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<blockquote data-quote="Caustic Burno" data-source="post: 798300" data-attributes="member: 694"><p>This is not a simple answer as technically ethanol has no octane. This is from one of my blending sources maybe it will explain it better. It increases the antiknock properties of gasoline. Gasoline is rated in octane. Normal octane has a -10 octane rating as a Iso-Octane (trimethylpentane) has 100</p><p></p><p>The designation "octane" on gasolines is not what you might think. As you</p><p>know, octane is a hydrocarbon with the formula C8H18. You also know that</p><p>this is not the same thing as ethanol. You may suspect that there is no</p><p>simple chemical reaction that converts ethanol into octane, and you are</p><p>quite correct.</p><p></p><p>The octane rating of gasoline relates to how it behaves as a motor fuel.</p><p>You may have noticed, at some point in your driving experience, that a</p><p>gasoline engine will occasionally "knock" or "ping" under a load, such as</p><p>when you try to accelerate up a hill in a high gear. This occurs because</p><p>fuel is igniting too soon in the cylinder, under compression but before the</p><p>spark is fired. This reduces the vehicle's efficiency, and also damages</p><p>the engine. (In a gasoline engine, that is. In a diesel engine, the fuel</p><p>has to ignite under compression because there is no spark.)</p><p></p><p>Different hydrocarbons have different tendencies to ignite in this way.</p><p>One that is prone to ignition under compression is heptane, C7H16. One</p><p>that resists early ignition, and thus makes a very nice gasoline, is</p><p>2,2,4-trimethylpentane, C8H18. This is a highly-branched isomer of octane,</p><p>and is commonly called "isooctane." A gasoline engine burning heptane will</p><p>begin to knock under a light load; burning isooctane, it will be</p><p>knock-free under much higher loads. If it burns a mixture of heptane and</p><p>isooctane, the load under which it begins to knock will be somewhere in</p><p>between.</p><p></p><p>If you are buying gasoline to put in your tank, you want to have some idea</p><p>of how "good" it is for your engine, and how well your engine will perform</p><p>with it. This is why the octane rating was developed. This is a method</p><p>for comparing a new gasoline formulation to a heptane/isooctane mixture.</p><p>The rating is made this way: run a standard engine on the new fuel, and</p><p>apply an increasing load to it. Record the load under which the engine</p><p>begins to knock. Then find the heptane-isooctane mixture that causes the</p><p>engine to knock uner the same load. The octane rating of the new gasoline</p><p>is the percentage of isooctane in the isooctane/heptane mixture that causes</p><p>engine knock at the same load as the new fuel.</p><p></p><p>But wait, there's more! You may notice that on the pump, where the</p><p>gasoline's "octane" is displayed, it usually says something like "(R +</p><p>M)/2." This is because different engines may give different octane ratings</p><p>to the same fuel. The fuel industry couldn't agree on a single standard</p><p>engine, so there are two: "R" and "M." The octane rating reported for the</p><p>gasoline is the average of the rating from these two engines.</p><p></p><p>Given all that background, now I can answer your question. There are many</p><p>compounds that can be added to gasoline to increase its octane rating, and</p><p>ethanol is one of them. THAT is how ethanol increases the octane. It's</p><p>just a matter of knowing what is meant by "octane."</p><p></p><p></p><p> Richard E. Barrans Jr., Ph. D.</p><p> Chemistry Division</p></blockquote><p></p>
[QUOTE="Caustic Burno, post: 798300, member: 694"] This is not a simple answer as technically ethanol has no octane. This is from one of my blending sources maybe it will explain it better. It increases the antiknock properties of gasoline. Gasoline is rated in octane. Normal octane has a -10 octane rating as a Iso-Octane (trimethylpentane) has 100 The designation "octane" on gasolines is not what you might think. As you know, octane is a hydrocarbon with the formula C8H18. You also know that this is not the same thing as ethanol. You may suspect that there is no simple chemical reaction that converts ethanol into octane, and you are quite correct. The octane rating of gasoline relates to how it behaves as a motor fuel. You may have noticed, at some point in your driving experience, that a gasoline engine will occasionally "knock" or "ping" under a load, such as when you try to accelerate up a hill in a high gear. This occurs because fuel is igniting too soon in the cylinder, under compression but before the spark is fired. This reduces the vehicle's efficiency, and also damages the engine. (In a gasoline engine, that is. In a diesel engine, the fuel has to ignite under compression because there is no spark.) Different hydrocarbons have different tendencies to ignite in this way. One that is prone to ignition under compression is heptane, C7H16. One that resists early ignition, and thus makes a very nice gasoline, is 2,2,4-trimethylpentane, C8H18. This is a highly-branched isomer of octane, and is commonly called "isooctane." A gasoline engine burning heptane will begin to knock under a light load; burning isooctane, it will be knock-free under much higher loads. If it burns a mixture of heptane and isooctane, the load under which it begins to knock will be somewhere in between. If you are buying gasoline to put in your tank, you want to have some idea of how "good" it is for your engine, and how well your engine will perform with it. This is why the octane rating was developed. This is a method for comparing a new gasoline formulation to a heptane/isooctane mixture. The rating is made this way: run a standard engine on the new fuel, and apply an increasing load to it. Record the load under which the engine begins to knock. Then find the heptane-isooctane mixture that causes the engine to knock uner the same load. The octane rating of the new gasoline is the percentage of isooctane in the isooctane/heptane mixture that causes engine knock at the same load as the new fuel. But wait, there's more! You may notice that on the pump, where the gasoline's "octane" is displayed, it usually says something like "(R + M)/2." This is because different engines may give different octane ratings to the same fuel. The fuel industry couldn't agree on a single standard engine, so there are two: "R" and "M." The octane rating reported for the gasoline is the average of the rating from these two engines. Given all that background, now I can answer your question. There are many compounds that can be added to gasoline to increase its octane rating, and ethanol is one of them. THAT is how ethanol increases the octane. It's just a matter of knowing what is meant by "octane." Richard E. Barrans Jr., Ph. D. Chemistry Division [/QUOTE]
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