jet fuel question

From:         stedder@tulsix.utulsa.edu (Stephen Tedder)
Organization: UTexas Mail-to-News Gateway
Date:         30 Jun 94 00:34:19 
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Well, a couple of weeks ago I made a rash statement about the relative
efficiencies of airliners and automobiles which have given rise to comment,
and deservedly so. It's particularly embarassing in view of the fact that
I have an excellent reference to the subject, The Physicist's Desk Reference,
from the American Institute of Physics, right over my desk.

The operative figures are as follows. Apparently these are for an average
over the US aircraft and automobile fleet. For automobiles the energy 
intensity of transport is shown as 2.77 megajoules/passenger-km.
For aircraft the figure is 3.49 MJ/passenger-km. In my defense I will
point out that in the early seventies, when I was learning about
such stuff, the average new car gas mileage was 13.8 mpg. (I am not
making this up, it's in the book too.) It is something like 30 today,
so that cars were probably consuming twice as much energy then. 

If you want a comparison, though, why not use buses, which are by far
the most efficient at 0.40 MJ/passenger-km. 

Some other figures of interest are the relative energy consumption of 
automobiles and airplanes as a whole. Transportation in general consumes
26.6% of the energy used in the United States. Of that amount, 49.5%
is consumed by automobiles and 8.4% by air transport, the remainder going
to trucks, marine, and rail transportation. So I think that my original
contention, that the use of energy by aircraft is minor compared to
automotive use, is correct.

Another interesting fact is that at a typical oil refinery, from 100 barrels
of crude oil, 45 barrels of gasoline are produced. The quantity of jet
fuel produced from the same 100 barrels is about 8 barrels. So about
5.5 times as much gasoline is produced as jet fuel, roughly consistent
with the energy consumption figures above.

In fact, we're so desperate to increase the gasoline output that we build
catalytic crackers to crack the heavy gas oil fraction into gasoline,
catalytic reformers to make reformed gasoline out of naphtha, and 
even polymerize some of the gaseous fraction for gasoline! In Europe,
only about 20% of crude oil is made into gasoline.

I can think of one other advantage of air travel. The major airlines have
had no loss-of-life accidents in the past three years, if I'm not mistaken.
What other transportation system can make that claim? 

One other note---please remember that the above energy intensity figures 
are for transportation as it is actually practiced, not for the ultimate
potential of either automotive or air transportation. So if we all had
Diesel Rabbits, the automotive figure would be better, especially if we
all rode around five to a Rabbit all the time! On the other hand, you
see more '70 Buick Electras with the 455 engine than you do Diesel Rabbits...

-- 
Steve Tedder stedder@tulsix.utulsa.edu
	     918 292-3301