The difference between a natural gas car and a petrol engine is the fuel system.
In the case of CNG (Compressed Natural Gas), the gas is compressed to about 250 bar and injected into the vehicle through a valve which leads to the storage cylinders. In said cylinder, the gas is stored at about 200 bar. As the gas is consumed, the pressure goes down.
The gas leaves the cylinder via line of high pressure to the engine compartment. The gas pressure is reduced (pressure regulator) to approximately atmospheric pressure.
Natural gas is mixed with air injection system and enters the combustion chambers. In the case of use of LNG (Liquid Natural Gas), natural gas is stored under low pressure, in cryogenic tanks.
The system is configured to receive and store LNG and supply it to the engine in gaseous form.