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Natural Gas/CNG



Compressed Natural Gas (CNG) is a substitute for gasoline (petrol) or diesel fuel. It is considered to be an environmentally "clean" alternative to those fuels. It is made by compressing methane (CH4) extracted from natural gas. It is stored and distributed in hard containers, usually cylinders.
Like oil (petroleum), this common fuel comes from underground. However, natural gas, as the name implies, is a gas much like air, rather than a liquid like petroleum. It has been found to be one of the most environmentally friendly fuels, and its popularity is growing.

Natural gas is mostly made up of methane. Methane is a hydrocarbon, meaning its molecules are made up of hydrogen and carbon atoms. Its simple, one carbon, molecular structure (CH4) makes possible its nearly complete combustion.
Because of its clean burning nature and the fact that it is not made from petroleum, as gasoline and diesel are, many auto makers around the world are developing vehicles to run on natural gas. Cars, vans, buses and small trucks generally use natural gas that has been compressed (called compressed natural gas or CNG) and stored in high-pressure cylinders.

Argentina and Brazil, in the Southern Cone of Latin America, are the two countries with the largest fleets of CNG vehicles. Conversion has been facilitated by a substantial price differential with liquid fuels, locally-produced conversion equipment and a growing CNG-delivery infrastructure.

In Germany, CNG-generated vehicles are expected to increase to two billion units of motor-transport by the year 2020. The cost for CNG fuels are 1/3 less than LNG fuels, in Europe.

A comparison of targets set by the EU until 2020 for alternative fuels:

2005: Biofuels- 2%; Natural Gas- no target set; H2- no target set; Total- 2%
2010: Biofuels- 5.75%; Natural Gas- 2%; H2- no target set; Total- 8%
2015: Biofuels- 7%; Natural Gas- 5%; H2- 2%; Total- 14%
2020: Biofuels- 8%; Natural Gas- 10%; H2- 5%; Total- 23%


Number of buses running on CNG:
Athens 416 buses
Torino 223 buses
Madrid 202 buses
Malmö 180 buses
Paris 90 buses


  • It is a safe fuel source that is commonly used in homes and businesses for heating, lighting and cooking.
  • Reduced polluting emissions (compared to diesel)
  • For each 100 kilometres covered, compared to diesel, CNG achieves a reduction of polluting emissions of between 82% and 98%.
  • Reduced noise (compared to diesel)
  • In case of redesigned gasoline/petrol engines with increased compression (13.5) there is also a substantial CO2 saving compared to modern diesel engines.



The use of natural gas like fuel in Spain started in the 90s, with the introduction of CNG technology in the urban buses fleets of several Spanish cities.
At present, Madrid, Barcelona, Sevilla, Málaga, Valencia, Burgos and Salamanca are the cities which use CNG in passenger urban transport. Moreover, in Madrid, Alcobendas, Pozuelo, Barcelona, El Prat, Tarragona, Reus, Oviedo and Vigo, the waste recollection companies have CNG fuelled trucks.
Nowadays, the number of CNG vehicles in Spain is 865: 480 trucks for waste recollection, 344 urban buses, 33 forklift trucks, 6 light trucks and 2 minibuses. On the other hand, LNG use like fuel in Spain is more reduced, limited to 22 trucks for waste recollection and 7 tankers.
It is expected that in a near future the number of CNG vehicles is incremented in 361 vehicles, because the existing plans of expansion of buses fleets in several Spanish cities.
Recently, the law 22/2005 of 18 of November, which incorporate several European directives about energetic products, established a tax for natural gas use like fuel of 1,15 €/GJ, which would allow a reasonable development of natural gas use like fuel in the light vehicles segment.

CNG buses in Barcelona

TMB (Barcelona Metropolitan Transport) started trials for the first time in 1995. The results confirmed the reduction of polluting emissions and noise produced by these vehicles compared with those propelled by diesel oil. In 2000, TMB signed an agreement with GAS NATURAL to introduce CNG as an operational fuel for the bus fleet. In this way, between 2001 and 2002, a fleet of 70 CNG vehicles was introduced. These are standard buses, of 12 m.length, supplied by IVECO and MAN (2001/2002 generation).
One part of the demonstration concerns the evaluation of the environmental and energy performance under real, generalized operating conditions. The other concerns the installation of equipment and specialization of one of the city’s biggest bus depots (Zona Franca I) for the maintenance of vehicles using this fuel type.

CNG is an attractive fuel because of its environmental advantages compared with diesel oil, since for a same use, it achieves a reduction of polluting emissions between 82% and 98% for each 100 kilometres covered. This significant reduction is more or lesssimilar for the following types of pollutant: NOx, CO, HC and solid particulates. However, the energy consumption of CNG buses is higher than the diesel alternative (standard bus type), amounting to a 46% increase.
In addition to the reduction in exhaust pollutant emissions, reductions in smell, vibrations and noise (both internal and external) have been measured by different surveys of bus passengers and drivers, indicating improved satisfaction with the new service.
In the Municipal Transport Company of Madrid there are currently 202 CNG vehicles. They began in 1996 with a fleet of 32 CNG buses and built a Refuelling Station. Nowadays the total number of CNG vehicles is 202 and the needed to enlarge the station up to 62 refuelling posts. With this measure they reduce significantly the NOx emissions.


Last year a Governmental Decision, no. 277/14.03.2006, approved the Technical Code for Compressed Natural Gas in Vehicules (NCGV). In 2007, Distrigaz Sud SA of Romania organised three round tables of discussions, dedicated to major decision makers from Romania, among them URTP, in order to prepare the next steps to be done, on short, medium and long term, so that CNG become soon a feasible alternative fuel for vehicles, including buses for passengers local public transport. Partnerships are intended to be established for both launching a pilot project dedicated to CNGV, and elaborating a national plan for accelerating CNGV promotion in our country.


