Transit buses running on compressed natural gas (CNG) reduce air pollution in cities as well as use a fuel that is usually less expensive compared to diesel fuel. Likewise, hybrid-electric buses (HEB) save fuel and reduce exhaust emissions. Both technologies reduce greenhouse gases, namely carbon dioxide, and our dependence of imported petroleum. So, why not combine the two technologies in a CNG hybrid-electric transit bus? To date, most HEBs use diesel engines, and a few cases, a gasoline engine.
Prototype CNG HEB
That is exactly what t he San Diego Metropolitan Transit System (SDMTS) and ISE Corp., also located in San Diego, are doing. New Flyer Industries, the California Air Resources Board (CARB), the South Coast Air Quality Management District (SCAQMD) and San Diego Air Pollution Control District (SDAPCD) are also partners in the project. For the prototype CNG-fueled hybrid-electric bus, SDMTS and ISE Corp. started with a 1997 New Flyer 40-foot CNG-powered low-floor bus. The bus has already traveled some 375,000 miles. More on SDMTS’s experience with CNG transit buses in a bit.
Currently, the CNG hybrid bus has been built and is being tested in simulated passenger service. The bus will eventually be operated in regular revenue service in San Diego, and used as a demonstrator to other transit agencies. Data from the testing will be used to compare with conventional CNG buses with respect to fuel economy, emissions and operating costs.
Versatile ThunderVolt Series Hybrid System
The existing Detroit Diesel Series 50G CNG engine was replaced by an ISE ThunderVolt Series Hybrid System. In a series hybrid, an internal combustion engine drives one or more motor/generators, which supply electrical power to electrical motors that drive the wheels. Excess electrical power is stored in either batteries or ultracapacitors. The batteries or ultracapacitors also store normally wasted energy during decelerating and braking that is recouped via regenerative braking. Energy from the batteries or ultracapacitors is used for high load demand situations such as while accelerating and for hill climbing as well as for “silent running” with the engine off.
A Cummins Westport B-Gas Plus 5.9L CNG engine is used in this ThunderVolt. The six-cylinder, 230-horsepower engine drives a Siemens 165 kilowatt motor/generator. Also used are dual Siemens AC induction 85 kilowatt electric motors connected to the wheels. A maximum of 2,400 pound-feet of torque is available at the driveshaft. This is the same electric motor propulsion used in other ISE hybrid-electric buses, of which more than 120 have been built. The versatile ThunderVolt system has been used with Cummins diesel engines and Ford V-10 gasoline engines in transit buses. For example, SDMTS has ordered 12 New Flyer buses that are fitted with ISE’s gasoline hybrid drive systems.
These are both 35- and 40-foot low-floor transit buses featuring an advanced aerodynamic design.
The CNG hybrid bus uses a liquid-cooled Cobasys Series 1000 NiMH battery pack. This is a departure from other ThunderVolt installations where ultracapitors are used to store electrical energy from the engine-driven generator and recouped by regenerative braking. While ultracapacitors offer better efficiency, they are limited to 45- to 50-foot hills. Batteries can capture more energy and provide more energy for climbing hills, an important issue for SDMTS. The batteries can provide a maximum of 240 kilowatt (321 horsepower) with 9.6 kilowatt-hours of maximum energy to allow silent, engine-off starts.
The CNG hybrid uses the same CNG tanks as existed originally on the 1997 New Flyer bus. Because of the CNG Hybrid’s improved fuel economy, range between fill-ups is increased by 10 to 20 percent. This is important because SDMTS has found that range has been a weakness of its pure CNG buses. This improvement comes from regenerative braking and by being able to use a smaller displacement engine that is operated at its optimum rpm range for best fuel efficiency.
Reducing parasitic losses has also increased overall efficiency resulting in 35 to 60 kilowatts (47 to 80 horsepower) in power savings. Improvements include using variable-speed electric cooling fans on the large-area, high-efficiency rooftop radiators for the air conditioning. Also, an electric, sealed air-conditioning system is installed and both the air compressor and hydraulic pump are now powered electrically.
Adding hybrid technology is expected to improve the already good reliability and maintainability SDMTS has experienced with its current fleet of CNG buses. For starters, being a series hybrid, there is no transmission or conventional drivetrain components to service or repair. Electric components like motors and generators could last the life of the bus. Regenerative braking has already proved that it can drastically increase the time between brake repairs and replacement.
CNG – A Success Story with SDMTS
At the recent 2008 Clean Heavy-Duty Vehicle conference in San Diego, Claire Speilburg, chief operations officer of the San Diego Metropolitan Transit System, outlined the agency’s experience with CNG buses. The SDMTS has been operating CNG transit buses for more than 14 years. SDMTS got its first four 40-foot CNG buses built by New Flyer Industries in 1994 in a pilot program with San Diego Gas and Electric. These were powered by Detroit Diesel Series 50G natural gas engines.
In 2000, SDMTS acquired transit buses using CNG-fueled Cummins C8.3L engines. It is now putting into service 81 new CNG powered buses – 55 40-foot and 26 60-foot buses. Powered by Cummins 8.9L ISL-G engines, they will replace retiring diesel buses. Before the end of 2008, 65 percent of the MTS fleet will operate on natural gas.
Of course, the CNG buses are cleaner than diesel buses; they are also quieter and preferred by riders and drivers alike. Following the CARB’s “Alternate Fuel Path,” SDMTS’s CNG buses meet California bus fleet alternative fuel clean air requirements.
They also get better fuel economy on a usually less expensive fuel, CNG. SDMTS’s CNG-powered buses have averaged a relatively constant 2.5 mpg over the last five years. This compares to 2.7 mpg for diesel-powered buses over the same period. This comparison is based on a diesel equivalent gallon of 138,000 British thermal units, where one therm of natural gas is equal to 100,000 Btu.
In order to fuel its CNG fleet, SDMTS has three refueling facilities that have been upgraded to provide up to 140 CNG bus fillings daily at each site. Most recently, it has installed a new CNG fueling facility at the MTS Kearny Mesa Division with two 1,260-scfm compressors that provide four-minute fill times. Maintenance has not been a problem with preventive maintenance intervals according to industry standards, which are 6,000-mile intervals.
Finally, the SDMTS is ordering more gasoline and diesel hybrid-electric buses. The total order could be for as many as 20 35-foot gasoline hybrid buses for delivery this fall, and up to 350 40-foot CNG or gasoline hybrid buses delivered over the next five years.
Besides saving fuel and cleaning the air, the San Diego fleet will continue to provide valuable data for comparing different alternative fuel options under real-world operating conditions.