Increased ridership, coupled with a focus on providing more environmentally sound means of public transportation, have led to a new trend in the transit bus industry. Just as more people are opting to use public transportation to save money, new regulations are requiring mass transit systems throughout the United States to take steps to ease their equipment’s impact on the environment. Further, an increased focus on environmental stewardship is driving bus manufacturers to switch from traditional rear-mount air conditioning to roof-mount systems. The result of these trends is an increase in overall energy savings and a reduction in carbon dioxide emissions, especially as many transit customers have opted for all-electric or hybrid-powered vehicles over conventional diesel-powered.

 “In the U.S., passengers took 10.7 billion trips on public transportation in 2008, the highest level of ridership in 52 years and a modern ridership record,” according to a report released March 9 by the American Public Transportation Association (APTA). “This represents a four percent increase over the number of trips taken in 2007 on public transportation, while at the same time, vehicle miles traveled on U.S. roads declined by 3.6 percent in 2008, according to the U.S. Department of Transportation.” Demand for mass transit increased as the public sought to save money in response to the deepening economic downturn.

The U.S. House Committee on Transportation and Infrastructure recently introduced a transportation bill that will affect highway, public transit, freight rail and intermodal projects across the United States. Like the U.S. Environmental Protection Agency’s (EPA) 2010 engine emissions regulations, the proposed transportation bill also will require transit industry stakeholders to take a greener approach, although some began embracing green technology well before the new standards were announced.

From the Back of the Bus
For many years, rear-mount air conditioners have been the most popular offering among leading transit bus manufacturers in North America primarily because there was room for the units in the engine compartment. It simply made sense to design and install HVAC units in the available space above the engine. However, an increased focus on environmental responsibility is changing preferences.

While rear-mounted air conditioning units can offer bus operators several benefits, there are three principal disadvantages when compared with rooftop systems:

1. Conventional bus air conditioning fluctuates with engine RPM, creating uneven and uncomfortable hot spots as the bus stops or idles. Hot spots are especially pronounced in the driver’s area, which is furthest from the rear of the bus and closest to the high heat-loaded windshield.

2. Large fans are needed to thrust cold air through the distribution ducts of a rear-mount system. The air has to be forcefully pushed forward to reach bus passengers, and large motors, which function at higher noise levels and consume more energy, are necessary to do that.

3. Rear-mount units are axle-weight challenged. In some municipalities, it is required that the weight of a bus be evenly distributed to protect the roads from excessive wear and tear. With the rear-mount localized in the back of the bus, overall weight of the bus can be difficult to balance.

While the advantage of the rear-mount system is that it can be serviced at ground level, it can be difficult to reach necessary parts on a rear-mount system due to the positioning of the many components in the engine compartment.

Additionally, in order to comply with the lower emissions standards set forth by the EPA, bus OEMs will have to find space to accommodate after-treatment devices, such as diesel particulate filters (DPFs) and urea injection systems, necessary to meet the new standards. This space may not be available in the already-crowded engine compartment, and if the engine compartment components cannot be rearranged, bus builders have only the bus roof to turn to for placement of the air-conditioning system.