fosters:

Nitrous Oxide doesn't burn....it is an Oxygen source that allows the fuel (whether that be gasoline, nirtomethanol, etc) to burn more quickly.

The atmosphere is only 21% Oxygen, and essentially 78% inert material...that material being Nitrogen. Nitrous Oxide raises that ratio of Oxygen to Nitrogen.

People think Oxygen burns, too, but it doesn't. All combustion is the result of the chemical combination (reaction) of oxygen with a fuel.

And Nitrous must be manufactured.....that takes energy.