Among the important ones are engine design, net energy per pound, net energy per gallon and the sulphur content of alternative fuel properties. Refer to chart below. Measuring a fuel's relative potential energy can easily be done by defining that fuel's Btu content. Ethanol requires few if any engine modifications to extract the maximum power from this fuel. Ethanol is reported to lower tailpipe emissions.
Ethanol is more difficult to ignite than gasoline, especially in cold temperatures.
Thus, for an engine to operate on 100 percent ethanol, the ignition and fuel systems may need modifications. Ethanol may be produced from agriculture crops and is considered a renewable energy resource. However, environmental considerations such as increased use of fertilizers and the associated problems with ground water runoff and limited supplies of ethanol when made from corn or other grain crops, hinders its widespread usage. When ethanol is used as a gasoline additive, antiknock compound and fuel extender, it becomes more economical with very positive results, especially from the emissions standpoint. It contains zero sulfur, thereby reducing tailpipe acids significantly.