Transportation Deployment Casebook/Hybrid-Electric Bus

Quantitative AnalysisEdit

The following analysis seeks to analyze the life cycle of alternative energy source bus technology, as defined by the American Public Transportation Association (APTA). The APTA does not collect their data by individual vehicle type from its member organizations, but rather categorizes the vehicles (specifically buses) by the number of energy sources used to provide driving power. In the case of Hybrid-Electric buses (HEB), the data is grouped together with other vehicles that use two or more different energy sources, which includes compressed natural gas buses (CNG) and electric buses, although HEBs are generally regarded as a more feasible option for the future. In addition to the nature of the categorization done by APTA, the organization is also only able to collect the specific (alternative or conventional diesel) vehicle data from their member organizations who opt to participate in APTA’s annual vehicle survey; the 2010 vehicle survey includes responses from 338 transit agencies within the United States. Furthermore, the data collected in the vehicle survey is used to estimate a total market share of alternative and conventional buses in the U.S. Finally, the estimated market share is multiplied against the actual total number of vehicles available for maximum use in the U.S. (APTA data) to calculate the estimated number of alternative energy source (heavy-duty) buses available for use in public transportation from 1992-2009.

Graph 1 displays the estimated number of alternative energy vehicles from 1992-2009. The visual representation of the data is that of an S-curve, which can be conceptually applied to the life cycle of many other transportation technologies.

Graph 1

In addition to the display of the estimated number of alternative energy buses available for use as heavy-duty public transportation vehicles in the U.S., an Ordinary Least Squares (logistic) regression was used to estimate the following logistic function: S(t) = K/[1+exp(-b(t-t0)]

Where: S(t) is the status measure, which is number of alternative energy buses t is time (annual, 1992-2009), t0 is the inflection time (year in which 1/2 of the saturation of the number of alt. energy buses, is achieved), K is saturation status level, b is a coefficient, measuring an amount of impact on the independent variable. Through the OLS logistic regression, multiple iterations are completed to find the 3 parameter estimated coefficients that create the “line of best fit,” and therefore have the highest adjusted R2 value in the logistic regression output. K, as previously mentioned, is the saturation level of the transportation technology. In the case of alternative energy buses, it represents the maturity of alternative energy bus systems in the market. After multiple iterations of tested values of K, the optimal results are listed in the summary table below (K = 41,000).

As reported in Table 1, the Adjusted R2 of .9361 is significant at the standard 90th percentile; the closer the R2 is to 1, the more statistically accurate the regression formula is in predicting the values for the status of the technology. In this particular application, the Adjusted R2 value, in conjunction with the value of the T Statistic, broadly states that the equation can explain approximately 94% of the variance in the number of alternative energy vehicles (and therefore inflection and saturation points) over time.

Table 1

The estimated inflection year, which indicates the point when the rate of growth of the vehicles begins to slow, is approximately 2009 (intercept of -341.94/-(b of .1701)). With these two estimates, the model is generally estimating that the infancy stage of the alternative energy bus technology could generally be considered between 1992 and 2009. The period following 2009 until the saturation level of 41,000 vehicles implies a general period of continued growth, and the time period following the saturation level of K=41,000 is that of maturity for the technology.

It seems particularly challenging to identify the accuracy of the equation, especially in a practical application, for multiple reasons. Of course, the estimation of the share of alternative energy vehicles is challenging, as well as the large categorization of hybrid-electric, electric, and compressed natural gas vehicles. Finally, the share of heavy-duty buses that these vehicles hold is certainly in a young stage, but the inflection year of 2009 is also likely to change once the 2010 number of vehicles in the market data is released. As a share of vehicles, the 2010 measure for alternative energy buses increased from 30.4% up to approximately 35%. However, with all of this aside, the R2 and T-statistics suggest that the equation is relatively accurate in explaining the variance in the number of alternative energy vehicles over time.

Graph 2, if only anecdotally, suggests additionally interesting information to keep the analysis of the life cycle of alternative energy vehicles in perspective. Although the equation estimates the inflection year of 2009, the data below shows the proportion of the market of alternate energy buses, as well as the number of vehicle total miles for alternative and conventional buses. Because the share of the market dropped in 2009 for alternative vehicles, and the number of vehicle miles decreased significantly, the data here suggests that an inflection year of 2009 may be too early (or late) of a prediction

Graph 2

Qualitative Analysis - Hybrid-Electric BusesEdit


Despite the data limitations in public transit buses present due to the lack of segmentation by vehicle type, this analysis shall focus largely on the main emerging technology within the alternative energy source sub-population, the Hybrid Electric Bus. However, a brief overview of the other significant alternative energy source buses will also be provided at the end of the analysis.

