Investigations on the Determination of the Service Methane Number of LNG

LNG is a fuel that is under increasing discussion for transport purposes. It differs from CNG because it often has a higher concentration of hydrocarbons > C4. This affects knocking in a negative way. The knocking properties of a gaseous fuel are characterized by the Methane Number (MN) which is defined as the methane content in a mixture of methane and hydrogen which has the same knocking properties as the gas under investigation. It was defined by AVL in the late 1960s. In contrast to the Octane or Cetane Number there is no standardized measurement procedure for the MN, because the equipment AVL used was unique and does not exist anymore. But AVL created a calculation methodology based on the large amount of data they had measured. There are several software implementations of this methodology. Further there are other algorithms which are not based on the AVL data. If an MN is measured on an arbitrary engine the result is called a Service Methane Number (SMN). It usually shows the same tendencies as the MN but different absolute values. For a set of LNGs the SMNs are measured on two single cylinder SI test engines of 200 and 600 cc swept volume. Different approaches to measure the SMN are investigated. Further, several calculation methods are compared with the measurements.