Story: I vividly
remember when my father finally gave up driving his car – it was a long and
traumatic struggle. The last time I rode in the car with my father driving, he
seemed to control the car ok, but every few minutes we had to remind him where
we were going, not a confidence building experience – I drove back on that
trip. The biggest obstacle to stopping was my step-mother, Nancy: she didn’t
want to be without all the benefits our society reserves for people with their
own cars. This was in spite of Charlestown providing free shuttle service
around the neighborhood, and modest charges for longer trips. My father
remembered only too vividly when he had to take the keys away from his father:
Grandpa picked up my father at the airport in Daytona Beach, and on the drive
back home did part of the route on a sidewalk.
The Autonomous Vehicles we have been talking about so far
are designed for moving within a building or a campus. To go longer distances
and to go faster, we need larger, more powerful, enclosed vehicles. Or do we?
Bicycles are among the most efficient forms of transportation, and you get some
exercise at the same time – unfortunately, bicycles, and motorcycles suffer
from weather and safety issues, and require skill to ride.
Story: Listening to
NPR on the radio one day, I heard a statistic which helped set me on the path
to this book: a freight train can carry a 1 pound load across the country on 1
tablespoon of oil. Could this be true? My car could barely get started with 1
tablespoon of oil. So I set out to check this claim.
Railroads must publish a standard report that includes the
annual number of revenue-ton miles, and the total amount of diesel fuel used. CSX reports 483 Revenue Ton-Miles / gallon. Converting to Pounds and Tablespoons
gives 3,773 miles/tablespoon. So a freight train could carry a 1-pound load,
such as a tomato, or apple, or can of beans, 3,773 miles on that tablespoon of
fuel. [https://www.csx.com/index.cfm/about-us/the-csx-advantage/fuel-efficiency/ accessed 3/29/2016]
We drive a Subaru Forester that gets about 30 miles /
gallon. Allowing for the 4.7% additional energy content of diesel fuel over
gasoline, this gives a comparable 648 feet / tablespoon. So if I drive to the
store and buy a tomato, I wouldn’t even get out of the parking lot on that
tablespoon of gas, and that's ignoring the extra fuel to start the car, and
that the fuel efficiency of accelerating and driving at low speed is much lower
than the 30 mpg average. The freight train has an efficiency advantage of 30,743:1.
So what is causing this incredible difference? First, the
Forester is also carrying me; if I went along with the tomato, packed in as
freight on the train, we have to divide 3773 miles by 201 pounds, yielding 18.8
miles, which is still a 153:1 efficiency advantage for the freight train. If we packed the Forester in
as freight as well, we have to divide by 3,701 pounds yielding about 1 mile, so the freight train still has efficiency advantage of 8.3:1.
Some conclusions from this analysis: Don’t drag along a 200
pound driver – Autonomous Vehicles fix that. Reduce the overhead weight of the
vehicle, 3,500 pounds is way too much – our goal will be for the vehicle to
weigh less than the intended load. The remaining 8.3:1 advantage is due to
factors such as economy of scale, reduced air drag, and reduced wheel friction
– we’ll look at those factors next.
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