WEBVTT - What does 'octane' mean?

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<v Speaker 1>m z y dot com. Welcome to brain Stuff from

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<v Speaker 1>how Stuff works dot com where smart Happens. Hi. I'm

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<v Speaker 1>Marshall Brand with today's question what does octane mean? If

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<v Speaker 1>you've read how car engines work, you know that almost

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<v Speaker 1>all cars use four stroke gasoline engines. One of the

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<v Speaker 1>strokes is the compression stroke, where the engine compresses a

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<v Speaker 1>cylinder full of air and gas into a much smaller

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<v Speaker 1>volume before igniting it with the spark plug. The amount

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<v Speaker 1>of compression is called the compression ratio of the engine.

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<v Speaker 1>A typical engine might have a compression ratio of eight

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<v Speaker 1>to one. The octane rating of gasoline tells you how

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<v Speaker 1>much the fuel can be compressed before it's spontaneously ignite.

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<v Speaker 1>When gas ignites by compression rather than because of the

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<v Speaker 1>spark plug, it causes knocking in the engine. Knocking can

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<v Speaker 1>damage and engine, so it's not something you want to

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<v Speaker 1>have happening. Lower octane gas like regular eight seven octane

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<v Speaker 1>gasoline can handle the least amount of compression before it

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<v Speaker 1>starts igniting. The name octane comes from the following fact.

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<v Speaker 1>When you take crude oil and crack it in a refinery,

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<v Speaker 1>you end up getting hydrocarbon chains of different links. These

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<v Speaker 1>different chain links can then be separated from each other

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<v Speaker 1>and blended to form different fuels. For example, you may

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<v Speaker 1>have heard of methane, propane, and butane. All three of

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<v Speaker 1>them are hydrocarbons. Methane has just a single carbon atom,

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<v Speaker 1>Propaine has three carbon atoms chained together, and butane has

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<v Speaker 1>four carbon atoms chained together. Pentane has five, hexane has six,

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<v Speaker 1>heptane has seven, and logically octane has eight carbon atoms

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<v Speaker 1>chained together. It turns out that heptane handles compression very poorly.

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<v Speaker 1>Compress it just a little and it ignites spontaneously. Octane,

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<v Speaker 1>on the other hand, handles compression very well. You can

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<v Speaker 1>compress it a lot and nothing happens. Eighty seven octane

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<v Speaker 1>gasoline is gasoline that contains eight seven percent octane and

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<v Speaker 1>thirteen percent heptane, or some other combination of fuels that

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<v Speaker 1>has the same performance as eight thirteen combination of octane

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<v Speaker 1>and heptane. It's spontaneously ignites at a given compression level,

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<v Speaker 1>and it can only be used in engines that do

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<v Speaker 1>not exceed that compression ratio. Do you have any ideas

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<v Speaker 1>or suggestions for this podcast? If so, please send me

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<v Speaker 1>an email at podcast at how stuff works dot com.

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