A hybrid electric bus (HEB) combines a conventional model of the internal combustion engine (ICE) with an electric system, used for propulsion, regenerative energy, or as an independent power source, depending on the alignment of the power-train system in the HEB. Series, parallel, and blended hybrids are the generally how the power sources are sub-categorized among HEBs; a series has no mechanical connection between the engine and drive axle. Instead, the engine sends power to the generator, which then powers an electric motor. Conversely, the parallel hybrid’s engine powers the generator, which can either charge directly drive the vehicle’s axle or charge the battery pack. The self-descriptive blended HEB is a combination of the two power alignment schemes, as displayed in Diagram A

Diagram A - Parallel and Series Hybrid Design



The hybrid electric bus has multiple environmental advantages over the conventional diesel bus. Perhaps the most widely cited environmental benefit of the HEB is the reduction of carbon dioxide, nitrous oxides, and hydrocarbon emissions that contribute to broad and local environmental and health problems. In addition to the resultant reduction in greenhouse gases and localized ozone, an air pollutant related to many human respiratory problems, HEBs also have lower levels of particulate matter and carbon monoxide emitted than conventional diesel buses. Similar to ozone, both of these pollutants can cause significant damage to the human respiratory and immune system, and high concentrations of the emissions are considered significant health problems.

In addition to the reduction of harmful emissions, the decreased use of petroleum based fuel offers a potentially significant cost reduction for transit agencies over time. According to the Federal Transit Administration, fuels costs are only second to labor costs in the operating expenses of a transit agency, and HEBs have recorded fuel economy increases ranging from 10% to 48%, relative to a conventionally powered diesel bus. The HEB system has a particularly positive effect on fuel consumption in urban driving situations and during rush hour. The regenerative breaking process converts kinetic energy from the braking process into energy that can be stored in the generator and eventually used to power the HEB. Because of this, this powering system is ideal for city buses, which brake and accelerate frequently.

Significant MarketsEdit

Within the U.S., most transit agencies are motivated to shift their vehicles in their fleet to alternative energy sources by local political actors, such as environmental advocacy groups, as well as environmental commitments to reduce emissions and improve air quality above or beyond EPA regulations. New York City, although it is inherently prone to higher levels of toxic emissions because of its congested nature, is an excellent example of a region or locality that chose to divert from conventional diesel buses due to pressure on the local and national level; the City had not achieved the air quality attainment goals set in the EPA’s 1990 National Ambient Air Quality Standards (NAAQS) and the increasing public awareness of health problems related to the harmful emissions, as well as growing state and national pressures to reduce emissions led NYC’s Metropolitan Transit Authority (MTA) to replace its entire fleet over a decade, and began this environmentally oriented endeavor in the mid 1990s. HEBs were not widely available until the late 1990s, so NYC opted for a significant investment in the natural gas re-filling infrastructure to support a fleet of Compressed Natural Gas (CNG) buses. However, since converting its entire fleet of heavy-duty buses away from conventional diesel power systems, NYC has also invested in HEBs. As part of the largest alternative energy bus fleet in the world and the largest transit bus operator in North America, MTA now regularly operates over 600 HEBs[2].

The California Air Resources Board (ARB) has also played a major role in influencing the growth of HEB use in the U.S. In response to EPA regulations regarding diesel fuel and engine performance, such as the Clean Air Act, the ARB declared the particulate matter emissions from diesel engines as a known human carcinogen in 1998. Since then, the ARB and California’s Office of Environmental Health Hazard Assessment have studied the health impacts and risks due to diesel emissions and implemented further rules to decrease emissions throughout the state, such as the 2000 Diesel Risk Reduction Plan. The measure, which aimed to drastically reduce diesel emissions by 2010, in addition to its specific 14 point plan, influenced the adoption of vehicles such as HEBs. The California Global Warming Solutions Act of 2006, signed by then Governor Arnold Schwarzenegger, has provided another incentive for the increased use of HEBs through the state’s urban areas; as part of the Act, the ARB is required to reduce greenhouse gases by 25% by 2020, and the board’s enforcement of the reduction required forced technology upgrades for heavy-duty conventional buses, such as the retrofitting of new and existing engines with particulate matter filters to reduce emissions. Additionally, the Act required the increased use of low sulfur diesel fuel. In addition to the adoption of HEBs in New York City, as well as early adoption in Los Angeles following a successful trial period in the notoriously polluted city, another trial adoption of the HEB technology in the U.S. did not have as much of a picturesque outcome. Cedar Rapids, Iowa proposed a cold-climate trial of the batteries needed to operate a hybrid-electric powertrain and electric buses to add to its public bus fleet to the Federal Transit Administration (FTA) in the early 1990s, and received over $10 million to purchase, operate, and maintain 5 HEBs and 4 electric buses. The FTA funding was also used for the construction of a facility, infrastructure for charging the batteries, and training[3].

