The use of Compressed Natural Gas (CNG) as a transport fuel is a mature technology and widely used in parts of the world. Although compressed natural gas is a fossil fuel, it is the cleanest burning fuel at the moment in terms of NOx and soot (PM) emissions.
CNG can be employed to power passenger cars and city busses. CNG passenger vehicles emit 5-10% less CO2 than comparable gasoline powered passenger vehicles. Generally, there is no benefit over diesel powered cars in terms of CO2 emission reduction. However, the NOx and soot emissions of CNG powered vehicles are substantially lower than from diesel powered vehicles. Thus for city busses, often diesel powered, the benefits of CNG lay in the improvement of the local air quality and not in the CO2 emission reduction. The introduction of CNG in the transport sector provides a good stimulus for biogas. Biogas has the potential to lower the CO2 emissions by almost 75%.
Natural gas, a fossil fuel comprised mostly of methane, is one of the cleanest burning alternative fuels. It can be used in the form of compressed natural gas (CNG) to fuel passenger cars and city busses or in the form of liquefied natural gas (LNG) to fuel heavy duty trucks. (For a discussion of Liquefied Natural Gas (LNG) see the topic Liquefied (Compressed) Natural Gas in trucks and cars).
There are both dedicated natural gas vehicles (NGVs) which run exclusively on natural gas, and bi-fuel vehicles, which have two separate fueling systems enabling the car to run either on natural gas or on a conventional fuel, i.e. gasoline or diesel (US DOE, 2009). Most current CNG passenger cars are bi fuel vehicles. For heavy duty application Liquefied Natural Gas is the preferred option, see the topic Liquefied (Compressed) Natural Gas in trucks and cars).
It is possible to retrofit a gasoline powered vehicle with a natural gas tank. However, these vehicles are in general not as fuel efficient as OEM natural gas powered vehicles. In addition, retrofitted vehicles have higher emissions of NOx and soot. Compressed natural gas is also ideally suited for city busses. In Los Angeles, for example, 95% of the city busses employ CNG.
Technically, natural gas vehicles function very similarly to gasoline-powered vehicles with spark-ignited engines. The natural gas is stored in high-pressure fuel cylinders on board of the vehicle. (US DOE, 2009)
The use of natural gas in transport is especially attractive in countries with significant own gas reserves.
Modern CNG vehicles are just as reliable as gasoline powered vehicles. Natural gas powered vehicles need extra space for fuel storage. In most modern CNG powered passenger cars the spare tire has been removed to fit the fuel tank. Still, CNG vehicles have a relatively short driving range, about ~ 50% shorter than vehicles powered by regular gasoline. The driving range of CNG passenger cars is limited within the range of 350 to 450 kilometers. Therefore use of CNG is mainly limited to passenger cars and city busses. Improvement of the vehicle range can be obtained with direct injection technology, taking advantage of the high octane number of natural gas, turbo charging and hybridization of the vehicle (IGI, 2009). These technological advances will also reduce the emission of CO2.
The introduction of natural gas as a transport fuel in most countries is hampered by the lack of refueling infrastructure with sufficient national coverage. The low penetration of CNG refueling stations is caused by the fact that the investment costs for a natural gas refueling station are significantly higher than for liquid fuels. (Roeterdink et al, 2010). These high investment costs slow down the growth of a CNG vehicle park because owners of fuelling stations will only offer CNG when there is a sufficient demand. However, consumers will only purchase such a vehicle when there are sufficient refueling stations. Combining CNG refueling stations for city buses with public fuelling stations may increase the initial efficiency of a CNG refueling station.
The use of Compressed Natural Gas (CNG) as a transport fuel is a mature technology and widely used in parts of the world. Worldwide there are almost 10 mln natural gas powered vehicles and more than 15.000 CNG refueling stations in 75 countries. The four leading countries in CNG usage are Pakistan (2,4 mln vehicles), Argentina (1,8 mln vehicles), Iran (1,7 mln vehicles) and Brazil (1,6 mln vehicles) (International Association for Natural Gas Vehicles, 2009).
In countries with significant gas reserves but little or no oil reserves, the use of natural gas in transport can increase energy security and lower the dependence on costly oil imports.
The use of natural gas as a transport fuel has two environmental advantages:
1) Natural gas is a clean burning fuel, with very low NOx and soot emissions, therefore substantially improving the local air quality.
2) Natural gas produces less CO2 for every unit of energy consumed by the vehicle.
The energy efficiency of a natural gas powered passenger vehicle is comparable to a gasoline powered passenger vehicle. Thus the relatively lower carbon content of natural gas causes 10 - 15% lower tailpipe CO2 emission compared to a gasoline powered car. No tailpipe CO2 emission reduction can be expected when a diesel powered car is replaced by a natural gas powered car. (RDW, 2010) However, when diesel powered passenger cars are replaced by natural gas powered passenger cars, the reduction in NOx emission is substantial, about 96%, see Table 1.
