time," Solomon says.

Environmentalists contend that the federal or state governments should cover additional costs transit agencies and school districts would incur by converting completely to alternative fuels. In the long run, they think that will bring CNG and hybrid prices down to levels that compete with cleaner diesels.

But some public fleet operators aren't convinced that CNG is that superior, and they also know they can't depend on Congress or their state legislatures to come up with the kind of money they'd need to get rid of diesels altogether. In New York City, mixing clean diesels with hybrids and CNG buses means that "by the end of 2003, we will have the cleanest transit fleet in the world," says Lowell, the MTA official. "Because clean diesel's so cost-effective, we can afford to do the entire fleet in three years. There's no way we could convert the entire fleet to natural gas in three years."

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