Prior to the hybrid-electric and electric testing, the moderately sized city of 100,000 had tested alternative fuels such as CNG, LP gas, hydrogen injection, ethanol, and bio-diesel since 1987. Following the implementation of the four electric buses in 1995, three HEBs were introduced in November of 1997, and the other two followed within three years. Amidst the six year evaluation process after the final HEB was added to the fleet (2000), an FTA analysis of the project anticipated that the city could overcome training, electronic, and mechanical issues it often had with the vehicles and see rewards from the significant investment. Despite the average fuel economy increase of 15% and improved emissions to help bring Cedar Rapids up to attainment of the NAAQS for ozone, issues such as battery size, financial feasibility of improved battery technology, maintenance, and reliability outweighed the benefits, and city officials sold the buses in an online public auction for $30,000 in 2008. The mothballed vehicles, which only had a collective 200,000 miles logged, had an added purchase requirement that required the winning bidder to haul the buses away, which seemed to imply that Cedar Rapids could not get the fleet out of its hands quickly enough.

Life Cycle of Hybrid-Electric BusEdit

Birth of Hybrid TechnologyEdit

Conceptually, the idea of powering a vehicle with an electric source has been privately tested for over a century. Electric vehicles were introduced into the London taxi market and even produced in Connecticut in the late 1800s, and continued to develop until approximately 1920. A hiatus from the electric based automobile in the U.S. from 1920-1965 ended with the introduction of multiple bills in Congress advocating for electric automobiles as a method to reduce air pollutants emitted by the conventional ICE in 1966 [4]. Eventually, the commercial development of the modern hybrid started growing in the late 1980’s and early 1990s. Toyota, who released the first commercially accepted hybrid automobile in 1997, the Prius, started development in 1993 [5]. 1997 was also the release year Toyota’s Coaster Minibus, which it claimed as the first heavy-duty HEB, but this release wasn’t quite as established as the market’s first. Other commercial developers claimed their release as the first in the HEB market; Gillig Corporation released its own HEB model in 1996.

Growth and Saturation of Alternative Energy BusesEdit

Despite Cedar Rapids’ negative account of alternative energy heavy-duty buses, vast improvements in technology, such as battery weight and lifespan, training, and maintenance knowledge have specifically contributed to an increase in the adoption of HEBs since their introduction into the market. International, private companies, striving to meet the increased demand for efficient vehicles with lower emissions, contributed largely to the initial increase in technology that would have helped a case like the FTA trial in Iowa. Cedar Rapids had specifically identified a battery that was significantly lighter than the one ton batteries it maintained; the batteries, which also had vast improvements in maximum operating length, were developed by a German company, but were simply too expensive due to a tariff at the time Transportation Research Board[6].

Although transit fleet managers continue to search for methods to lower costs, decrease emissions, and satisfy related interest groups, the Diesel Emissions Reduction Acts (DERA) of 2005 and 2010 may be encouraging less growth in the alternative energy heavy-buses, including HEBs. The 2005 Act provided $200 million/year for five years for congressional appropriations to encourage a major reduction in diesel emissions. Although the $10 million in appropriations were never fully funded, $300 million was added to the incentive program in 2009 as a one-time stimulus, and the 2010 Act reauthorized the bill for another five years, which matches the actual $500 million that was appropriated from 2005 through 2009[7].

A significant outcome of DERA was, of course, a reduction in diesel emissions throughout the U.S. that ultimately becomes a significant investment in public health; every $1 spent by the DERA program (which often is a match to local and state expenditures on the diesel emission reduction) is estimated to yield between $13 and $28 in improved health (benefits) to individuals due to decreased cancer, asthma, and other health improvements related to strengthened respiratory and immune systems[8]. The practical implications of the incentive program, however, shifted the focus away from decreasing or eliminating the use of petroleum-based fuels. Particulate matter filters, which have decreased diesel emissions to a level almost identical to HEBs, were developed and retrofitted to conventional diesel bus fleets at a significantly lower cost than the investment otherwise required to adopt alternative vehicles, such as HEBs, into a fleet.

