Source: CLEAN AIR INITIATIVE website
Natural gas (85 to 99 percent methane) is a fossil fuel, clean burning, cheap and abundant in many parts of the world. Because natural gas is mostly methane, Natural Gas Vehicles (NGVs) have much lower non-methane hydrocarbon emissions than gasoline vehicles, but higher emissions of methane. Since the fuel system is closed, there are no evaporative emissions and refueling emissions are negligible. Cold-start emissions from NGVs are also low, since cold-start enrichment is not required; this reduces both VOC and CO emissions. NOx emissions from uncontrolled NGVs may be higher or lower than comparable gasoline or diesel vehicles, depending on the engine technology, but are typically slightly lower.
As a substitute for conventional diesel engines with high sulfur fuel, NGVs should have somewhat lower NOx and substantially lower PM emissions.
There are three types of natural gas vehicles, which can all be manufactured specifically to operate on natural gas or converted from conventional vehicles (1):
- 1.Bi-fuel, where the vehicle can operate either on natural gas or gasoline
- 2.Dual-fuel, where the vehicle operates either on diesel only or diesel and natural gas, with the combustion of diesel used to ignite the natural gas (1). For more information on this option, see the "In-Use Buses: Conversions" section.
- 3.Dedicated, which operate entirely on natural gas.
Most vehicle manufacturers offer the dedicated option only, because of higher reliability.
Most compressed natural gas (CNG) buses are equipped with a diesel engine modified to a spark-ignition engine that is optimized for the use of natural gas. Gas cylinders are installed that can be refueled at gas stations with compressed natural gas. Such engines can be of two basic types:
- Stoichiometric - this type enables the use of a three-way catalytic converter as in common gasoline cars.
- Lean burn (high air/fuel ratio) - this type is 10-20% more fuel efficient than stoichiometric engines (3), but also has higher NOx emissions. A three-way catalytic converter is unsuitable, but oxidation catalysts can be employed. The great majority of heavy-duty natural gas engines are of this design (3).
In general, CNG buses are between 17% and 41% less fuel efficient than conventional diesel buses(4)(5). They have a substantially lower driving range than diesel buses (4) - e.g. in (4) CNG buses are described as having a driving range of about 300 miles (of course depending upon the capacity of the gas cylinders) compared to a little more than 400 miles for diesel buses.
