Driving higher return with fuel-grade butanol at the refinery Driving higher return with fuel-grade butanol at the refinery Since the advent of the combustion engine, gasoline and automobile manufacturers have utilized additives as a means of improving overall fuel characteristics. Since the advent of the combustion engine, gasoline and automobile manufacturers have utilized additives as a means of improving overall fuel characteristics. Initial efforts focused on improving the fuel’s octane rating, lowering the propensity for engine knocking. Tetraethyl lead (TEL) proved to be an inexpensive additive capable of meeting this objective, and it remained prevalent in the U.S. until the 1970s, when concerns over lead toxicity resulted in the gradual shift to unleaded fuels.