 It is because of CNG need a source of ignition for combustion in diesel engine.A flexible-fuel vehicle (FFV) or dual-fuel vehicle (DFF) is an alternative fuel automobile or light duty truck with a multifuel engine that can use more than one fuel, usually mixed in the same tank, and the blend is burned in the combustion chamber together.These vehicles are colloquially called flex-fuel, or flexifuel in Europe, or just flex in Brazil.