# A bucket of gasoline...and a cigarette

**URL:** <https://boards.straightdope.com/t/a-bucket-of-gasoline-and-a-cigarette/223828>\
**Category:** Factual Questions\
**Created:** [January 10, 2004, 3:03pm UTC](https://boards.straightdope.com/t/a-bucket-of-gasoline-and-a-cigarette/223828 "2004-01-10T15:03:55Z")\
**Posts on this page:** 2\
**Page:** 2

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**Author:** ![antechinus](https://avatars.discourse-cdn.com/v4/letter/a/fbc32d/32.png) [@antechinus](https://boards.straightdope.com/u/antechinus)\
**Post date:** [January 13, 2004, 5:09am UTC](https://boards.straightdope.com/t/a-bucket-of-gasoline-and-a-cigarette/223828/21 "2004-01-13T05:09:51Z")

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The concentration of the vapour above the fuel has to be within the **EXPLOSIVE LIMITS** of the fuel for ignition to occur. For petrol this is about 1- 8% in air, with **FLASH POINT** -7[sup]o[/sup]C.

Flash point might seem low, but this is the temperature above which an ignition source near the surface (where the explosive limits are just right) of the liquid will cause ignition.

It is the vapour that burns. The heat generated from this forms more vapour, and off it goes…

So it is very likely that point in the vapour concentration gradient above the fuel will have the right explosive limits for ignition. Thus the fuel will very likely ignite.

Autoignition temperature (as per ghostmans link) is the temp that the stuff will ignite _without_ an external source of ignition. I think this is a misleading way of approaching the problem, since there is a point where the flashpoint becomes the determination of whether the fuel will ignite before direct contact with the fuel is an issue.

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**Author:** ![Zaphod7](https://avatars.discourse-cdn.com/v4/letter/z/df788c/32.png) [@Zaphod7](https://boards.straightdope.com/u/Zaphod7)\
**Post date:** [January 13, 2004, 5:19am UTC](https://boards.straightdope.com/t/a-bucket-of-gasoline-and-a-cigarette/223828/22 "2004-01-13T05:19:07Z")

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we use to play with petrol a lot when i was younger, being bored a lot and sick of the beach.

we found that a lot of the time it took a fair bit to get the betrol to light, although often the vapours lit quickly the petrol its self didnt, we mainly found that if we used old milk cartons with the top cut off and put slightly in the ground to keep it up right would be a bitch to light.

best results were a beer bottle and rag, those suckers lit quickly and created a nice little explosion when they landed. awww wasnt being young and foolish great…

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