Commercially available "propane" fuel, or LPG, is not pure. Typically in the USA and Canada, it is primarily propane (at least 90%), with the rest mostly butane and propylene (5% maximum), plus odorants. This is the HD-5 standard, (Heavy Duty-5%maximum allowable propylene content) written for internal combustion engines. LPG, when extracted from natural gas, does not contain propylene. LPG, when refined from crude oil  does contain propylene. Not all products labelled "propane" conform to this standard. In Mexico, for example, the butane content is much higher.