The Transition to Zero-Emission Bus Fleets in Megacities

Growth in urban populations demands investment in affordable and accessible transportation. Increases in economic activity demand more efficient and clean transportation. And growth in greenhouse gas emission—and the threat of an irreversible change to climate conditions—demands low-carbon modes of transportation. Urban bus fleets provide solutions to all of these challenges and can become the spark of a broader transformation for heavy-duty vehicles in general.

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A soot-free bus in Jakarta, Indonesia (photo: Scania)

This article appeared in the April 2019 issue of EM Magazine, a publication of the Air & Waste Management Association

Despite their affordability, higher energy efficiency than gasoline powertrains, and low-carbon missions per passenger, approximately 80 percent of new buses sold today still utilize old diesel engine technology. Diesel exhaust is a known human carcinogen, according to the International Agency for Research on Cancer. As much as 75 percent of the fine particulate matter emissions of a pre-2007 diesel bus engine in the United States, for example, consists of black carbon (or soot), an ultrafine particle that serves as a universal carrier of toxic organic compounds and penetrates into the deepest regions of the lungs. Black carbon is also a super climate pollutant, capable of warming equivalent to 3,200 times the effect of carbon dioxide over a 20-year period.

Soot-Free Engines

A “soot-free” engine meets Euro VI or equivalent (e.g., US 2010) emission levels, which can achieve a 99 percent or greater reduction in diesel black carbon emissions in real world driving when compared against all previous generations of diesel engine technology. A diesel engine can deliver soot-free emissions through a combination of ultra-low-sulfur diesel fuels (i.e., 15 parts per million sulfur or less) and a ceramic wall-flow diesel particulate filter. Other soot-free engines include a port-fuel injection engine powered by compressed natural gas or low-carbon biogas, or a zero-emission electric drive engine.

Commercially available Euro IV and V diesel buses are not considered soot-free solutions, as they rely on a combination of in-cylinder particulate emissions control and oxidation catalysts, and lack particulate trapping and burning capabilities. Manufacturers such as Volvo and Scania have recently developed Euro V products with a diesel particulate filter for the Colombia and Iranian markets, respectively, whose soot free emissions performance should be validated.

The long-term trajectory for urban bus fleets is the zero-emission technology pathway. Dedicated electric drive engines are a commercially viable option that deliver very low or in some cases zero carbon emissions. These include battery-electric, trolleybus electric, or fuel-cell electric engines. The high operational savings from these very efficient engines may translate in some cases to lower overall total cost of ownership compared with today’s conventional combustion engine technology.

Zero-Emission Technology Pathway

We take a look here at examples from two major cities following a dual soot-free and zero-emission technology pathway. In the near-term they are taking advantage of locally available fuels in order to deploy soot-free engines needed to mitigate the air quality and near-term climate hazards caused by older technology diesel engines. And over the long term they are implementing a plan to deliver a zero-emission bus fleet.


A new hybrid bus in London (photo: Volvo)

London Pursues a Dual Path in Low-Emission Zones

London became in 2017 the first megacity to publicly commit to reaching by 2030 the World Health Organization air quality guideline of 10 μg/m3 for fine particulate matter (PM2.5). This decision by the local government was the result of air quality monitoring data that shows that 95 percent of Londoners live today in areas with PM2.5 concentrations more than double the future target. Fifty percent of the emissions come from the transport sector. To achieve its clean air goals, the City of London has adopted an ambitious plan that includes a fully zero-emission bus fleet by 2037 and the adoption of the first ultra-low emission zone staring in 2019.

In the near-term, the transport authority of London has started delivering soot-free Euro VI buses for routes located in low-emission bus zones. These zones are located outside Central London, in critical areas that are emissions hotspots where buses are large contributors to the problem. As of today, 7 of the 12 low-emission zones are served by Euro VI buses. Central London, where air quality is most critical, is considered an ultra-low emission zone (ULEZ) and zero-emission bus zone by the local government, where only Euro VI and battery electric buses (BEBs) are being deployed. Approximately 3,000 Euro VI diesel buses will be operating in this type of zone by the end of 2019, and 250 BEBs will be in operation by 2020. About 30 percent of new Euro VI buses are hybrids, providing lower emissions and fuel consumption than a dedicated Euro VI diesel engine. Older in-use buses are also targeted for retrofits that would set them at Euro VI levels.

