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This probably doesn't warrant a new thread but it's all i could think of.
My problem is this: Cars emmissions are presented as g/km of CO2. When i get my car tested, its emmissions are measured in ppm. How do i correlate the two? Is there a formula i can use?

I'll take a stab at this. Using a bunch of figures from google, so take with the appropriate amount of salt. ;)

PPM is measuring the percentage of the exhaust that is _____ emission.
Most engines run at about 12.5:1 air-fuel ratio. Given MPG, we can estimate the air pumped through the engine per mile / km. Apply the percentage from the PPM to that to get a rough g / km number.

50 mpg (gas) = 0.05 gal / mile gas
* 12.5 = .625 gal / mile air
+ .05 gas = .63 gal / mile exhaust
150 ppm = .015%
.015% of .63 = .00945 gal / mi emissions
air weights about 1.3 g / liter = 4.91 g / gal
.00945 * 4.91 = .046 g / mile emissions

Tier 2 / Bin5 specifies NOx of .05 g / mi.

I may be WAAAAAAAY off on all of this. :confused:

Wow, thanks for the (in the end) simple enough calcs. However, car adverts proclaim their cars to put out, say 150g/km of CO2. That seems an awful lot!

Hi VTEC-e:

___Really quick, CO2 output is a measure of fuel consumption and vice versa. It depends on the duty cycle of the test just as FE is different for a city driver than a highway driver in the same vehicle.

___Andrew, impressive back of the envelope math skills :D

___Good Luck

___Wayne

Wikipedia sez one litre of gas produces 2360g of CO2. That's roughly 1,2m³. At 50 mpg (4,7 l/100km) that's about 110g CO2/km or 56cm³/km.

However, I don't think it's as easy as PaleMelanesian thinks to convert between ppm and g/km. For you'd need to know how much air passes through your engine while traveling one km. That depends on your engine displacement h, the average cycles/s c, the gear ratio k and the circumference of your tires r. Something like

Volume of air = (1km / r) * c * h

At least that's what I think.

I looked at this a while back: http://www.simi.ie/showcontent.asp?SubsectionID=1
I found lots of cars with the same extra urban figure but slightly different CO2 figures. Some of this was due to some cars being diesel so thats a moot point. However, within the gasoline cars there were differences. All i can think of is how some cars are tuned for power while retaining the same engine size. But then power should increase fuel consumption...
I suppose the consumption is tied to CO2 but varies slightly, perhaps due to better or worse engineering in the engines.
For example:
Alfa romeo 147---------------1.6 engine-6.3L/100k-194g/km
Alfa romeo 159 sportwagon-1.8 engine-6.3L/100k-187g/km
I know the 147 is a sporty car, while the 159 sportwagon is an estate with a more aero shape but its got a bigger engine, same economy and less emissions.
I'm sure if i plugged all the data into a spreadsheet or something i might see a pattern but i hardly know how to and haven't the time.

ollie

Well, I'm guessing that the simple explanation is probably right.

g/km is presented to 3 significant digits.
L/100k is presented to 2 significant digits.

So, they round to the same L/100k.

-mr. bill