Different chemistry. Petrol must be burnt quite rich in the cylinder - remember in first year chemistry in high school learning how to use a Bunsen burner? Open

Different chemistry. Petrol must be burnt quite rich in the cylinder - remember in first year chemistry in high school learning how to use a Bunsen burner? Opening up the air valve to get a bright blue flame, closing it down to get a cooler yellow flame? Anyway if you burn hydrocarbons in a fuel-rich environment you get soot and carbon monoxide - there's too much carbon for the amount of oxygen available. Propane wants to burn leaner, and burns with a bigger, slower, and hotter bang. You can run a far leaner mixture without pre-ignition (pinking or knocking) because it's got a higher octane number, something like 115RON. You don't get anything like as much soot, you don't get any carbon monoxide, and because of the different proportions of carbon and hydrogen you get more water vapour and less carbon dioxide per mile travelled.