[SGVLUG] [OT] Especially for Tom & his Prius.... [my rebuttal, then I'll shut up]

[Dustin Laurence]

On Fri, Jul 14, 2006 at 03:41:46PM -0700, David Lawyer wrote:
> On Mon, Jul 10, 2006 at 11:59:45PM -0700, Dustin Laurence wrote:

> > For ordinary fossil fuels like gasoline and diesel only MPG matters.
> 
> Wrong.  A gallon of diesel fuel has about 11-12% more carbon in it
> than a gallon gasoline.

David,

I'm not sure if there is a point to replying, since you utterly missed
the thrust of the conversation.  I wasn't comparing gasoline to diesel
here, the question was how to reduce the greenhouse emissions of a given
engine.  You can only do that by burning less fuel, since CO2 *is* one
of the two major end results of hydrocarbon combustion).  And if I were,
in that context we'd correct by carbon content--the BTU difference would
be totally irrelevant except indirectly, by affecting MPG and thus
emissions.  If you wish to be pedantic then (since, if I recall I was
discussion greenhouse emissions) the relevant figure of merit is moles
of carbon dioxide per mile.  And even that ignores less important
greenhouse effects of other gases in the exhaust, so that would probably
be something like moles of carbon dioxide *equivalent* per mile
(probably only useful for comparing with more exotic fuels, granted).

And finally, in that context it wasn't necessary to be accurate enough
to see much difference between gasoline and diesel anyway.

> Furthermore, diesel has slightly more carbon per BTU content than
> gasoline.  Transportation Energy Data Book Ed. 24, Table B.16 shows
> Distillate fuel (diesel) at 19.95 million-metric-tons-carbon per
> quadrillion-BTU.  Motor gasoline is only 19.34.

I.e. only 3% greater.  You have demonstrated why it would have been an
irrelevant distraction in this context, if it'd come up at all.

> When comparing diesel and gasoline MPG, you need to convert the diesel
> MPG to equivalent gasoline MPG by reducing the diesel MPG figure by
> about 10%.

Only true if you need to compare at constant BTU content.  Since that is
not the case here, there is no need to convert (to repeat, if we were
comparing gas and diesel--which we weren't--we'd do so at constant
carbon content, not constant BTU).

Dustin
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