All automobiles sold in North America are designed with full warranty protection to operate with ethanol-blended gasoline at a concentration of up to 10% ethanol, without engine modification. (This contrasts with propane and compressed natural gas use where extensive modification is required.)
If the ethanol concentration increases well above 10%, for example to 20% ethanol, or even neat (near 100%) ethanol as is used in Brazil, a higher engine compression ratio is desirable to take advantage of the ethanols higher octane rating (compared to normal gasoline). Higher compression ratios can, in turn, mean greater pressure on engine bearings, engines must be designed for this. (Racing cars commonly use ethanol, methanol or other alcohols, as the principal source of high octane fuel.) However, such concerns are not a factor with the 5% or 10% ethanol blends sold in North America.
Ethanol burns more cleanly and at a slightly cooler temperature than normal gasoline. This means longer spark plug life and fewer carbon deposits in most engines.
Research results and experience with 10% ethanol-blended gasoline have shown that actual fuel efficiency is essentially identical to that of regular (ethanol-free) gasoline. Yet ethanol contains less caloric energy than gasoline, which should, theoretically, result in poorer fuel efficiency. The discrepancy results from the greater efficiency with which ethanol-blended gasoline is burned during engine operation.
Octane is a measure of how well a fuel resists premature combustion, or knocking. Gasoline with too low an octane rating converts fuel to heat rather than power, making for less efficient fuel usage and reduced engine life.
Two octane numbers are often reported, the Research Octane Number (RON), and Motor Octane Number (MON). Isooctane (a flammable, colourless, liquid hydrocarbon used for comparing ignition characteristics) has a value of 100 for both RON and MON. Octane values at the retail gasoline pump are calculated as the average of RON and MON. When gasoline is blended with ethanol at the 5-10% level, its octane number increases more than would be expected from ethanol alone. The blending octane rating of ethanol, at about 118, is as high as for any organic (non-metallic) octane additive.
Volatility is a measure of the tendency to evaporate. To burn properly in an internal combustion engine, the fuel must be well vaporized. The higher a fuels volatility, the greater its level of evaporative emissions. If volatility is too low, there can be starting problems at low outside temperatures; if too high, there can be premature vaporization, particularly in hot weather, causing engine stalling. The Reid Vapour Pressure (RVP) is the measure of the fuels vapour pressure at 37.8 degrees C, and is an important measurement of volatility.
Volatile Organic Compounds (VOCs)
Volatile Organic Compounds (VOCs) are hydrocarbons which evaporate from gasoline and which mix with nitrogen oxides in the presence of sunlight and heat to produce ozone. VOCs can be emitted from the tank (especially during refilling), from tailpipe exhaust, and from a warm engine after being shut-off.
Particulates are emissions of soot and particles of partially combusted fuel components. They are of particular concern in compression ignition (diesel) engines. Ethanol can dramatically reduce particulate emissions.
Water Solubility is infinite in alcohols (ethanol, methanol), but negligible in gasoline. Ethanol eliminates problems associated with moisture in gasoline (hence, the reason ethanol is used as gas-line antifreeze). However, when water content reaches the 0.4 - 0.8 weight percentage, water and ethanol separate from the gasoline. They float above the gasoline in the tank, which can cause engine stalling. This is also the reason that pipelines and refilling station tanks must have excess water cleaned out before using ethanol.
Solvent - Ethanol, like other alcohols, is an excellent solvent for many compounds. Therefore, deposits which have built up over time with conventional gasoline, may break free after using ethanol-blended gasoline. Filters are installed at gas pumps and blenders to remove such impurities. Your car fuel filter may need to be changed after the first few tankfuls of ethanol. After this, the ethanol will maintain a clean system.