I have vague memories of a discussion here about the amount of energy
used by various forms of transportation in common units, but couldn't
find it, so I pulled some numbers together:
You get about 31,000 (kilo)calories from burning a gallon of
gasoline, so a car getting 20 mpg burns about 1,550 calories/mile; a
car getting 30 mpg burns about 1,033 calories per mile; a car getting
60 mpg burns about 517 calories per mile; a car getting 60 miles per
gallon with 4 people in it burns about 129 calories per person mile.
For comparison, an average bike rider burns about 35 calories per
mile. The 60 mpg car would have to carry 15 people in order to match
that efficiency per person mile! An interesting picture, but I don't
expect to see it! Also, the bike rider calories are coming from food,
not fossil fuels. Of course fossil fuels are used to grow and
transport the food, so, of course, that's not the whole story.
As for hybrids, the standard Prius, getting it's advertised 55 mpg
uses exactly the same amount of energy as any other vehicle getting
55 mpg -- a hybrid is one of several ways to increase mileage, and it
makes no difference which are used -- hybrid gas/electric, high
efficiency gasoline, high efficiency diesel, etc. Therefore I agree
that tax incentives for hybrid vehicles are nothing but political
pablum -- tax incentives for vehicles that get better than 50mpg
would at least be defensible as being related to the stated goal.
On the other hand the people making plug-in Priuses claim to getting
60 miles on a fully charged 9kWH battery pack, or 150 WH/mile or
about 130 (kilo)calories/mile (the stock 1.5kWH battery pack would
then come to about a 10 mile all-electric range). That's about 1/4
the energy used by the same car run in the standard configuration.
This has promise... except the electric power grid doesn't have the
capacity to charge all those batteries (if everyone switched). A pure
hybrid, with the gasoline engine doing nothing but charging the
battery, and the electric motor driving the wheels should be able to
come close to the plug-in hybrid efficiency (the gas engine is
probably a bit less efficient at generating electricity than the
powerplant, but doesn't have grid transmission losses). Too bad none
of the car companies are making any of those. The car manufacturers
keep saying they need to drive the wheels with the gas engine because
the electric motor doesn't have enough power... while talking about
grand plans for fuel cell powered cars that will have to be powered
by electric motors, because that's all there will be... and don't
even mention hydrogen.
Anyway, if your goal is to reduce the amount of fuel burned and CO2
produced, going from a 20 mpg car to a 60 mpg car reduces your impact
by a factor of 3; going from a 20 mpg car to a plug-in electric
reduced your impact by a factor of 12; reducing the amount you drive
reduces your impact by whatever the percentage of reduction is,
potentially all the way to zero, and can be combined with vehicle
changes; riding a bicycle instead of driving a 20 mpg vehicle reduces
your impact by at least a factor of 44 on a pure calorie/mile basis
(but how many bicycle riders average 15,000 miles per year?).
I find the numbers helpful to keep things in perspective... and to
remember that my total bicycle related expenses are less than the
insurance on a car would be...
Jayf
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