How do they do that? is a series for The Source that explores the technology that helps keep Metro running and passengers and other commuters moving. Some of it applies directly to the trains, buses and freeways and some of it runs in the background — invisible to nearly everyone but essential to mobility in our region.

How do they refuel a CNG bus?

You can’t just pull into Costco to fill up a compressed natural gas (CNG) bus. When Metro began shifting its fleet of diesel buses to CNG, special stations had to be constructed at each of Metro’s 11 bus maintenance facilities to refuel Metro’s 2,200 buses that run not on gasoline but on gas.

With the retirement in January, 2011 of Metro’s last diesel bus the conversion was complete, making Metro the first major transit agency in the world to operate only alternative clean-fueled buses. Further, CNG is American produced, meaning that Metro is not dependent on fluctuating and unstable foreign oil supplies.

To refuel, a bus pulls up to a service bay and hooks up to a dispenser that looks something like a gas pump, with a hose and a nozzle that mates to the bus. A service attendant attaches the nozzle that automatically locks on during fueling. The nozzle is connected to a tube that carries the gas from a compressor housed nearby in a sound deadening compartment. The natural gas is compressed to 3,600 pounds per square inch as it is dispensed into the bus using a large electric motor or engine driven compressors.

The fueling system incorporates several safety features to prevent fire or explosion. Smoking is not allowed near the stations and all electrical systems are sealed to prevent sparks. CNG is only flammable if mixed with the correct ratio of air so the fuel in the cylinders in each bus is not explosive. Buses hold roughly from 17,000 standard cubic feet of gas (for a 40-foot bus) to 27,000 (for an articulated bus) and take only a few minutes to fuel.

CNG costs roughly $1.50 per diesel gallon equivalent, including compression and dispensing costs. This is less per gallon than diesel but cost is not the reason for the conversion to CNG. In fact, CNG buses cost about 10 to 15 percent more to operate than standard diesel engine buses, largely because of increased maintenance costs. But the move to clean air vehicles, ordered by Metro’s Board of Directors in 1993, was decided because the health benefits of running a clean-air fleet are immeasurable.

Compared with diesel buses, Metro’s CNG fleet reduces cancer-causing particulate matter by more than 80 percent. And because of the switch from diesel to CNG, Metro avoids emitting nearly 300,000 pounds of greenhouse gas emissions per day.