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CNG Buses - an Assessment
By Prof. Dr. Rudolf Petersen, Wuppertal Institute, Germany

The use of Compressed Natural Gas (CNG) in urban buses provides certain advantages and disadvantages compared to conventional diesel use. The international literature collected in this Information Pool discusses various aspects of different fuels and vehicles. The papers - mainly published by research institutions, transit authorities and industry associations - allow to draw preliminary conclusions about the suitability of CNG buses in urban transport in Latin American Cities.

The reference level with respect to environmental impact as well as investment and operation cost is defined by state-of-the-art diesel buses without particulate traps and catalyst aftertreatment. In this text, these are referred to as "conventional diesel buses".

Because of the currently minor role of gasoline engines in heavy-duty vehicles, this concept is not discussed in the available papers.

The main results regarding a comparison between conventional diesel buses and CNG buses can be summarized as follows:

  • CNG bus engines operating at stoichiometric conditions with closed-loop control and three-way-catalysts provide the best results in terms of gaseous emissions. The minority of CNG engines operating at lean mixtures suffer from higher NOx emissions.
  • The main environmental advantage of CNG buses compared to diesel buses is the absence of diesel particulate emissions (1) (2) (3).
  • CNG bus engines produce less noise than diesel engines (2).
  • Energy consumption of CNG buses is significantly higher than that of diesel buses - in a range of 20% to 30% consumption penalty (1) (4) (2).
  • CNG buses demand significantly higher investments the figures differ widely (1) (4) 3).
  • Higher investment cost and higher energy consumption of CNG buses can partly or even fully be compensated, if fuel tax incentives are provided, and subsidies for investments are given by the governments.
  • Today's CNG buses can be as reliable as diesel buses (1).
  • Fuelling facilities for CNG buses are significantly more costly, both in terms of investments and operation, compared to equipment for diesel buses (1) 3).
  • A shift from diesel to CNG in urban bus companies demands comprehensive planning and education efforts. A minimum number of about 20 to 50 CNG buses seems to be necessary to justify additional expenses for facilities and training of the technical staff. Bus drivers should be trained as well in CNG bus driving.
  • In mountainous areas, complaints have been made about lack of engine power of CNG buses, but this seemed to be either a problem of older bus models or even of individual perception of bus drivers.

The papers published agree on the better environmental performance of CNG buses compared to diesel buses but also point at existing problems associated with CNG bus use, and necessary preconditions for a broader use:

  • Fuel tax differentiation has to be maintained over a long planning period.
  • Fuel efficiency of CNG bus engines needs further improvements (2).
  • High investment costs of buses and stationary equipment may be reduced in the future due to larger series and standardization but will clearly remain a disadvantage compared to diesel (1).
  • Over the next five to ten years, the comparative advantage of CNG buses in terms of environmental impact may be reduced by advanced diesel aftertreatment technology. When particulate traps and catalytic NOx reduction (either by lean NOx catalysts, NOx adsorbers or SCR) will be introduced in the US and in Europe, diesel buses may be more or less considered as comparably clean. A necessary prerequisite will be the availability of high-quality diesel fuel with extremely low sulfur content (10 to 15 ppm at maximum) (5) (4) (2).

On the other hand, future CNG engine technology has the potential to reduce the energy consumption penalty and cost differential. Catalytic DeNOx aftertreatment will allow for improved fuel efficiency at lean operation. In general, there still is a large potential for fuel savings in gas engines because of the shorter development period and intensity compared to diesels. The basic disadvantage in energy consumption, resulting from the fact that CNG bus engines are equipped with spark ignition and have therefore lower compression ratios than diesel engines, can be compensated for at least to a certain degree by direct-injection technology. DI-NG engines are currently under development but it is unclear when they will reach series production status.

Important policy considerations related to natural gas buses are discussed in "Breathing Clean - Considering the Switch to Natural Gas Buses".

See Also
Cleaner fuels - CNG
General topics
Alternative fuels and energy sources - Compressed Natural Gas (CNG)

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