How did the Amar Jawan Jyoti keep burning for 50 years at India Gate?

The Amar Jawan Jyoti, which was merged with the flame at the National War Memorial at India Gate, was burning for around 50 years without being extinguished. It was kept burning with the help of LPG cylinders till 2006, following which it was changed to piped natural gas (PNG). One LPG cylinder could keep one burner alive for 1.5 days.