In 2013, the Regional Transportation District (RTD) faced a big decision. Advancing technologies, combined with a boom in natural gas production, presented an opportunity to replace 200 aging buses, running on diesel fuel and ripe for replacement, with buses that would run on compressed natural gas (CNG).
Air quality is often poor on the Front Range, and it violates the Environmental Protection Agency (EPA) standards on occasion, partly because of vehicle emissions. Cities are increasingly aware of the savings on health care and improvements in quality of life they can reap from cleaning up their diesel fleets. On the other hand, cost-efficient public transportation is important to customers and taxpayers.
So in 2013, RTD pondered: Would a switch to gas make sense?
This was a reasonable question on the surface. When natural gas is burned, it releases little pollution compared to diesel. Smog-choked cities in Europe, where diesel cars are more common than in the U.S., are plagued by diesel exhaust’s nitrous oxides (NOx), volatile organic compounds (VOCs) and particulates, toxic chemicals that stick to soot. Many chemicals in particulates are also known to cause cancer. Cities in the U.S., like Denver, also suffer from these pollutants, but they are less concentrated in the gasoline that fuels our cars.
However, diesel emissions from heavy-duty vehicles still cost billions of dollars per year in the U.S. in asthma attacks, emergency room visits, lost work days and premature deaths, according to the EPA. Most vehicles also emit lots of carbon dioxide, too, the main cause of climate change.
Since 2000, however, the EPA has been tightening regulations on our most-used diesel vehicles, including heavy-duty trucks, machinery and buses. Today, almost every diesel engine runs on what’s called “clean” diesel — a fuel type that contains almost no sulfur, an engine lubricant that was removed because it created acid rain. Using this “ultra-low-sulfur” diesel also makes it possible for engineers to attach various other devices to capture harmful pollutants, removing as much as 90 percent of them from exhaust. Those features are now required for new vehicles.
When the RTD fleet managers crunched the numbers in 2013, they saw a clear choice. New diesel engines could burn just as cleanly as CNG engines — but at a fraction of the cost. Many scientists came up with similar answers. So apart from a few small-batch experiments with diesel/electric hybrids, RTD doubled-down on diesel.
RTD does still operate a few old diesel buses, ones that engulf bus stops in black clouds of exhaust as the buses idle and accelerate. But frequent transit users may have noticed that this unpleasant experience is becoming less common. In the last few years, RTD has managed to get rid of its oldest and dirtiest buses.
Lou Ha, RTD’s manager of technical services, wrote in an email that about 85 percent of their diesel fleet now has the latest emissions controls. He also wrote that the remaining 15 percent will be replaced next year. If the diesel/electric hybrid or pure electric bus experiments pay off, Ha suggested that RTD may pursue those avenues further.
But RTD isn’t the largest operator of diesel vehicles on the Front Range. School districts are the biggest players, employing thousands of diesel buses to ferry children to and from school. And while they all run on clean diesel, most of those buses aren’t new, which means that many of them do not have the latest emissions controls.
Bob Young, director of transportation for Boulder Valley School District (BVSD), says that on a tight budget, school districts across the country haven’t had many good options for getting rid of their older and more polluting diesel buses. But they’ve certainly tried. In 2001, BVSD decided to experiment with compressed natural gas. But after 15 years of trying to operate a few dozen CNG buses, Young says RTD’s decision not to go for gas makes a lot of sense.
“I haven’t had anything but problems with it,” he says.
Young agrees with RTD’s calculations, which showed that not only do CNG buses cost tens of thousands of dollars more than diesel models, but they require a sophisticated and enormously expensive network of fueling stations.
Those stations can easily cost $1 million each to install. They compress natural gas into a liquid form, which means the buses’ fuel tanks also need to withstand unusually high pressures, making them heavier and more expensive. The list of problems continues, including issues with high altitude and long refueling times.
The school district also owns two diesel/electric hybrid models, but these have a hard time climbing up the mountains. They’re also more expensive to purchase and maintain. Young oversees a fleet of 254 buses, and a hybrid could cost $50,000 more than a new diesel bus. Albert Samora, Young’s fleet manager, says considering repairs and reliability issues, they have barely seen any savings from the more fuel-efficient hybrid engine.
So far, Young and Samora say they’ve had the most success with cutting emissions by working with the diesel vehicles they have. That’s meant implementing new policies aimed at reducing how long the buses idle, for example, which is when they’re burning least efficiently. But it’s also meant tacking on partial emission-reducing technologies to buses made before the latest standards.
In 2006, BVSD joined with districts across the Front Range to participate in a program to add some of these technologies to their fleets. Organized by the Regional Air Quality Council, the program allocated about $1.3 million of federal money to upgrade about 900 diesel school buses. Since then, Young says his team has been steadily adding more controls to older models and purchasing newer ones.
But if Young has his way, BVSD will replace all 221 of its diesel buses with buses fueled by propane — and he wants to do it by 2030. “If we were serious about emissions,” he says, “we’d get rid of these old buses.”
While the EPA hasn’t approved these propane engines for use in public transit buses, many school districts across the nation are switching to buses fueled by propane because propane is cleaner than diesel, often cheaper, and it’s easier to store than CNG. Due to these cost savings, Young says he can afford to put in the extra $5,000 or so investment that each propane bus costs, even if his grant applications fall through. Young’s first shipment of 10 propane buses is due to arrive any day now. But he has a long way to go before he can retire all of his older diesel buses.
However, for all of the improvements that clean diesel technology offers, one ugly problem remains unsolved. No amount of filters, scrubbers, catalysts or converters can remove the climate-changing carbon dioxide from bus exhaust.
New standards from the EPA have certainly helped diesel engines become more efficient and to produce less carbon dioxide. Natural gas and propane, a byproduct of natural gas production, also emit less carbon dioxide when burned — though many studies that include processing and transport show that natural gas can be worse for the climate than burning gasoline or coal.
With innovation and incentives, electric buses might prove to be the solution. But they’re only carbon-free if their electricity comes entirely from renewable sources like wind and solar. And in Colorado at least, that may not be the case anytime soon.