Muni Hybrid Buses

The San Francisco Municipal Transportation Agency (SFMTA) has the goal of reducing its fleet greenhouse gas emissions to thirty percent below 1990 levels by 2014 and becoming 100 percent emission-free by 2020. The SFMTA recently took another step forward in reaching these goals through the purchase of 86 Daimler-Chrysler Commercial Buses of North America (DCCBNA) Orion VII low floor diesel hybrid electric buses (DHEBs).


The SFMTA unveiled the first of Muni’s new hybrid buses on April 11, 2007. San Francisco Mayor Gavin Newsom and the manufacturer, Daimler-Chrysler, announced that San Francisco will have the third-largest hybrid bus fleet in the nation.

What is a Hybrid bus?

The Muni hybrid buses are essentially electric buses just like Muni’s electric trolley coaches. Rather than get their electricity from overhead wires, they use a small diesel engine (5.9 liter Cummins ISB found in pick-up trucks) to turn a generator that, together with traction batteries, supply the necessary electrical energy to move the bus through the streets of San Francisco. Muni’s hybrid buses are “series hybrids” meaning there is no mechanical connection between the engine and wheels: The engine turns a generator that produces electricity to power drive motors that propel the electric bus (note that “parallel” hybrid bus, more similar to a Toyota Prius, uses a blend of mechanical and electrical power to accelerate, rather than just electricity). The drive control system on Muni’s series hybrids operates the diesel engine at its optimum emission and fuel economy settings. Traction batteries supply energy for acceleration, hill climbing and other peak load conditions. This reduces the diesel engine speed (rpm) fluctuations and helps minimize engine emissions and increases fuel economy.

A hybrid bus can also recover and store braking energy. During vehicle deceleration, the control system changes the traction motor into a generator. The traction motor/generator is then used to help slow the vehicle as the traction motor/generator stores braking energy in the traction batteries. This increases the vehicle’s fuel economy and brake life. Other major transit agencies including New York City, Boston, Washington DC, Toronto and Seattle have adopted hybrid bus technology within their respective public transportation systems.

Zero Emissions 2020 Plan

The “Zero Emissions 2020” Plan commits the City and County of San Francisco to a clean air policy for public transit. In coordination with the SFMTA and the San Francisco Department of Environment, Zero Emissions 2020 focuses on the purchase of cleaner transit buses including hybrid diesel-electric buses. The plan also discusses purchasing and upgrading to hydrogen fuel cell buses when they become commercially available for large fleet purchases.

Hybrid buses are generally recognized as a “bridge” technology to fuel cell buses. AC Transit and VTA are in the process of demonstrating and testing prototype fuel cell buses (three buses are operating at each location). Muni is closely following the progress of these programs and tentative regional plans may allow for Muni to operate and evaluate some of these fuel cell buses in San Francisco. In addition to the purchase of hybrid buses as called for in the SFMTA Zero Emissions 2020 Plan, Muni will fuel all of its diesel-engine vehicles with biodiesel (B20) by the end of 2007.

Hybrid Emission Reduction

Hybrid buses will replace Muni’s oldest diesel buses, some dating back to 1988. The hybrid buses emit 95 percent less particle matter (PM, or soot) than the buses they replace, they produce 40 percent less oxides of nitrogen (NOx) and they reduce greenhouse gases by 30 percent.

There are two main “green” features: 1) improved air quality, and 2) improved energy savings.

  • Muni hybrid buses reduce emissions and use less fuel through more efficient use of fuel energy and the recuperation of brake energy.
  • An increase in fuel efficiency of 30 percent is anticipated--which will result in savings of at least 20,000 gallons of diesel fuel over 12 years.
  • Regenerative braking allows energy to be stored in batteries that would otherwise be lost as heat energy in braking.
  • In addition the hybrid drive system is adaptable to new technology developments that could lead to further emission reductions and fuel savings.
Hybrid Technical Reliability
  • Hybrid buses are a major step forward for bus technology in terms of reliability.
  • Hybrid-electric drive systems are being recognized by the transit industry as a means of facilitating several important transit bus design goals, including lower maintenance requirements and therefore reduced operating expenses.
  • Hybrid-electric propulsion allows for significant reductions in transmission and brake maintenance, fewer service bays and maintenance spares may be needed than with a similarly sized fleet of motor buses.
  • Hybrid-electric drive systems are setting reliability records at agencies that have similar transit duty cycles (Manhattan) and terrain (Seattle) to San Francisco Muni.
  • The braking capabilities of the hybrid-electric bus will dramatically lower wear rates and repair on mechanical service brakes.
Hybrid Benefits to Customers and Operators
  • Muni’s low floor hybrid buses provide many benefits and features for customers and Operators.
  • The hybrid low floor design (no steps) makes for easier access, shorter loading and disembarking times for customers and Operators.
  • Hybrids are smoother, provide quicker acceleration and create less engine noise--a benefit for customers and bus operators.
  • Customers will notice a more spacious and accessible interior.
  • It is a seamless transition for the driver to operate a conventional diesel bus and a hybrid bus.
Cost Advantage

The Muni hybrid buses, made by Daimler-Chrysler, cost roughly $500,000 (approximately $150,000 more than a conventional diesel bus). Fuel and maintenance (operating) cost savings over the life of the bus are expected to help recover the higher initial (capital) cost. Specifically, operating cost savings are expected through the following features:

  • 30 percent increased fuel economy
  • Extended brake life
  • No transmission to service
  • Less moving parts
  • Less engine wear and tear
  • Less expensive engine

SFMTA’s Hybrid Fleet

SFMTA Muni will have a fleet of 56 forty-foot and 30 thirty-foot hybrid buses by July 2007 making San Francisco the third-largest hybrid bus fleet in the nation. SFMTA is also working internally on a plan to buy additional thirty-foot hybrids. Currently, more than half of Muni’s vehicles are zero emissions, including 40 percent of Muni’s buses.

  • Hybrid buses: 86
  • 56 forty-foot hybrid vehicles
  • 30 thirty-foot hybrid vehicles
  • Diesel buses: 495
  • Electric trolley buses: 333
  • Metro streetcars: 151
  • Historic streetcars: 26
  • Cable cars: 40