As of last September the number of CNG fuelling station in Italy, is 598. Another 72 facilities are under construction.

In Rome the supply of 400 CNG buses has been completed in december 2007, now all these buses are in operation. A depot has been completely adapted to host and refill the buses.

In Italy, the Directive 2003/30/EC was implemented by the Decree 30/05/2005 n. 128, followed by the Law 11/03/2006 n. 81.
The Decree 128 lays down targets on biofuels and renewable fuels consumption, expressed in percentage as to the overall diesel oil and petrol present on the national market:
  • 1% by 31/12/2005;
  • 2,5% by 31/12/2010.
More ambitious targets as well as fiscal incentives and other measures are expected in the Financial Law 2007, currently under approval.

CNG Financial and legal issues in Italy

The use of CNG for vehicle propulsion in Italy is quite widespread. A policy milestone was represented by the framework agreement set up in 2001 among the Italian Ministry for the Environment, FIAT and Unione Petrolifera (oil producers trade association). The agreement established roles and duties of different parties in the future development of CNG for vehicle propulsion. In particular, FIAT undertook to put on the market new CNG fed models, while the Government had to provide for laws, incentives and resources to support the market itself. As a result, during the last five years, several measures and incentives have been put into action at national and local level, in order to increase the number of vehicles and the fuelling infrastructures.

The Financial Law 2007 is going in the same direction, by the introduction of new incentives for the CNG (and LPG) vehicles diffusion. A contribution of 1500 € is given to individuals purchasing a new CNG/LPG car and 50 M€ are provided in the next three years for the transformation of conventional vehicles into CNG/LPG vehicles.


It is forbidden to enter an underground parking lot with CNG-powered cars.


In Bulgaria three type of engine conversion is going on:

1) Dedicated, Mono-Fuel or Monovalent - conversion of conventional diesel engine by using spark ignition system (mainly trucks and buses). A dedicated engine uses natural gas as its only fuel source. A dedicated engine has the advantage of being 'optimised' to operate on natural gas, thus ensuring maximum efficiency and optimum emissions results;

2) Bi-fuel or Bivalent - Bi-fuel engines operate on either natural gas or gasoline. Bi-fuel engines are available only as an aftermarket conversion (more than 95% of taxi fleets in cities with CNG refueling stations are bi-fueled);

3) Dual-fuel - A dual-fuel engine utilizes a mixture of natural gas and diesel, with the natural gas/air mixture ignited by a diesel “pilot”. The diesel is injected directly into the combustion chamber, while gas is introduced into the air intake by carburetion or by gas injection. The mixture of natural gas and diesel varies according to the load and the duty cycle of the engine, ranging anywhere from 80% gas down to 0% gas. At lower engine loads, diesel use tends to be higher whereas at higher engine loads it is possible to use a higher proportion of gas. Dual-fuel engines are usually the result of a conversion of a diesel engine and have the advantage of not being totally dependent on natural gas for fuel supply. Thus, if a vehicle runs out of natural gas or is away from an available NGV fuel source, it is able to operate solely on diesel. In the Bulgarian capital Sofia more than 100 buses are converted to dual-fuel operation.

At the “Laboratory on Ecological Problems of Engines” at University of Rousse a research work with gas mixture known as Hythane contains 20% hydrogen and 80% natural gas is going on. At this ratio, no modifications are required to a natural gas engine, and studies have shown that emissions are reduced by more than 20%. Mixtures of more than 20% hydrogen with natural gas can reduce emissions further but some engine modifications are required.


Concerning CNG, some fleets are presently using it, namely, CARRIS ( Lisbon bus company - 40 buses), STCP ( Oporto bus company - 300 buses), TUB ( Braga bus company - 16 buses), VALORSUL (waste collection trucks from Lisbon ’s waste management facilities). Thus, there are some small-scale on-going projects on public and private companies. However, in the whole county there is only one available CNG fuelling station for citizens - the same that is used by TUB, in Braga (north of Portugal ). CNG private vehicles have a reduction up to 40% of automobile purchase tax. Taxis have already a 70% reduction on automobile purchase tax, but CNG Taxis have a cumulative reduction of 40% of the remaining 30%. The Portuguese government announced that soon there would be a big fleet of CNG taxis in Lisbon, using the Lisbon BUS company (CARRIS) refueling station.


For the use in vehicles only LPG is available in Slovenia at only 12 publicly accessible refuelling points. In Slovenia CNG is mainly used for heating, it is not used as a motor fuel in vehicles at all.

Supply of vehicles

Conversion of spark ignition motors for flexible use of patrol or LPG is done at about ten mechanical shops in Slovenia. In Slovenia only Subaru, Fiat and Volvo dealers offer also bifuel cars.

Incentives and laws

There are no incentives or laws that would encourage the use of LPG, CNG or biogas in vehicles.

Economic aspects

Depending on the model and age of the vehicle, conversion of a car to bifuel propulsion costs from about 1000 to 2000 €. But the cost is quickly repaid since the price of petrol is about 2 times as high as that of LPG (on May 3rd 2007 LPG costs 0,575 € while the price of petrol is 1,061€).


416 CNG busses are used in Athens for public transport purposes. In 2001 the Urban Transport Organisation of Athens purchased 295 CNG busses, while 121 new busses were bought during 2004. There are two CNG refuelling stations in Athens region (Ano Liosia and Anthousa)serving only CNG busses operated by the Urban transport Organisation of Athens.

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