It is not just moving AIR. HOT AIR helps - and so does CARBON DIOXIDE. So you want an engine that is running hot and efficient - Carbon MONOXIDE is NOT what you want in the tank, as it can still burn. When I need to weld a fuel tank or oil pan or similar device I fill the container with CARBON DIOXIDE from a fire extinguisher. Dry Ice can also be used. Carbon Dioxide is heavier than air so will displace any oxygen in the container - it will not support combustion, and it will not absorb great amounts of heat, making sldering/brazing/welding difficult. As long as the repair site is below the level of the CO2, the repair is safe. NEVER weld or braze on the TOP of the container because it is POSSIBLE there is trapped air/fuel in a pocket at the top. With CO2 leaking out of the repair site there is no chance of fuel-gas fronm the torch getting into the tank unburned and "going off" as some bozo in a former thread on this subject suggested as the cause of a possible explosion.