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

[David Lawyer]

> 
> > On Mon, Jul 10, 2006 at 11:59:45PM -0700, Dustin Laurence wrote:
> > > For ordinary fossil fuels like gasoline and diesel only MPG matters.
> > 
> On Fri, Jul 14, 2006 at 03:41:46PM -0700, David Lawyer wrote:
> > Wrong.  A gallon of diesel fuel has about 11-12% more carbon in it
> > than a gallon gasoline.
I was wrong.  It's actually a little over 14%.  Sorry.
> 
On Fri, Jul 14, 2006 at 07:47:58PM -0700, Dustin Laurence wrote:
> David,
> 
> I'm not sure if there is a point to replying, since you utterly missed
> the thrust of the conversation.

I didn't read the thread in sequence (and still haven't read most of
it), but I still think my points are an important contribution to the
discussion.  In fact later in the same posting, you did mention that
there's a difference in the carbon content of gasoline and diesel so I
think I'm right on topic.  And I do recall seeing discussions of MPG
for gasoline and diesel in the thread.

> 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.

Even if this was 0%, diesel would still contain about 11% more carbon
per gallon of diesel than a gallon of gasoline since it has about 11%
more energy (10% is what I remembered since gasoline has 10% less
energy per gallon).  The 3% makes it even worse for diesel on a per
gallon basis.

> > 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).

Depends what we're comparing.  What about energy-efficiency?  There's
the problems of fuel prices, peak oil, and energy depletion as well as
the problem of global warming, and all these problems are related.  For
energy intensity (the inverse of energy efficiency) the US Gov't uses
BTU per vehicle-mile (or BTU per passenger-mile).

For comparing a diesel car to a gasoline car where both are the same
energy efficiency (per vehicle-mile):  The diesel has 3% more CO2
emissions.  The diesel's miles per gallon is 11% higher even though
it's no more energy-efficient than the gasoline auto.  But a naive
person might erroneously think that since the diesel got 11% more
miles per gallon it was both more energy efficient and put less CO2
into the atmosphere.  It isn't and doesn't.

			David Lawyer