Revision 3 - Amendment 1 03 series of amendments - Date of entry into force : 27 December 2001 Uniform provisions concerning the approval of compression ignition ( C.I . ) and natural gas ( NG ) engines as well as positive-ignition ( P.I . ) engines fuelled with liquefied petroleum gas ( LPG ) and vehicles equipped with C.I . and NG engines and P.I . engines fuelled with LPG , with regard to the emissions of pollutants by the engine