Fuel is a passive for Viper which gives her 100 Fuel. Activating Poison Cloud or Toxic Screen requires Viper to consume Fuel at a rate of 1 every 0.15 seconds (6.667/sec). Viper can have both abilities active and still consume fuel at the same rate. If her fuel levels run out, the abilities automatically deactivate and Viper must wait for her fuel levels to replenish, which will happen at a rate of 1 every 0.2 seconds (5/sec).