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  Chicagoland MG Club:Tech Tips

Reformulated Gasoline and the Older Automobile

Reformulated gasoline is now being sold in many areas of the country. Questions regarding this new fuel have been researched and answered by our friend and customer Dr. Peter Gillespie, who holds advanced degrees in organic chemistry and metallurgy-perfect qualifications for dealing with this subject.

What is reformulated gasoline?

In very simplistic terms, reformulated gasoline is gasoline in which pollution-producing high-octane aromatics are replaced with oxygenates. It may be considered a "second generation" unleaded gasoline.

What are oxygenates?

Oxygenates are just preused hydrocarbons; they contain oxygen, which cannot provide energy, but their high blending octane provides reasonable anti-knock value, making them good substitutes for pollution-causing aromatics such as benzene, toluene, and xylene. They may also reduce the smog forming tendencies of the exhaust gasses. Common oxygenates are: methanol, ethanol, methyl tertiary butyl ether (MTBE), tertiary amyl methyl ether (TAME), and ethyl tertiary butyl ether (ETBE).

Oxygenates are added to gasoline to reduce the reactivity of emissions, but they are effective only if the hydrocarbon fractions (HC) of the base fuel are carefully modified to utilize the octane enhancing and volatility properties of the oxygenates. If the hydrocarbon fractions are not correctly modified, oxygenates can increase the undesirable smog-forming and toxic emissions. Oxygenates do not necessarily reduce all individual exhaust toxins, nor are they intended to. For example, it has been reported that reformulated gasoline can cause slight increase in aldehyde and nitrogen oxides emissions.

Oxygenates have significantly different physical properties than hydrocarbons, and the levels that can be added to gasoline are controlled by the EPA, with waivers being granted for some combinations. The change to reformulated gasoline requires oxygenates, but also that the hydrocarbon composition must be significantly more modified than the existing oxygenated gasoline to reduce unsaturates, volatility, benzene, and the reactivity of emissions.

Oxygenates that are added to gasoline function in two ways. They can replace high octane aromatics in the fuel which are responsible for disproportionate amounts of carbon monoxide and unburned hydrocarbon exhaust emissions. Oxygenates also cause engines without sophisticated engine management systems to move to the lean side of stoichiometry (the theoretically correct air-fuel ratio), thus reducing these emissions. Two percent oxygen can reduce CO by 16% and HC by 10%.

Oxygen in the fuel cannot contribute energy, consequently oxygenated fuel has less energy content. For the same efficiency and power output, more fuel has to be burned, and the slight improvements in efficiency that oxygenates provide on some engines usually do not completely compensate for the added oxygen.

Will oxygenated gasoline damage my vehicle?

Unless your vehicle was designed to operate on unleaded gasoline, damage such as valve seat recession may occur with reformulated gasoline at a faster rate than with earlier formulations of unleaded fuel. The mechanism of valve seat recession is the transfer of minute iron particles from exhaust valve seats to the exhaust valve faces, in which they become imbedded. When these surfaces come in contact, the abrasive iron particles grind away the seat, and even more iron particles are transferred. This erosion of the valve seat causes the head of the valve to recede deeper into the cylinder head. This not only upsets the gas flow, but reduces the valve clearance, resulting in difficult starting, poor running, increased fuel consumption, and potential engine damage. If your exhaust valve clearances keep tightening up, suspect valve seat recession. SAE test reports have shown that engines run at low speed (under 2500 rpm), light load, and cool engine temperature (160-1800F) show very little evidence of this problem when run on unleaded fuel. However, an increase in any of these three conditions can lead to rapid acceleration of valve seat erosion. Hardened valve seat inserts and stellite-faced exhaust valves will prevent valve seat recession under even arduous conditions. The stellite valves may advantageously be used by themselves if the valve seats have not recessed.

Damage should not occur in vehicles designed for unleaded gasoline if the reformulated gasoline used is correctly formulated, and the appropriate octane grade is used. In the first year of mandated oxygenates, it appears that some refiners did not carefully formulate their oxygenated gasoline, and drivability and emissions problems occurred. Most brands are now carefully formulated to avoid these problems. Some older activated carbon canisters may not function efficiently with oxygenated gasoline, but this is a function of the type of carbon used.

Because oxygenated fuel has less energy content than "traditional" gasoline, cars without sophisticated fuel management systems will run lean. The minor improvements in combustion efficiency which oxygenates may provide cannot compensate for the 2+% of oxygen in the fuel that does not provide energy. Richer mixture settings and slightly retarded ignition timing will compensate for this, but fuel consumption will increase slightly. If no compensation is made, the engine will run hotter, and may be prone to "pinging" (knock), which is physically damaging to internal engine components.

Most stories of corrosion problems date back to the earlier days of methanol-containing "gasahol", and are derived from corrosion of light metals (notably aluminum and magnesium) caused by anhydrous methanol. However the addition of either 0.5% water to pure methanol, or corrosion inhibitors to methanol/gasoline blends will prevent this. Oxygenated fuels may either swell or shrink some elastomers (rubber and plastics) on older cars, depending on the exact formulation of the fuels. If you observe or suspect any such corrosion or damage, talk to your local gasoline supplier. Most currently available reformulated gasolines do not seem to cause corrosion problems.

Be prepared to try several different brands of reformulated gasoline to identify the most suitable brand for your vehicle, and be prepared to change again with the seasons, as seasonal changes are made to gasoline. Refiners can choose the oxygenate they use to meet the regulations, and may choose to set some fuel properties differently from their competitors.

Can I use unleaded reformulated fuel in my older car?

Yes, providing the octane rating is appropriate. Seasonal changes in formulation and vapor pressure will probably have a greater effect than in the past, requiring appropriate periodic tuning adjustments. Hardened exhaust valve seat inserts and stellite exhaust valves are advised if valve seat recession becomes a problem.

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