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# stale fuel what happens, and fuel density

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Discussion and questions related to the course Understanding AFR

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hi guys

a couple of questions first one is what chemically happens to fuel when it goes off does it lose some of its octane content what actually happens to it?

2. as fuel temp increases its density decreases so lets say at a pulse width of 5ms and a fuel temp of 10 degrees and a afr of 14.7, then we had the same pulse width of 5ms but a fuel temp of 50 degrees now how much would the afr change due to the decrease in the fuels density?

3. and also what happens to the fuel when the density decreases does the fuel mass become smaller or something? what happens

cheers guys

G'day Tony.

1) I don't have an answer to that one, except that 'It smells weird, and is only good for my lawnmower'

2) My quick google (https://www.quora.com/How-much-more-dense-is-gasoline-in-50-degree-F-weather-than-it-is-in-80-degree-weather) leads me to believe that gasoline will be approximately 5% less dense at 50 degC than at 10degC, meaning for the same volume, you actually have 5% less mass of fuel. So for the same pressure across the injector, and intake air temperature you'd want to increase your injector duty cycle by 5% to get the correct mass of fuel to maintain your 14.7:1 air to fuel mass ratio. A higher fuel temperature will effect the wall wetting, and charge mixing too, so its not quite this simple, but that calculation is good enough to get things in the ballpark.

3) Temperature is how we measure the thermal energy of something, which is how much its atoms are vibrating. Cold things have very still atoms that can pack together tighter, meaning the same number of atoms (mass) can fit in a smaller space (volume). When things get hot, those atoms start vibrating and pushing on one another, and one of two things happens. If the space they occupy is restricted (fixed volume), the pressure increases. If the space they occupy is not restricted, the pressure will remain the same, but the volume will increase.

Have a google of the ideal gas law, PV - nRT. One of my physics lecturers put it best I reckon, he said to think of a school assembly with everyone sitting in orderly rows. You can pack a bunch of students into the assembly hall that way. Now, close the doors and turn on some sick beats to get everyone dancing (apply heat). There isn't enough space for everyone anymore and you'll be elbowing your neighbour while doing the white man shuffle. If you open the doors, people will spill out and take up more space then they were before, but everyone will have room to move.

Not that I ever danced at school, because, you know... Nerd ;-).

awesome thanks zac

Thats a great analogy Zac, i will use that. Thanks. :)