Q. Why does the CNG cylinder heat up?

Remember CNG is compressed natural gas. This is a flammable gas that can be used for cooking or transportation fuel. Let us assume we need to fill up a 25 kg CNG cylinder. The size and volume of this cylinder are obviously fixed. The CNG of 25 kg in nature would occupy a huge amount of space. Therefore in order to store it in a limited-sized container it will need to be pressurized. Remember the ideal gas equation given the same volume as pressure increases so does the temperature. Therefore when a fixed-sized 25 kg cylinder is being filled it needs to be done so at a higher pressure. When this is done then automatically the temperature will also increase. That is why the CNG cylinder heats up.