Table 1: Emission of CO2, NOx and PM10 by passenger cars (source: TNO, 2009)
|Fuel||CO2 [g/km] ||NOx [g/km] ||PM10 [g/km] |
Heavy duty diesel engines of busses emit considerable amounts of NOx and, when not yet equipped with particulate matter filters, also considerable amounts of soot (PM10). Especially city busses emiting these air polluting substances in urban areas may significantly affect public health. Table 2 shows the emission of the substances CO2, NOx and PM10 for three types of technologies used for busses (Kadijk, 2008).
Not all technologies used in CNG busses will contribute to a better local air quality. Lean burn CNG busses, which have CO2 emissions similar to diesel busses, also have similar NOx emissions. However, CNG busses employing stoichiometric combustion (one to one oxygen/fuel ratio) have a 60% lower NOx emission than diesel busses, but stoichiometric CNG buses have about 22% higher tailpipe CO2 emissions.
Table 2: Emission of CO2 , NOx and PM10 by busses (Kadijk, 2008)
|Technology||CO2 [g/km] || NOx [g/km] ||PM10 [g/km] |
|Diesel bus||864 - 1076||3,4 – 4,6||~0,3 (< 0,03*)|
|Lean burn CNG bus|
4,1 – 4,5
|Stoichiometric CNG bus ||1040 – 1440||1,2 – 2,2||< 0,03|
*employing a PM filter
Well-to-Wheel CO2 (WTW) emissions
Taking into consideration the energy needed to extract and refine the fuels, the corresponding Well-to-Wheel (WTW) CO2 emissions of natural gas are much lower than for gasoline and diesel.
As mentioned before the energy efficiency of a natural gas powered passenger vehicle is comparable to a gasoline powered passenger vehicle. Therefore, the Well-to-Wheel CO2 emission of gasoline and natural gas listed in Table 3 can be directly compared. The Well-to-Wheel CO2 emission of a natural gas powered vehicle are about 25% lower than from a gasoline powered passenger car.
The energy efficiency of a diesel power car is about 15% better than that of gasoline and natural gas powered cars. However, the overall WTW CO2 emission reduction of natural gas compared to diesel is still about 12%.
Table 3: WTW CO2 emissions (Hanschke at al, 2009)
WTW CO2 emission
Potential for further CO2 emission reductions
When using first-generation biofuels, the CO2 reduction potential of CNG powered passenger cars and busses is considerably higher than for gasoline and diesel powered passenger cars and busses. This is due to the high CO2 reduction potential of biogas compared to ethanol and biodiesel. According to several studies, using 100% biogas will for example lower the CO2 emission of CNG busses to about 300 g/km (i.e. 75%), using 100% first-generation biodiesel will lower the CO2 emission only to about 600 g/km (i.e. 38%) (Kadijk, 2008; European Parliament, 2009). Similar reduction potentials can be expected for passenger cars.
The purchase price of natural gas powered vehicles is in general higher than the purchase price of comparable gasoline and diesel powered vehicles. Depending on the country the additional price ranges from 3 to 30% (International Gas Union, 2009). However, the lower price of CNG will compensate for the extra purchase cost.
European Parliament (2009). Directive 2009/30/EC of the European Parliament and of the Council, available at http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2009:140:0088:0113:EN:PDF 
Hanschke, C.B., Uyterlinde, M.A., Kroon, P., .Jeeninga, H. and Londo, H.M. (2009). Duurzame innovatie in het wegverkeer. Een evaluatie van vier transitiepaden voor het thema Duurzame mobiliteit, ECN report no. 08-076, 2009.
International Association for Natural Gas Vehicles (2009). Natural Gas Vehicle Statistics, available at http://www.iangv.org/tools-resources/statistics.html 
International Gas Union (2009). Natural Gas for Vehicles - Global Opportunities for Natural Gas as a Transportation Fuel for Today and Tomorrow. Available at http://www.madegascar.eu/fileadmin/dam/madegascar/downloads/Global_Opportunities_for_Natural_Gas_as_a_Transport_Fuel.pdf 
Kadijk, G. (2008): Praktijkemissies EEV stadsbussen. TNO report no. MON-LTR-033-DTS-2008-01497
Roeterdink, W.G., Uyterlinde, M.A., Kroon, P., and Hanschke, C.B. (2010). Groen Tanken: Impassing van alternatieve brandstoffen in de tank- en distributie infrastructuur, ECN report no. 09-082, May 2010.
RDW (2010). Brandstofverbruiksboekje 2010. available at http://www.rdw.nl/nl/voertuigeigenaar/auto/kopen_en_verkopen/milieu_en_verbruik/ 
US DOE (2009). Alternative Fuels and Advanced Vehicles Data Centre – What is a Natural Gas Vehicle?, available at http://www.afdc.energy.gov/afdc/vehicles/natural_gas_what_is.html