Just use compressed air, a scuba tank may get you to the store and back.
I saw somewhere that a guy built a car engine out of stainless steel and used hydrogen and oxygen as fuel, the waste product was water, (no outside air was used) thus the need for stainless steel.
The bad part was that the hydrogen kept escaping the tank, this due to it leaking through the sides of the gas cylinder, so it had to be a rear engine design with the hydrogen tank up front, this also turned out to be a bad design as the car crash experiments all wound up exploding.