Although preliminary figures for 2010 from the APTA signal an increase in the market share of alternative fuel heavy-duty buses in the U.S., the DERA seemed to perpetuate some lock-in issues present with conventional diesel technology[9]. The high costs associated with the infrastructure, training, and battery replacement and disposal, which is still a significant and untested variable in the cost structure of alternative heavy-duty vehicles like the HEB, are simply too overwhelming to justify a major switch in bus technology and new capital investment for transit agencies. Instead, the upgrades in technology on the existing diesel vehicles are widespread and much less expensive for agencies to implement.

Although the saturation model suggests that the inflection year for alternative energy heavy-duty buses compared to conventional buses is 2005, technology such as the HEB seems to have more of an uncertain future than the model may suggest. Heightened awareness to the extraction of oil, burning of petroleum based fuels, and imminent international pressure for the U.S. to join the European Union and other prominent nations in the drastic reduction of greenhouse gas emissions may have impacts on the alternative heavy-duty bus market that simply cannot be predicted at this point.

Moving forward with HEB technologyEdit

FTA research, in combination with improvements in international trade agreements that will improve U.S. access to new and future technology, show a hopeful future for alternative energy heavy-duty bus development. February of 2010 saw the signing of the first science and technology agreement between the United States and Germany, which provides a framework for cooperation and will bring multiple research entities together to address “cross-cutting scientific issues” [10]. Additionally, the countries signed a memorandum to initiate further research cooperation to address energy, climate change, and health issues, among others. Multiple German companies have produced innovative research and developed advanced batteries and bus technologies that have been helpful in advancing HEBs in the U.S., and this agreement facilitates an even larger sharing of technological advances in the future.

Not unlike any other rapidly evolving technology, the hybrid-electric bus has its drawbacks. As demonstrated through the FTA trial in Cedar Rapids, Iowa, intricate maintenance management, technical training, battery weight, and the technical design of powertrain, including the battery, are all invaluable in the successful implementation of HEBs. As technology continues to improve (ex. the dramatic reduction in the size and weight of the battery), the higher maintenance and training costs inherent in the relative infancy of a technology are equally stressed by a 2010 FTA assessment of hybrid-electric technology. Additionally, the complexity of the electric drive components will likely increase overall maintenance costs. However, as the technology matures in the long run, maintenance costs and unfamiliar complexities will naturally decrease, and have the potential fall below similar costs related to conventional diesel buses because of decreased repairs on transmission and brake linings [11].

Hybrid Electric Buses vs. Other Alternative Energy BusesEdit

CNG buses were the dominant alternative energy bus technology when HEBs entered the market, and maintain a presence in the bus fleet; the APTA Fact Book shows an estimation of 11% of the total bus fleet as of the end of 2004. Related to the earlier discussion of New York City’s CNG fleet, many transit agencies are more reluctant to deploy CNG buses because of the overhead infrastructure costs related to establishing a fueling location for the vehicles and installing compressors. At the time of New York’s initial investment, CNG was the main commercially viable clean fuel alternative to the conventional diesel bus[12]. Although the life cycle cost of a CNG bus usually falls below that of a HEB, the fuel efficiency is also lower than HEBs and conventional diesel buses. Although they become more attractive when liquid fuel prices increase but the price of natural gas remains stable, pricing trends for different fuels usually follow one another.

An alternative to HEBs an CNG buses, electric buses also have a small presence in the alternative fuel market for heavy-duty buses, but have similar drawbacks to HEBs regarding the battery technology. Although they provide an even smoother and quieter ride compared to the HEB, which is an improvement over the noise and vibration of the conventional diesel ICE, pure battery buses have energy storage capacity and battery cost issues that have stopped the buses from being a viable vehicle for most applications of public transit. In addition to these problems of sufficient traveling range, it is also challenging for transit agencies to take their buses out of operation for recharging. At best, pure electric buses are seen as niche market and useful for those who need a bus without emissions and may be benefited by the lack of noise; parks and indoor uses are the most viable applications for pure electric buses today.

Ultimately, fuel cell buses are seen as the long-term goal by many transit agencies, health advocates, and environmental advocates. However, doubts as to when the technology advances will actually materialize are fairly widespread. HEBs, though, are seen as a bridge from conventional diesel to a future technology. Although many issues still need to be addressed, they are regarded as a “here now” technology that also has many benefits.


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Last modified on 14 November 2012, at 18:27