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CNG bus
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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 ( 2 ). 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 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 pressure gas stations. with compressed natural gas. Sucg engiones 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 miles (of course depending upon the capacity of the gas cylinders) compared to a little more than 400 miles for diesel buses.

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There is a large potential to achieve low emissions when CNG is used as the fuel. Some emission reductions compared to conventional diesel buses have been published in the literature. Since different sources make different assumptions regarding the exact nature of the comparison, it is essential to consult the original sources for further information.

  • Particulate matter (PM): reductions of 60% to 97% compared to conventional diesels with high sulfur fuel ( 4 ) ( 2 ) ( 1 ). (Note that advanced diesel buses equipped with diesel particulate filters and burning ultra low sulfur fuel have comparable or even lower PM emissions than CNG buses.) It should also be noted that the number of ultrafine particles from a CNG fueled bus can also be quite high under certain driving modes.
  • Nitrogen oxides (NOx): reductions of 25% to 86% compared to conventional diesel ( 4 ) ( 2 ) ( 1 ).
  • Carbon monoxide (CO): reductions of 52% to 84% compared to conventional diesel ( 2 ) ( 1 ). Other data indicates that CO emissions from CNG could be higher than from diesels.
  • Total hydrocarbons (HC): significantly higher for CNG ( 6 ) ( 7 ). Over 80% of these emissions are composed of methane, which has a low potential to react in the atmosphere to create ground-level ozone (summer smog) ( 2 ), but it is a potent greenhouse gas (see also below).
  • Regarding further parameters: natural gas vehicles have significantly low noise levels and engine vibration ( 2 ).
  • Unless equipped with an oxidation catalyst, CNG vehicles normally have much higher aldehydes emissions than a typical diesel. (W22)
  • "Life cycle analysis suggests greenhouse gas emission savings relative to gasoline, and possibly small savings relative to diesel" ( 1 ).
  • "For Panel Vans and heavy-duty vehicles >3.5 tonnes, total GHGs are comparable or slightly increased when compared to diesel operation" ( 2 ).
  • "While more research is required, greenhouse gas emissions from CNG buses appear to be similar to those from diesel buses on a total fuel cycle basis, even though they emit more methane. Natural gas buses have inherently lower carbon dioxide emissions than diesel buses" ( 8 ).
    In summary, GHG emissions from NGVs will be approximately 15 percent to 20 percent lower than from gasoline vehicles, since natural gas has a lower carbon content per unit of energy than gasoline. NGVs have about the same GHGs as diesel fuel vehicles, with lower CO2 emissions offset by higher CH4 emissions.

It should be noted that these emission and greenhouse gas values do not necessarily justify a choice in favor of CNG buses. The relevance of the relative emission savings and the parity in greenhouse gas emissions must be carefully analyzed in the light of the individual situation. Thus for instance, the total emission reductions achieved through a particle filter on a conventional diesel bus may be perfectly sufficient to alleviate a high PM level problem. It must, however, be pointed out that a particle filter require diesel fuel quality with low sulfur content. Other factors, such as cost effectiveness, may then be decisive regarding the choice of system (see also "CNG Buses - an Assessment").

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"One measure of the reliability of a bus is the average number of road calls. When the driver cannot complete his or her route and calls for a replacement bus, a road call (which encompasses events from engine failure to running out of fuel) is recorded" ( 9 ). In 1996, the National Renewable Energy Laboratory (NREL) carried out a "Vehicle Evaluation Program" for the US Department of Energy (DOE), using the number of road calls to assess bus reliability of various alternative fuel buses ( 9 ). Those calls involving engine/fuel system-related components, including "out of fuel" calls, were treated separately from those that involved other parts of the bus (e.g. doors).

"Buses that ran on CNG in Miami had about four times as many alternative fuel system-related road calls per 1,000 miles as their diesel counterparts. Most were engine or fuel system-related, including 9 out of 81 for running out of fuel. These buses have very low mileage because there is no convenient fuel station on the premises. In contrast, the buses at Tacoma have more than four times the mileage accumulation as the Miami buses. The Tacoma engines are also one model year newer. Their road call rates are identical for CNG and diesel. … Overall, we can conclude that CNG buses are potentially as reliable as diesels. It is important to note that the manufacturer is now selling newer, more advanced CNG engines than the ones used at Miami or Tacoma" ( 9 ).

According to ( 5 ), "CNG buses are only 50 - 75 % as reliable as comparable diesel buses"; ( 1 ) also states that they "are often less reliable".
It should be noted that dedicated CNG buses are now (2003) commercial technology, sold by major vehicle manufacturers with the same guarantees as for conventional buses.

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Factors determining the incremental cost associated with the use of natural gas engines relative to diesel in HDDVs include the following ( 4 ):

  • The incremental cost of a natural gas engine and associated equipment (gas cylinders, piping, valves etc.) as compared to an equivalent diesel engine;
  • The cost related to the fueling infrastructure required for use of CNG or LNG;
  • The cost differential between CNG/LNG and diesel fuel; and
  • The operating and maintenance cost related to the use of natural gas vehicles as compared to diesel vehicles.
    CNG buses cost more to purchase than diesel buses ( 1 ). Operating expenses are also significantly higher ( 5 ). The exact costs depend on the individual situation, but different estimates are available in the literature.

According to the International Energy Agency (IEA), a CNG bus costs "US$ 25,000 to 50,000 more than comparable diesel bus (less in developing countries)" (see also IEA's cost comparison table in the tabular overview). According to ( 8 ), a typical diesel bus costs between US$ 250,000 and US$ 275,000, a natural gas bus between 15 and 25 percent (i.e. about US$ 40,000 to 65,000) more.

In the program descriptions Cleaner Bus Fleets in New York City and CNG Buses in the United States, some cost experiences in the United States are provided, including fuel, maintenance and operating costs (see also documents ( 5 ), ( 11 ), ( 12 ), ( 13 ) and ( 14 )).
For more in-depth information on cost implications of CNG, see also ( 4 ), ( 15 ), ( 2 ), ( 8 ), ( 6 ), ( 9 ).

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Over 1.2 million natural gas vehicles - including light duty vehicles - are in use worldwide in over 40 countries with Argentina (450,000), Russia (>300,000) and Italy (300,000), Canada and USA (70,000) operating the largest fleets ( 2 ).

A report on the international experience with natural gas transit buses has been published by the International Association for Natural Gas Vehicles (IANGV) ( 16 ). All in all about 3,500 natural gas buses - most of them CNG - were operating in the USA in the year 2000, giving a market penetration of about 8%. Furthermore, natural gas buses account for 18% of current new bus orders ( 16 ).

The city of Beijing, China has recently introduced approximately 1500 CNG buses.

Within the Info Pool, the following projects are described which feature CNG buses: CNG Buses in the United States, CNG Buses in Delhi and Cleaner Bus Fleets in New York City.

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The main barriers to the use of natural gas vehicles are their high capital cost and the lack of a refueling infrastructure ( 2 ) ( 8 ). These may be reduced through increased market penetration. Introduction of CNG vehicles and fueling and service infrastructure should ideally be dealt with together in a coordinated approach.

In many countries, diesel fuel is subsidized relative to other fuels. The combination of high fuel efficiency for diesel fueled buses along with the low price of diesel fuel, often results in much higher operating costs for CNG relative to diesel.
According to ( 1 ), the disadvantages of natural gas include the following:

  • Greater difficulty in distribution and storage
  • Shorter driving range
  • Greater weight of the fuel tank (gas cylinder)
  • Longer refueling time, especially if using a slow fill refueling system
  • Backfire in the inlet manifold.

These factors should be considered in the further technological development.

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