Quasi-monovalent drive 

Quasi-monovalent concepts run primarily on natural gas, but also have a petrol tank as a mobility reserve with a maximum of 15 litres capacity. Exclusively at outside temperatures lower than minus 10 degrees and immediately after filling with CNG, a petrol-based start is necessary. After a short time, the vehicle switches to running on gas, once the conditions for smooth natural gas operation are met.

See also:

Schematic diagram of the quasi-monovalent drive in a Volkswagen