CNG (compressed natural gas) is natural gas which is compressed and then filled into the gas tanks of a vehicle running on CNG (an NGV – natural gas vehicle) as fuel for the vehicle’s engine.
The energy density of natural gas at normal physical density is low when compared to liquid fuels. In order to increase the amount of gas energy that a vehicle’s tanks can hold, gas is compressed to the pressure of 200 bar. This is the reason for the names of CNG (compressed natural gas), CNG car, CNG engine, CNG filling station, etc.
The gas tanks of a vehicle are filled to the nominal pressure of 200 bar at the temperature of +15 °C (nominal pressure – standard-defined maximum filling pressure). Nominal pressure is a value that depends on temperature: if the temperature is below +15 °C, the nominal pressure is also below 200 bar and vice versa. The gas formula provides an explanation to this.
The general gas formula describes the state of ideal gas via the parameters of pressure p, volume V, temperature Tand substance quantity n (or particle count N or mass m).
Gas formula: p • V = n • Rm • T , where Rm is the gas constant.
While the volume (capacity of gas bottles), the substance quantity and the gas constant do not change, the temperature and the pressure are in linear correlation.
If filling was not dependant on temperature, but instead would follow the set pressure, then it would be dangerous at low temperatures and inefficient at high temperatures. If filling to 200 bar at a low temperature, we would fill the tanks with more gas than prescribed and as the temperature would increase later, the pressure in the vehicle’s gas tanks could increase over the allowed limit. And filling to 200 bar at a high temperature, we would fill the tanks with less gas than prescribed and the vehicle’s mileage would decrease.
CNG (compressed natural gas) must not be confused with other fuel gases like liquefied gas.