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:

Manufacturer	Caterpillar
Type	G 3508 TA-54
Principle	8-cylinder combustion engine,              4-stroke Otto engine
Displacement	34,5 dm3
Bore	170 mm
Stroke	190 mm
Maximum power	472 kW
Rated speed	1400 revs / min

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:

Manufacturer	Linde
Gross volume	810 dm3
Net volume	~ 670 dm3
Weight (empty)	900 kg
Storage temperature	-138°C
Storage pressure	4 bar
Insulation	vacuum

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.