# Fuel consumption

Question: I'm trying to determine the fuel cost per mile for a Honda FCX fuel cell vehicle. According to their statistics (see http//:www:llhondacorporate.com/kxjspecs.html), the vehicle will get 220 miles on 3.75 kg of hydrogen at 5,000 psi. Commercial sources quote hydrogen at about \$2.50 per "hundred cubic feet." Can you tell me how many cubic feet of hydrogen would be in 1 kg?

There is one important factor to remember. The density of hydrogen will vary with the pressure of the gas and also its pressure. Lets assume that the temperature of your 5000 psi. hydrogen gas cylinder is 20oC.

Now the density of hydrogen at 15 psi. (roughly normal atmospheric pressure) and 20 oC is approximately 0.083 kg/cubic metre or 0.0026 kg/cubic foot. You can therefore work out the density at the pressure you mention (5000 psi).

This comes to about 28 kg/cubic metre or 0.81 kg/cubic foot.

Therefore 1 kg of hydrogen at a pressure of 5000 psi. will have a volume of about 1.24 cubic feet.

However the cost depends on whether the price that you quote for hydrogen is when it is at a pressure of 5000 psi. or at atmospheric pressure (15 psi.)

If we assume that the pressure is 5000 psi.
So for your \$2.50 you would get 100 cubic feet or 81 kg of hydrogen. This would mean that the vehicle could travel [81/3.75]x220 = 4750 miles. This seems unlikely. Amazing value for money!

If we assume that the pressure is 15 psi.
So for your \$2.50 you would get 100 cubic feet or 100x0.0026 = 0.26 kg of hydrogen. This would mean that the vehicle could travel [0.26/3.75]x220 = 15.3 miles. This does not look so good.