On Tue, 21 Jan 2003, David Forbes wrote:
> >>Date: Sat, 18 Jan 2003 23:40:58 -0800 (PST)
> >>From: Tom Jennings
> >>Subject: chemistry
> >>
> >>Know any chemists? I need to figure out how to calculate (or
> >>decently approximate) pounds of carbon dioxide produced from
> >>combustion of:
> >>
> >>- gasoline
> >>- LPG/propane.
> >>
> >>... per unit volume (mole, I could deal with even) fuel. Rule of
> >>thumb, table lookup, etc fine by me.
>
> Tom,
>
> The chemist says to ignore the H2O and concentrate on the CO2 when
> balancing the eqn, since hte carbon is the deciding factor. His
> assumption is that gasoline is mostly octane, so the basic eqn is:
>
> C8H22 + 8O2 => 8CO2 + 11H2O
>
> The mol weight of C8H22 is 96 + 22 = 118
> The mol weight of CO2 is 12 + 32 = 44
>
> Thus the ratio of CO2 to C8H22 is 44*8 / 118 = 3.0
>
> So you get three pounds of CO2 for every pound of gasoline.
>
> The same math done for propane is:
>
> C3H8 + 3O2 => 3CO2 + 4H2O
>
> Mol wt of C3H8 is 36 + 8 = 44
> Mol wt of CO2 is 44
>
> Thus the ratio of CO2 to C3H8 is 44*3 / 44 = 3.0
>
> So you also get three pounds of CO2 for every pound of propane.
>
Tomj wrote back:
Thanks for these numbers.
For gasoline, this calc works out to 18 lbs/CO2 per gallon gasoline; stats
from the web pin it at 20. Similar assumptions about carbon are made.
I also find varying numbers for BTU/gal, etc, so I decided to punt and
calc everything as ratios (LPG vs. gasoline). It turns out the calcs
below agree whath what I find on the web (EPA souce, etc) pretty closely,
so for comparison it's good enough.
The short story is: LPG is better in carbon/mile about 5% over gasoline.
Eh.
Calculating via BTU/mile (easy; miles per gallon, calc gallons per mile;
BTUs per mile is simple proportion).
LP is a lot lighter than gasoline (6.00 vs 4.20 lbs/gal) so:
Constants:
BTU/Gal lbs/gal Lbs/C gal
Gas 114000 6.00 18.00
LPG 84300 4.20 12.60
I got varying numbers depending on the source, but the ratios were always
the same, so I picked a set I liked.
My car:
14.4 MPG LPG, 12-month average highway mileage
19.47 MPG gasoline equiv (BTU/mile equiv)
14.4 MPG LPG= .069 gal/mile * 12.60= .88 lbs C/mile
19.47 MPG gas= .051 gal/mile * 18.00= .92 lbs C/mile
.88 / .92= 4% less C/mile
Eh. Nothing to get too excited about. Not what I had hoped.
tomj
[Aug 2003 note: The above conclusion is skewed by the fact that
this car, in best-case trim in 1963 (196 ci inline six, 3-speed
overdrive transmission) never got better than 18 mpg; my car has
a slightly larger, newer engine, an automatic transmission and
3.31:1 axle (higher-numerical ratio that the std/OD). My guess
is this car sans LPG and in 1963 trim would get 17 mpg highway.
The 14.4LP/19.47gas includes a closed-loop controller.]