Natural gas engines are engines that are equipped with a spark ignition system like gasoline engines and are fueled with natural gas. As well as methane, which is the main constituent, natural gas also contains other substances as secondary components and/or impurities that considerably affect its suitability as fuel.
Requirements, properties, parameters
The knock resistance is represented by the methane number which is determined by comparison with defined mixtures of a known methane number. In practical terms however, the methane number is usually calculated from the composition.
The knock resistance of commercially available natural gas qualities with an appropriate composition is significantly greater than that of comparable gasoline qualities. Methane numbers that are as high as possible are still desirable in order to avoid higher fuel consumption. The automotive industry demands at least 75 as adequate value.
The sulfur content of natural gas should be a low as possible. The same technical fundamentals apply as for gasoline, see sheet 126.0 "Sulfur in gasoline".
Solid foreign matter and water may lead to problems arising with the fuel supply. In addition, water can trigger corrosive processes; in turn, resulting corrosion products can impair fuel supply. Therefore, refueling at well-managed filling stations is preceded by a drying process. Compressor oil can also cause problems if large quantities thereof accumulate in the fuel injection nozzles.
Standardization, quality designations
The above-mentioned parameters are far from being complete, standardization of natural gas for engine applications is currently being worked on. This standardization will be based on general gas standards such as specified in DVGW worksheet G 260 ("DVGW" = German Technical and Scientific Association for Gas and Water), and appropriate modifications and additions that are relevant for use in vehicles will be incorporated. Changes to DIN 51624 "Automotive fuels ---Compressed natural gas---Requirements and test methods" in progress.
This quality description includes the concepts of L gas and H gas, with H Gas having a higher content of energy and L gas having a lower content of energy. Both gases are basically suitable for use with MB engines. Other gas qualities may be available in foreign countries, whereby the basic requirement calls for at least 80 % by volume methane in natural gas to ensure that a sufficient calorific value is available.
These gases are identified at the gasoline pumps as follows:
∅ = 85 mm to 100 mm
∅ = 85 mm to 100 mm
Natural gas is normally odorless. For safety reasons, an odor (odorizer) is mixed with the gas. This should preferably be sulfur-free.