FCS overall efficiency by using stored cathode oxygen during down-transients

1. A method for utilizing a pressurized volume of oxygen in a cathode plumbing of a fuel cell system including a fuel cell stack, said method comprising:

calculating an air/oxygen balance that is based on an air balance and an oxygen balance in the cathode plumbing, wherein calculating the air/oxygen balance includes assuming that an oxygen mole fraction in an air stream exiting the fuel cell stack is equal to an oxygen mole fraction that is staying in the stack;

determining the number of moles of oxygen available for fuel cell chemical reactions using the calculated air/oxygen balance; and

drawing current from the fuel cell stack using the moles of oxygen available for fuel cell chemical reactions.

2. The method according to claim 1 wherein calculating the air/oxygen balance includes calculating the air/oxygen balance at a beginning of a power down-transient of the fuel cell stack.

3. The method according to claim 1 further comprising determining a maximum current draw based on the determined number of moles of oxygen that are available for fuel cell chemical reactions, said maximum current draw being determined based on half of the determined number of moles of oxygen that are available.

4. The method according to claim 1 wherein calculating the air/oxygen balance includes calculating the air/oxygen balance based on oxygen flowing into the fuel cell stack and oxygen flowing out of the fuel cell stack.

5. The method according to claim 1 further comprising monitoring a fuel cell stack voltage while drawing current from the fuel cell stack using the moles of oxygen available.

6. The method according to claim 5 further comprising ending the draw of current from the fuel cell stack using the moles of oxygen available if the fuel cell stack voltage drops below a predetermined threshold.

7. The method according to claim 5 further comprising recalculating the air/oxygen balance and drawing a new current from the fuel cell stack based on a determined number of moles of oxygen available for fuel cell chemical reactions if fuel cell stack voltage does not drop below a predetermined threshold.

8. A method for utilizing a pressurized volume of oxygen in a cathode plumbing of a fuel cell system including a fuel cell stack, said method comprising:

calculating an air/oxygen balance that is based on an air balance and an oxygen balance in the cathode plumbing at the beginning of a power down-transient of the fuel cell stack, wherein calculating the air/oxygen balance includes assuming that an oxygen mole fraction in an air stream exiting the fuel cell stack is equal to an oxygen mole fraction that is staving in the stack;

determining a first number of moles of oxygen available for fuel cell chemical reactions using the calculated air/oxygen balance;

drawing a first current from the fuel cell stack using the first number of moles of oxygen available for fuel cell chemical reactions;

recalculating the air/oxygen balance that is based on an air balance and an oxygen balance in the cathode plumbing after the first draw of current is complete;

determining a next number of moles of oxygen available for fuel cell chemical reactions using the calculated air/oxygen balance; and

drawing a next current from the fuel cell stack using the next number of moles of oxygen available for fuel cell chemical reactions.

9. The method according to claim 8 further comprising stopping current from being drawn from the fuel cell stack when the determined number of moles of oxygen available for fuel cell chemical reactions is approximately zero.

10. The method according to claim 8 further comprising determining a maximum current draw based on the determined number of moles of oxygen that are available for fuel cell chemical reactions.

11. The method according to claim 10 wherein the maximum current draw includes a safety margin, said safety margin being determined based on half of the determined number of moles of oxygen available for the fuel cell chemical reactions.

12. The method according to claim 8 further comprising monitoring a fuel cell stack voltage while drawing current from the fuel cell stack using the moles of oxygen available.

13. The method according to claim 12 further comprising ending the draw of current from the fuel cell stack using the moles of oxygen available if fuel cell stack voltage drops below a predetermined threshold.

14. A system for utilizing a pressurized volume of oxygen in a cathode plumbing of a fuel cell system including a fuel cell stack, said system comprising:

a controller that is programmed to perform the following:

means for calculating an air/oxygen balance that is based on an air balance and an oxygen balance in the cathode plumbing, wherein calculating the air/oxygen balance includes assuming that an oxygen mole fraction in an air stream exiting the fuel cell stack is equal to an oxygen mole fraction that is staying in the stack;

means for determining the number of moles of oxygen available for fuel cell chemical reactions using the calculated air/oxygen balance; and

means for drawing current from the fuel cell stack using the moles of oxygen available for fuel cell chemical reactions to charge a battery.

15. The system according to claim 14 wherein the means for calculating the air/oxygen balance calculates the air/oxygen balance at a beginning of a power down-transient of the fuel cell stack.

16. The system according to claim 14 further comprising means for determining a maximum current draw based on the determined number of moles of oxygen that are available for fuel cell chemical reactions and a safety margin, said safety margin being determined based on half of the determined number of moles of oxygen available for the fuel cell chemical reactions.

17. The system according to claim 14 wherein the means for calculating the air/oxygen balance calculates the air/oxygen balance based on oxygen flowing into the fuel cell stack and oxygen flowing out of the fuel cell stack.

18. The system according to claim 14 further comprising means for monitoring a fuel cell stack voltage while drawing current from the fuel cell stack using the moles of oxygen available.

19. The system according to claim 18 further comprising means for recalculating the air/oxygen balance and drawing a new current from the fuel cell stack based on a determined number of moles of oxygen available for fuel cell chemical reactions if fuel cell stack voltage does not drop below a predetermined threshold.