BPS Memorandum Circular No. 04 Series of 2009, Timetable for Implementation of PNS/UN ECE 115:2006, Uniform Provisions concerning the approval of : (a) Specific LPG (Liquefied Petroleum Gases) and (b) CNG (Compressed Natural Gas) Retrofit Systems to be installed in motor vehicles for the use of LPG or CNG, respectively, in their propulsion system