Both in road and rail transportation, internal combustion engines running on natural gas are discussed as an alternative to diesel propulsion and several prototypes and test series have been realized.

Natural gas engines

In most cases, combustion engines running on natural gas are derived from diesel motors by some modifications, e.g. cylinder heads and the camshaft are usually modified and spark plugs and an electronic motor management are added.

The gas engine used in the LNG-fuelled shunting locomotive at DB AG has the following characteristics:

Source: DB AG 2001

Fuel storage

Since under normal environmental conditions (20 °C, 1 bar) natural gas has a very poor energy content per volume compared to diesel, natural gas has to be condensed to obtain an acceptable energy density for mobile applications.

Three technologies are discussed in this context:

LNG (liquefied natural gas): LNG offers high energy density and is therefore the best choice in term of autonomy and range. Drawbacks lie in cryogenic station-based storage (requiring -160 °C) and a very expensive distribution system. LNG reaches 50 % of the energy density of diesel.

CNG (compressed natural gas): Very mature technology. Even though high pressure requires compressors (200 bar) when refueling, experience from bus sector show that CNG is a simple and viable technology. LNG reaches 25 % of the energy density of diesel.

ANG (adsorbed natural gas): emerging technology. Tank contains some adsorbent material (e.g. active coal) which trap methane molecules by adsorption. This way storage performance is comparable to CNG, but at lower pressure than in the compressed form. However, there are still some technological challenges and issues to be resolved. ANG could become the most promising solution in the future.

Technical data of the gas tank used in the LNG shunting locomotive at DB AG:

Source: DB AG 2001

Distribution network

Whereas diesel has to be produced from oil, natural gas occurs as such in nature and almost does not need any treatment.

CNG: Pipelines to refilling station (usually low pressure) and compression at refilling station.

LNG: Transport of LNG to refilling station and cooling at refilling station

Technical data of the natural gas fuelling station constructed at Munich main station:

• 11 m³ tank for LNG
• 3 m³ tank for liquid nitrogen
• Vacuum-insulated LNG transfer pipe
• Gas pump

Manufacturer of gas engines

MTU etc.

DB AG made several test series on natural gas propulsion:

CNG vehicle at Usedomer Bäderbahn, LNG vehicle for shunting locomotive.

SNCF has a project for a regional railcar running with natural gas, using a 228 kW MAN gas engine.

Medium (Considerable optimisation expected on the basis of present materials and construction principles and improvement by < factor 2 in all fields)

Further progress in ANG technology to be expected.

Higher engine powers (500-2000 kW) needed for railway applications. There is an need for further R&D. Engines for 1500 kW and more presently only exist for stationary applications. Engine control has to be adapted to railway situation with constantly changing operation points.

• Provide clean alternative to diesel propulsion in terms of pollution and noise.
• Provide alternative fuel in view of limited petrol reservoirs

Costs

Low cost compared to other diesel alternatives (e.g. fuel cell).

Infrastructure

Solutions are possible without heavy installations (simple links to the existing distribution networks)

Resource supplies

Longer supply range than diesel (cf. General criteria - Benefits).

Emissions

Although today’s diesel vehicles generate quite low emissions, the key strength of thegas-powered version is to cut emissions down to levels never achieved before by an internalcombustion railcar.

Engine power

Available natural gas engines can only cover the low end of power classes needed for railway applications. More powerful gas engines only exist for stationary applications.

Technological know-how

There is virtually no experience with natural gas technology in railways, neither for propulsion system nor for fuel supply and handling.

Additional supply infrastructure

One problem is the need for an alternative supply system which can become costly, especially since there is no experience with natural gas infrastructure. Therefore a favoured application would be for a closed fleet, e.g. on an island or at a shunting station. Interoperability is problematic even on a national scale as long as natural gas infrastructure is confined to isolated parts of the network.

Operation range

Another problem is the lower operation range due to lower energy density of the fuel. Storage technology needs further progress. Energy content per volume for CNG and LNG solutions is still low compared to diesel (50% for LNG, 25% for CNG). The CNG vehicles tested in Usedom, Germany, had an operation range of about 500 - 600 km as compared to 800 - 900 km for equivalent diesel vehicles. This disadvantage can be substantially reduced if dedicated vehicles are designed.

Initial investment

Compared to diesel, initial investment for conversion to natural gas technology is still high.

Resource limitation

Although natural gas has a higher time range than diesel, it is a limited resource and therefore again a temporary solution.

DB AG gives the following success factors for a wide-spread introduction of natural gas propulsion:

• Availability of natural gas engines in higher power range for locomotives as well as for under-floor integration (for MUs).
• Further development of gas engine technology to improve efficiency at low load
• Reduction of higher costs of gas technology as compared to diesel technology
• Development of supply infrastructure
• low price for natural gas and long-term calculability (taxation)

LNG shunting locomotive at DB AG

DB AG has refitted a class BR 360 diesel-hydraulic shunting locomotive with a natural gas engine (Caterpillar G 3508 TA-54) and a 810 liter LNG tank system (by Linde). To fuel the vehicle an LNG station was constructed near Munich main station.

