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Old and Slow
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Post by Old and Slow » Fri Apr 12, 2019 5:33 pm

I'd be interested in seeing an answer - I believe there is a "standard" small car driving cycle that is used for the speed/mileage part of the calculation, but then.....


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Post by Alan SS1 » Fri Apr 12, 2019 8:01 pm

Old and Slow wrote:
Fri Apr 12, 2019 5:33 pm
I'd be interested in seeing an answer - I believe there is a "standard" small car driving cycle that is used for the speed/mileage part of the calculation, but then.....
fag packet calculations
45 mpg is about one tenth litre per mile or 0.06 litre / km
assume 0.8 as petrol density, gives 0.05 kg / km or 50g fuel per km

if we assume fuel is about C8H18 with 12.5 O2 gives 9 H2O and 8 CO2
mass balance shows 3 kg of CO2 per kg of fuel

so my reckoning is 45mpg is 150g/km ?

as they say up here 'me heed is burstan' trying to rake up old Chemistry calcs :shock:


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Post by philhoward » Fri Apr 12, 2019 8:53 pm

That’s probably not far off the mark, Alan. My previous company car was 111g/km and would apparently do almost 70mpg. Add in a fudge factor for aerodynamics and a weight correction factor.

New car is either 74 or 78g/km depending on which set of test results you use - but I’ve had 83mpg from it myself (when we had that warm spell). They reckon it will do a bit more than that, so your correlation still holds about right.


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Post by Old and Slow » Sun Apr 14, 2019 5:52 pm

Well done...
So perhaps we can say that 3.3 times the mpg is approximately the pollution in g/km.
Although as Jim said, the pollution is not just carbon.
Thanks.


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Post by philhoward » Sun Apr 14, 2019 6:12 pm

Old and Slow wrote:
Sun Apr 14, 2019 5:52 pm
Well done...
So perhaps we can say that 3.3 times the mpg is approximately the pollution in g/km.
Although as Jim said, the pollution is not just carbon.
Thanks.
Try a different calculator.. :wink: 22mpg would be about 300g/km, not 66g. I think you’ve got the numerator and denominator in the wrong place.


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Post by Old and Slow » Sun Apr 14, 2019 10:34 pm

Mental arithmetic is driving me mental!!!
According to Alan, 45mpg gives 300g/km, so a smaller mpg should result in a bigger rate of pollution - GOT IT! 135000 divided by mpg gives g/km.
But this means 22 mpg should be nearer 600g/km?
Got to rest my brain now.


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Post by philhoward » Sun Apr 14, 2019 10:52 pm

“Alan SS1” wrote:so my reckoning is 45mpg is 150g/km ?


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