There are some forklift trucks whose engines run from propane-and sometimes these are run indoors-and-at the Food Lion out here-one night I stopped off there to get some breakfast goodies after my mid shift-and someone was polishing the floor with a buffer whose motor ran from propane-sounded like someone driving a motorcycle thru the store.In the case of both the forklift and buffer-the propane fuel has special additives in it to reduce the toxicity of the gas-but ventilation is still required. I used the forklift with the propane tank for power.also I have seen some "pack" style emerg gen sets for home use whose engine runs from a propane tank that you have to send out to be filled.In this case the genset sits outside the house.