On Tue, 21 Jan 2003, David Forbes wrote:

> >>Date: Sat, 18 Jan 2003 23:40:58 -0800 (PST)
> >>From: Tom Jennings <tomj@wps.com>
> >>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.]