CNG in general
CNG is a natural product. It evolved from organics matters over 600 Million years ago. Today it is drawn from domestically drilled gas wells or in conjunction with crude oil production. CNG is used in its primary gasiform state. Since it does not have to be transformed into any secondary energy such as fuel oil or electricity, the user can utilize it right away and in addition no environmental pollution through any complex transformation occurs. Stocking, ordering or delivery dates are not necessary in connection with CNG.
Natural gas consists of about 90% methane. In its natural form natural gas does not smell. Therefore, the gas is odorized prior to distribution in order to detect possible leakage. Gas can therefore be smelled already at a concentration of 0.3%. As CNG requires a concentration of about 5% to 15% to combust, 0.3% is far below the dangerous combustion level.
Physical attributes of CNG
If the cylinder is depleted and refilled with CNG, the cylinder will get warm. This is nothing to be concerned about. If a gas is put under pressure, the density of the molecules will increase, and therefore the temperature will rise. After a while it will adopt the temperature of its environment again.
Contrariwise the cylinder cools down while driving. When gas expands the density of the molecules decreases and the temperature drops; a nice side-effect in a warm climate like Singapore.
These physical attributes also have an effect on the total storage capacity of the cylinder when refueling. If the temperature increases, the pressure in the cylinder increases as well. The dispensers at the filling stations automatically stop dispensing CNG, once a pressure of 200 bar is reached. If a cylinder can theoretically accommodate 18 kg CNG under standard conditions (200 bar pressure, 15° Celsius), the cylinder will carry a bit less than 18 kg. Practically this means that the cooler the cylinder and the temperature around the cylinder is the more kg of CNG can be pumped into the cylinder.
A CNG car can be left in the sun without concerns, as this heat is never sufficient enough to heat up the cylinder to a critical point. The cylinders are tested and can sustain a pressure of up to 500 bar - a pressure dimension, which is usually never reached in our daily environment.
CNG measured in Kg
CNG is measured in the mass unit kg and not in liters or m³, both measures for volume. One cubic meter of CNG under 10 bar pressure has just a fraction of the energy value than one cubic meter of CNG under 200 bar pressure. However, one kilogram of CNG has always the same calorific value, no matter whether it has a volume of 500 liters, or just a volume of 60 liters - under 200 bar pressure.
1 kg CNG = 1.51 liters of Petrol