How Do Transit Agencies Decide on What Vehicles To Purchase?
The biggest capital expense for transit agencies is buses (recall the difference between capital and operating funding). As buses in the United States are expected to last at least twelve years before being replaced, poor decisions in the coach procurement process are not only costly but can doom the agency for years. Agencies make the purchasing decision based on the following factors: size, propulsion system, high or low floor, and manufacturer. Recall in Part 1 we looked at size . In this part we will examine the choice of propulsion system.
2. Propulsion System
Propulsion system basically means what kind of fuel will power the bus. Currently in the United State buses can run on the following fuels: diesel, natural gas (both compressed and liquid), hybrid (both gasoline/electric hybrids and diesel/electric hybrids), and electricity from overhead wires (trolley buses). Buses that operate using only electric batteries and hydrogen fuel cells are currently being tested, but the technology has not yet advanced to the point where these technologies can be widely deployed.
Diesel
The majority of transit buses today run on diesel fuel, and for good reason: diesel engines are a proven commodity that has been around since the 1800s, and diesel fuel is readily available. Diesel buses cost around $300,000, depending on what options are included, and average 2 - 3 miles per gallon.
Although traditionally diesel engines have been very polluting, significant improvements have greatly reduced the amount of emissions they produce. Despite these improvements, they still pollute more than the other propulsion systems we will discuss, which means that they are gradually being replaced by these other systems, particularly in large cities that have air quality problems. For example, the South Coast Air Quality Management District in Southern California has banned the area's transit systems from buying new diesel buses.
Natural Gas
After diesel, the second most used propulsion system is natural gas. Although when natural gas was first used as a fuel liquid natural gas was fairly common, concerns about explosions both on the bus and at the fueling station have led to compressed natural gas being more popular.
Since natural gas does not have as much energy per unit amount as diesel fuel, fuel economy tends to be lower on natural gas vehicles. For example, a recent examination of the performance of buses using different propulsion systems in New York City found that while diesel buses averaged 2.33 miles per gallon, natural gas buses averaged only 1.7 miles per gallon.
In addition to the lower fuel economy, transit agencies who wish to convert to natural gas have to construct natural gas fueling facilities that can cost as much as $10 million per garage. Natural gas buses also cost about $30,000 more than comparably equipped diesel buses.
Why then do transit agencies purchase buses powered by natural gas? The main reason is that natural gas buses eliminate almost no emissions. While they do emit more pollutants than the other propulsion systems we will soon discuss, the amount they emit is far less than for an equivalent diesel bus; this reduction helps transit agencies meet environmental protection laws.
Hybrid
A more recent development in propulsion systems, hybrid bus engines combine a fossil-fuel burning engine, which burns either gasoline or diesel, with an electric engine. In much the same way that a Toyota Prius uses both the electric and the gasoline engine during the accelerating process and switches solely to the electric engine when the traveling speed is reached, so too do hybrid buses take off using both the gasoline or diesel engine and the electric engine and switch to the electric engine after coasting speed is reached.
Although diesel-electric buses offer better fuel economy, many transit agencies who have bought hybrid buses prefer the gasoline-electric hybrid as the gasoline engine provides better acceleration from take off and also emits fewer pollutants than the diesel-electric hybrid. Hybrid buses tend to cost from $100,000 to $200,000 more than a similarly equipped diesel bus, and can come in between $300,000 and $500,000.
The main advantages of a hybrid bus are a reduction in emissions and an increase in fuel economy. In the same New York Transit study which showed that diesel buses were averaging 2.33 miles per gallon, the hybrid buses in the study were averaging between 3 and 3.22 miles per gallon for a 30 - 40% increase in fuel economy. As the price of diesel and gasoline settles into a new normal of $4.00 per gallon, this increase in a fuel economy is a powerful motivator for transit agencies to purchase hybrid buses.
Trolley Buses
In today's era of strict environmental regulation, trolley buses are the perfect propulsion mode: after all, if you do not factor in how the electricity is produced, they emit no emissions whatsoever. However, only six transit agencies in North America currently operate trolley buses (although many systems in Europe and Asia operate them): Vancouver, B.C.'s Translink; King County Metro of Seattle, Washington; the San Francisco Municipal Railway; the Miami Valley Regional Transportation Authority of Dayton, Ohio; the Southeastern Pennsylvania Transportation Authority of Philadelphia; and the Massachusetts Bay Transportation Authority of Boston. For those cities that do not currently operate trolley buses, the cost of installing the infrastructure is likely to be prohibitive. In addition to the cost, there may be community opposition to the overhead wires, and there is difficulty in modifying the route if changes need to be made. Trolley buses themselves are also significantly more expensive than other kinds of buses; Muni of San Francisco recently entered into a contract with Electric Transit, Inc. for $234.5 million for 273 buses, or over $850,000 per bus. Fortunately, trolley buses last longer than diesel buses and require less maintenance.
Electric and Fuel Cell
In addition to the above, testing continues to be done on buses that operate solely using electric batteries. Several electric buses operate along Foothill Transit's Route 280 in Azusa, CA: these buses recharge themselves at recharging stations located at one of the terminal layovers. The largest obstacle to the widespread deployment of all-electric buses is range; current battery power limits these buses to short routes that have frequent charging opportunities. Until buses are able to go 200 - 300 miles without recharging, electric buses will remain a niche market.
Fuel cell buses are typically powered by hydrogen; the cell functions much like a battery, and produces no emissions besides heat and water. Due to technological problems and logistical problems (i.e. no hydrogen stations next to gas stations along the highway), fuel cell technology is likely to be downplayed in the future in favor of all-electric bus technology. Currently, only Whistler, B.C. operates city-bus sized fuel cell buses in North America.
Conclusion
Although diesel buses are the dominant propulsion system in North America, environmental considerations will likely cause more and more transit agencies to switch over to natural gas, hybrid, and eventually all-electric technologies. Go on to Part 3 of the series, which examines whether transit agencies should buy high or low floor vehicles and what manufacturer they should buy from .
