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.