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

[Dustin Laurence]

On Sat, Jul 15, 2006 at 08:22:15PM -0700, David Lawyer wrote:
> > 
> 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.

You started a totally different discussion without any context for the
switch.  That's very frustrating.  It's much more frustrating when it
happens far enough along that nobody can quite remember what you're
replying to.

> ...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.

Grr.  No one said you were wrong.  What I said was that it wasn't what
we were discussing.  If you want to change direction slightly, then make
it clear that's what you are doing so we can all follow along at home.

> > 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?

What about comfortable seats?  We *could* have compared those too, but
we *weren't*.  I'd be annoyed if someone dragged that in without any
context either.

Let's make this very simple.  The context was (if I can even remember)
greenhouse emissions and how they differ from the kinds of pollutants
that bodies like the EPA and CARB have classically recognized.  Tom
pointed out how "green" the Prius is.  I replied that it *is* indeed
quite good when pollutants are measured, *much* better than a diesel VW.
The reason is that there is a fair amount of chemistry you can do on the
exhaust stream to reduce those pollutants, and the Prius works extra
hard on emissions, while the diesel can't even have a catalytic
converter because of the (current) sulfur content of US diesel.

Key point: currently, a diesel (not that particular diesel--any diesel)
simply can't play in this area (yet), and this is the area that Toyota
advertising emphasizes.

Then I pointed out that greenhouse emissions are a very different
situation, and you can't play games with chemistry (yet--theoretically,
cars could perhaps sequester carbon dioxide, but there is no practical
system for many good reasons).  CO2 is an endpoint of combustion--in
fact, one thing the catalytic converter does is finish the combustion
process, producing *more* CO2 from CO and unburned fuel.  For any given
vehicle, it's greenhouse emissions are proportional to it's fuel usage.
You can't clean up a given engine's greenhouse emissions by bolting on
extra stuff--you have to burn less fuel in it.

Finally, I pointed out the implication--the Prius is not light years
ahead of other cars on greenhouse emissions--it is good, due to low fuel
consumption, but not different than other cars with similar consumption.

Key point: diesels, and any other efficient engines, can play this game
on equal terms.

In the context of that discussion, 3% or 10% or 15% is irrelevant; we're
discussing technologies, qualitative effects, and factor-of-two
characteristics.  It's a simple concept:

            Toxic Pollutants       Greenhouse gasses

Advanced
Gasoline,       Good                    Good, best in traffic
example:
Prius

Advanced
Diesel,         Poor, at least          Good, best on highway
example:        until designed
VW TDI          for ULSD

I didn't even get into the fact that the VW performs this well while
being a different class of vehicle with different design goals and very
different performance (the VW is a kick to drive).

If you want to discuss calculating the carbon emissions of particular
cars, which is certainly a reasonable thing to do, at least provide
warning that you're veering off onto a new subject.

> For comparing a diesel car to a gasoline car where both are the same
> energy efficiency (per vehicle-mile):

Gosh.  You're comparing a poor diesel to a better gasoline engine,
because ordinarily the diesel will have *better* energy efficiency.
It's inherent in the cycle.  (This is *in addition to* the greater
energy content of the fuel you correctly point out.)  I'm not going to
discuss biased comparisions--it's pointless.  A 6-71 diesel gets fewer
MPG than a model airplane engine too.

If you want a fair comparison, you may compare a diesel with 40% better
mpg than the gasoline engine.

Dustin

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