Chicagoland MG Club: Tech Tips

Chicagoland MG Club

Chicagoland MG Club

General Tips

MGA Tips

MGB Tips

Midget Tips

MGT Tips

MMM Tips

Tech Session
Photo Archives

  Chicagoland MG Club:Tech Tips

Gasoline: Back to the Basics

We all use gasoline, but rarely give it much thought, coming to accept the lack of lead, seasonal volatility changes, local differences, and the presence of "gasohol". But most of us still have some concerns about how these affect our cars' performance and what adverse affects they may have on the components of our cars' fuel systems. Now that we have become used to unleaded gasoline, gasoline has again been reformulated to meet the requirements of the Clean Air Act Amendments of 1990.

In order to understand the whys and wherefores of gasoline reformulation, it is necessary to know something about gasoline. Modern gasolines are complex mixtures of components to satisfy the requirements of all different types of gasoline engines, operating requirements, and climatic conditions with only a few different grades of gasoline - a miracle of chemical engineering we often overlook.

Octane is the measure of a fuel's ability to resist detonation (engine knock, or ping). For automotive gasoline, this is expressed as the familiar anti-knock index (or "pump octane") which we see on the gasoline pumps. This is the average of two different measurements of octane which reflect the octane rating of a particular fuel under different conditions. The octane requirement of any particular engine is dependent on factors such as load conditions, altitude/barometric pressure, temperature, humidity, air/fuel mixture, and ignition timing. Using a grade of gasoline of higher octane rating than necessary does not give better fuel economy or greater power; it just costs more. Using gasoline of too low an octane rating can result in engine-damaging detonation.

Volatility is probably the next most important property of gasoline. This is the fuel's tendency to evaporate, and must be carefully controlled for proper driveability. Winter gasoline is made more volatile than summer gasoline to provide good cold starting and warm up performance. Summer gasoline is formulated to minimize vapor lock and hot starting problems. Less volatile gasoline is also provided for high altitude areas, where less temperature is required to produce a given rate of evaporation. From a pollution control standpoint, increased volatility produces greater evaporative losses, which lead to smog formation, while severely decreased volatility results in poor starting and warm up characteristics (requiring overly rich air/fuel mixtures to run) which increase exhaust emissions.

Corrosivity must be controlled to ensure that fuels do not adversely affect fuel system components. Some cars experienced corrosion problems with some types of gasoline which contained methanol. This is no longer a problem, as what little methanol that is currently used in gasoline is combined with other components to eliminate this problem.

Sulpher Content must be limited to prevent excess exhaust emissions, engine deposits, formation of acids within the crankcase, and to reduce emission-caused acid rain.

Phase Separation temperature specifications are used to determine the water tolerance of gasoline containing ethanol or methanol, as these alcohols can absorb considerable amounts of water.

Additives are used in very small percentages to improve the quality of fuels. Detergents remove fuel system deposits; deposit control additives and fluidizer oils control intake valve deposits; anti-icers prevent fuel line freezing; corrosion inhibitors minimize fuel system corrosion; anti-oxidants enable longer storage with minimal gum formation; metal deactivators minimize the effect of metallic components of gasoline, and lead replacement additives minimize valve seat recession.

Oxygenates are being used more and more, and in increasing percentages as a component of gasoline blends. Their use has been mandated in certain areas since as early as January 1988 to reduce carbon monoxide emissions, and have since been mandated for use in the fall of 1992 for many areas throughout the country.

While most components of gasoline are hydrocarbons, fuel oxygenates are alcohols and ethers, composed of hydrogen, carbon, and oxygen. Their advantages in gasoline include high octane ratings, clean burning, reduction in CO emissions, and that they are replacing environmentally dangerous and health hazardous aromatic hydrocarbons such as benzene, toluene, and xylene. The most common fuel oxygenates now in use are methyl tertiary butyl ether (MTBE) and ethanol.

MTBE is currently present in approximately 25% of all gasoline sold in the U.S. It does not increase the volatility of most gasoline, and is not as sensitive to water as the alcohols. At 15% volume in a blend, MTBE raises the octane by up to 3 octane numbers. MTBE is produced from methanol and isobutylene, and eliminates the unfavorable characteristics of high volatility, questionable materials compatibility, and low water tolerance associated with the use of straight methanol.

Ethanol has been used in gasoline since the 1970s, when it was used as a gasoline extender during times of gasoline shortages. These blends were known as gasohol. After that, it has been used as an octane booster, and most recently, as a means of reducing CO emissions. A blend containing 10% ethanol by volume will show an increase in pump octane of 2.5 to 3 octane numbers. Ethanol has the advantage of being easy to produce, and is a renewable energy source produced by fermentation of agricultural products, primarily corn

Other oxygenates include tertiary amyl methyl ether (TAME), which is produced from methanol and isoamylene, ethyl tertiary butyl ether (ETBE), made from ethanol and isobutylene, and blends of methanol plus various cosolvents to offset the undesirable effects of straight methanol.

From the standpoint of operating a car, the effects of reformulated gasoline are surprisingly few, and usually minor, if noticed at all. Unlike early methanol blends, current formulations seem to cause no problems with corrosion, water tolerance, elastomer swelling and deterioration, and volatility-induced problems. Older cars may encounter fuel filter clogging when first using ethanol blends, due to the solvent effect of the ethanol on fuel system deposits. This is easily rectified by changing the fuel filter, and has the very positive effect of cleaning all the accumulated grunge from your fuel system! Fuel mileage on older cars will often show a slight increase, due to the leaner burning and more efficient combustion characteristics of the reformulated fuel blends.

On newer cars, fuel economy may decrease typically by no more than approximately 1.5 to 2%. The benefits to those of us who drive our old cars are double - less pollution and better gas mileage. That combination is hard to beat.

Back to General Tech Tips

©1999, 2001 Chicagoland MG Club, All rights reserved.