plus the air/fuel mix in a combustion engine is compressed before ignition, usually to 10 or 12 bar, so you will need a motor or co2 to force the ram down to get the compression needed for an efficient burn/expansion. Just injecting and burning will work, but you wont get ic engine sorts of force from it without precompressing the mix.

not saying it wont work, but it is not as easy as it sounds.