[SGVLUG] [OT]Hybrids and trains (was fuel prices and the dollar)

[David Lawyer]

On Wed, May 14, 2008 at 08:12:26AM -0700, Dan Borne wrote:
	[snip]
> I am still confused thought how a plane is more efficient than a
> train; doesn't a jet engine burn 10, 11 gallons of fuel a second.

Your figures must be high since this would imply under 3 pass-miles
per gallon with 300 passengers at 500 mi/hr.  Trans. Energy Data Book
indicates the statistics show about 38 pass-mi/gal for 2000 after
taking into account cargo hauling and using the heat content of
automobile gasoline for comparison.  The figure would be even higher
if you gave it in terms of gallons of jet fuel (kerosene) which has 8%
more BTU/gal than gasoline.  (US Dept. of Energy still uses BTU
instead of joules).

A clue as to why it's high is revealed by looking at the 1973 "Fact
Sheet" for the Pratt & Whitney JT9D-7 turbofan engine (with "high"
bypass ratio -- the ones today are even higher in bypass).  For this
engine, takeoff thrust at sea level is about 47k lbs. while max cruise
at sea level is 36k lb.  But at 35,000 ft. and Mach .8, max thrust is
only 11k lbs.  Although TSFC (thrust specific fuel consumption)
increases from 0.336 lb(fuel)/hr/lb(thrust) to 0.62, the lower max.
thrust at 35,000 ft. means less fuel flow.  But a plane isn't apt to
use max. thrust unless they're behind schedule.  And the range of thrust
of 35,000 ft. is 6k to 11k with a TSFC of about 0.6.  So if we take an
av. of 8.5k lbs of thrust,  it implies 5.1k (0.6 x 8.5) lb/hr fuel flow
or 0.2 gal/hr.  Now at max. cruise at sea level this is 36k lb
(thrust) x 0.336 = 12 lb/hr fuel flow or 0.47 gal/hr.  An aircraft
like the Boeing 747 with 4 such engines will thus have 0.8 gal/hr at
35,000 altitude cruise or almost 2 gal/hr at max. sea level cruise.

Now with 300 persons in a 747 at 500 miles/hr and 0.8 gal/hr we'll get
over 30 pass-mi/gal (since we got over 3 with 10 gal/hr).  But
aircraft engine efficiency has improved since 1973, and hence 38
mi/gal in 2000.  This is about the same as Amtrak if you account for
some of the "jet" fuel being allocated to non-passenger cargo.

Two more points.  Railroads don't have routes along a great circle
path like airlines do which favors air over rail.  I didn't account
for this in my letter.  Also, the value of ones time has a high energy
content and since a train takes longer, the longer time spent on the
train has an energy cost.  But if we were prohibited from flying, the
longer time on the train would have an energy benefit since it would
discourage people from travelling so much (unless the train was very
fast).

I'll try to answer the other comments too.
> 
> 2008/5/13 David Lawyer <dave at lafn.org>:
> 
> > I've submitting the following letter to the editor of the LA times.
> > Not sure they will print it.
> >                        David Lawyer
> > =======================================================================
> > Re: "Civilization's last chance", May 11
> >
> > The implication that hybrid autos and trains (instead of airplanes)
> > could make much of a difference in global warming is simply wrong.
> >
> > Hybrids use internal combustion engines no more energy-efficient than
> > the engines on non-hybrids.  They also waste a lot of energy converting
> > the gasoline motor power to electricity, charging a battery, and then
> > withdrawing energy from the battery to power an electric motor to move
> > the car.  All this energy conversion wastes energy and adds to the
> > weight of the car.  The reason hybrids get good mileage is not because
> > they are inherently efficient, but because people don't know how to
> > efficiently drive a non-hybrid and because laws and car design, etc.,
> > impede one from efficiently driving it.
> >
> > To efficiently drive a non-hybrid, one needs to get a "brake specific
> > fuel consumption map" for their engine, which the auto companies don't
> > supply.  Then use the map to apply the optimal amount of torque at any
> > given rpm, but autos have no torque meter.  Efficient driving will
> > mean doing a lot of coasting but coasting in neutral is illegal.
> >
> > As for trains vs. airplanes, it turns out that they are about equally
> > energy efficient, and we don't save energy by taking the train.  See
> > the U.S. Dept. of Energy's "Transportation Energy Data Book": edition
> > 26, table 2.14, and account for the fact that about 15% of fuel for
> > passenger aircraft goes to transport freight in the cargo hold.  For
> > details see my "Fuel-Efficiency of Travel in the 20th Century"
> > http://www.lafn.org/~dave/trans/energy/fuel-eff-20th-3.html#air_eff<http://www.lafn.org/%7Edave/trans/energy/fuel-eff-20th-3.html#air_eff>
> >
			David Lawyer