The official definition of octane is: the measure of a fuel's ability to resist "knocking" or "pinging" during combustion, caused by the air/fuel mixture detonating prematurely in the engine.
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Most vehicles are designed to run on 87 octane, but others require a higher octane fuel. For example, the 2016 Honda Civic requires fuel with a minimum octane rating of 87 while a high-performance vehicle like the 2016 Mercedes-Benz E350 requires at least 91 octane.
The best way to know what kind of octane your car needs is detailed in the owner’s manual, or a label on the inside of the fuel cap cover. In some vehicles, it is indicated near the fuel gauge on the dash.
The Department of Energy states that: “Higher octane fuels are often required or recommended for engines that use a higher compression ratio and/or use supercharging or turbocharging to force more air into the engine. Increasing pressure in the cylinder allows an engine to extract more mechanical energy from a given air/fuel mixture but requires higher octane fuel to keep the mixture from pre-detonating. In these engines, high octane fuel will improve performance and fuel economy.”
Oil companies liked to use fancy petroleum based synthetic octane enhancers called aromatics. While these aromatics do cause your octane to increase they are often harmful to the environment. One such was MTBE which was eventually banned due to its toxic content.
According to the EPA’s Urban Air Toxics report to Congress, current aromatics like benzene, toluene, ethyl benzene, and xylene have cancer-causing emissions since they emit particulate matter and aromatic hydrocarbons that can damage the immune, respiratory, neurological, reproductive, and developmental systems. And to top it all off, aromatics are expensive to produce and increase your fuel costs.
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In terms of its octane rating, ethanol has a rating of 113. As mentioned above, fuels with a higher octane rating reduce engine knocking and perform better. Also, almost all gasoline in the US contains 10 percent ethanol. When you mix 10 percent 113 octane ethanol with 85 octane gasoline it increases the octane two points to the normal 87 octane most consumers use. So the higher the ethanol content, the higher the octane. The octane rating for E15 (15% ethanol) is 88 octane and E85 (85% ethanol) is 108 octane.
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Moreover, since ethanol is cheaper than those synthetic aromatics, gasoline blended with ethanol reduces the price at the pump. As detailed in a study released earlier this year by the University of Illinois, ethanol is 35 cents to $1 cheaper than benzene, toluene and xylene.
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The Oak Ridge National Laboratory, Argonne National Laboratory and National Renewable Energy Laboratory recently found that vehicle efficiency would increase 5 percent for E25 and 10 percent for E40, making mid-level ethanol blends the optimal fuel for future cars. READ MORE
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