Greenhouse gas emission

Natural gas offers the lowest "carbon content" per energy unit of all mainstream fuels, about 25 % lower than diesel fuel. This advantage is however partly compromised by bad fuel economy (11 to 28 percent worse than diesel) which is shown by test runs with CNG and diesel busses (urban driving cycle comparable to railways). Compression or liquefaction reduces the carbon dioxide advantage even more. Emissions of natural gas itself, e.g. from leakage and refilling losses, contribute to greenhouse effect.

According to a study by IFEU, the maximal carbon dioxide benefit can only be achieved if fuel economy for natural gas is as good as for diesel traction, which is doubtful. In the worst case there is a disadvantage for CNG traction; in the best case only a slight advantage (cf. Figure 1). If LNG was applied, the emissions from the prechain would be about equal.

Figure 1: Greenhouse gases in CO₂ equivalents

Source: IFEU 2000
				Other environmental impacts: positive
					Pollutants The toxic emissions are much lower than for diesel propulsion. In contrast to the diesel engine, the combustion for the natural gas engine is soot-free, whereby the use of a three-way catalytic converter is possible. Comparison of relative emissions from natural gas and diesel engine: MAN diesel engine      D 2866 MAN gas engine        E 2866 Hydrocarbons       100 % 32 % CO 100 % 16 % NOX 100 % 3 % Particulates 100 % < 5 % Source: Althammer, Hattensperger 1998, IZT calculations. Noise Natural gas engines emit less noise than diesel engines. At 1 meter distance, the gas version of the MAN 2866 shows a noise emission of 97 dB(A) compared to 105 dB(A) for the diesel version. Resource availability According to current assessments, natural gas reserves will last much longer than oil reserves: Natural gas: 60 years based on current exploitation rates and the assumption that no additional reserves are exploited. ~ 170 years based on current exploitation rates and estimates of reserves exploitable in the future. The gas reserves would be even much higher than this if the huge gas hydrate reservoirs could technically be exploited. According to the Intergovernmental Panel on Climate Change (ZEIT 2001), the gas hydrate reservoirs are approx. 12.000 billions of carbon tons (as opposed to < 4000 billions for coal, and less than 1000 for both natural gas and oil). Diesel: 40 years based on current exploitation rates and the assumption that no additional reserves are exploited. Up to 80 years based on current exploitation rates and estimates of reserves exploitable in the future.
	Economic criteria
				Vehicle - fix costs: medium
					Investment is medium to high depending on whether conventional engines can be converted or whether dedicated engines have to be designed.
				Vehicle - running costs: significant reduction
					Energy costs Although energy efficiency of natural gas propulsion is poor in comparison to diesel propulsion, low gas prices may lead to reduced running costs. Different DB sources give savings in fuel costs between 15 and 30 %. This is partly due to favourable taxation and might change in the future. Maintenance costs Maintenance and inspection increased.
				Infrastructure - fix costs: high
					Investment in new supply structure necessary. If good national gas infrastructure is in place (e.g. in France), no costly installations are needed.
				Infrastructure - running costs: increased
					Parallel diesel and natural gas infrastructure will increase running costs.
				Scale effects: low
					Low (no mass markets to be followed and no critical mass to be reached within railway markets) Since natural gas does not take off as a diesel or gasoline alternative in road transportation, rail industry currently cannot follow mass market.
				Amortisation: > 5 years
					According to DB AG, with 30% reduction of running costs refit will pay off in 5-10 years.
	Application outside railway sector
				Status of development outside railway sector: in use
					Busses and cars equipped with natural gas propulsion are available on the market. Both monovalent (natural gas operation only) and bivalent (both gas and gasoline operation possible) gas vehicles exist. Most cars can be converted to natural gas operation.
				Time horizon for broad application outside railway sector: in 2 - 5 years
					(no details available)
				Expected technological development outside railway sector: dynamic
					Considerable optimisation potential in engine technology.  Further progress in ANG technology to be expected.
				Market potential outside railway sector: highly uncertain
					More than 1 million vehicles with CNG propulsion are in use world-wide, predominantly in the GUS States, Italy, Argentina and New Zealand. The further diffusion of the technology depends on infrastructure (natural gas stations) and taxation as well as progress in other clean vehicle technologies (fuel cells etc.). The wide-spread success of natural gas propulsion is uncertain.
	Overall rating
				Overall potential: interesting
				Time horizon: mid-term
					A conversion of Diesel fleet to natural gas propulsion does not meet any long-term technological barriers. Gas engines will be available in most power classes for railway applications in the future. Storage solutions and range are expected to improve in mid-term. Investment costs are still high but running costs may be substantially reduced if price advantage over diesel remains stable. Advantages of natural gas propulsion over diesel include reduced toxic emissions and higher resource reservoirs. However, in terms of CO2 emission, natural gas is about equal to diesel. From an energy efficiency point of view, natural gas propulsion is therefore neutral. More generally, the technology lacks a striking advantage to justify the currently high transition costs. However, in some areas natural gas propulsion could offer an interesting mid-term alternative to diesel propulsion.

References / Links:  DB AG 1999;  Althammer, Hattensperger 1998;  Chabas et al. 2001;  Xin et al. 1997;  Schweiger et al. 1997;  ZEIT 2001;  DB AG 2001;  b-verband gas und wasser.de

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Related projects:  Energy chains of alternative fuels;  Natural gas propelled railcar;  Shunting locomotive with LNG propulsion;  Railcar with CNG propulsion

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date created: 2002-10-09