The Economic Commission for Europe of the United Nations (UN/ECE) â?Emissions of C.I. and P.I. (NG and LPG) engines Uniform provisions concerning the measures to be taken against the emission of gaseous and particulate pollutants from compression-ignition engines for use in vehicles, and the emission of gaseous pollutants from positive-ignition engines fuelled with natural gas or liquefied petroleum gas for use in vehicles