The world’s first ULEZ will enter into effect in April 2019 in Central London and be expanded to the Greater London area by 2021. For buses, the ULEZ design requires that diesel heavy-duty vehicles meet Euro VI standards or face a fine of £300–£100 if standards are not met. Other vehicle types such as passenger cars and light commercial vehicles are also included in the ULEZ.

Santiago Leads the Bus Technology Transition in Latin America

Santiago, Chile is a prime example where data-driven decisions by proactive regulators can spur technology change and result in measurable improvements in local air quality. In the 1990s, Santiago, home to 7 million people, was in non-attainment for PM10, ozone, and carbon monoxide. This resulted in an estimated 4,000 early deaths annually at an annual social cost between US$670 million and US$1,900 million. The government resolved in 1997 to improve air quality through an air quality management plan, including better fuel quality and vehicle emission standards with a focus on public transport development and fleet renewal. Santiago became the first Latin American city in 2009 to adopt ultra-low-sulfur diesel and Euro V for its bus fleet. These and other actions have since led to reduced PM and ozone precursor emissions by almost 70 percent through 2015.

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A zero-emissions bus in Santiago, Chile

Constant growth in vehicle fleet and vehicle activity resulted in daily non-attainment of the PM2.5 standard between 2011 and 2014. Regulators and environmentalists sprang into action again, resulting in a new air quality management plan for the city in 2017. This latest plan required both the immediate transition to soot-fee Euro VI technology for all new buses, as well as the roll out of the first fleet of zero-emission buses. Santiago again leads the way as the first Latin American city to require Euro VI for its buses. Already 500 Euro VI buses have been ordered and are being manufactured in Brazil, and some have entered into operation as of January 2019.7 More than 3000 Euro III buses will be replaced with new Euro VI buses over the coming years.

Besides a massive and swift transition to soot-free diesel technologies, Santiago is accelerating the adoption of battery-powered electric buses. The government, in coordination with environmental non-government organizations and private sector efforts are now developing a strategy to move to 25-percent electric drive buses by 2025. Today, Santiago is receiving 200 battery-powered electric buses produced both by BYD Auto and Yutong and privately financed by the electric utility companies Engie and Enel. This model of BEB financing demonstrates the shifting finance and business models that will be needed to bring new bus technology into conventional bus fleets. An additional 500 buses will be added in a next phase.

Besides immediate environmental and climate benefits, which are expected to reach 500,000 tons of carbon dioxide reductions, the total cost of ownership of these electric buses on some routes can be 20 percent lower than previous diesel buses. The transition to electric bus operation for a system with the reach of Transantiago requires proper planning, including bus and route selection, duty cycle determination, and vehicle simulation. Such planning has been necessary in partnership with international technical experts to develop route-based bus performance models. In this way, Santiago will be able to deploy the most suitable bus technology to meet the operational requirements of its bus routes.

The Technology Transition Begins Now

Bus fleets are targets for large-scale investment. They can provide affordable, low-carbon, and energy-efficient transport in major urban areas. But despite their advantages, the vast majority of new buses today utilize older technology diesel engines that continue to contribute to harmful air pollution and other environmental hazards. Continued technology stagnation, particularly in rapidly developing emerging economies, will accelerate the health and environmental burden of bus fleets in urban areas as investments in bus fleets continue to grow.

When megacities such as London and Santiago adopt a strategy to deliver lower air pollution and energy consumption in the transportation fleet, they invest in clean technology for their heavily utilized urban buses. The results can be surprising: in London today a Euro VI diesel bus produces lower aggregate emissions than a Euro V or Euro VI diesel car. These results are proof that investments in public transport deliver large-scale public benefits. Fleets must prepare for constant technology change over the coming decades. Our current and future environmental challenges demand near-term incorporation of soot-free technology and a long-term transition to zero emissions.

Ray Minjares is Clean Air Program Lead and Francisco Posada is Senior Researcher, both with the International Council on Clean Transportation (ICCT). E-mail:

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