A heat engine in a road vehicle is best measured in work per distance, i.e.
J/m. A fuel tank filled with any burnable stuff such as gas, gasoline or
hydrogen, contains certain amount of combustible energy that is converted to
the kinetic energy that propels the vehicle (with about 3/4 loss). The amount
of energy in a fuel is expressed usually as the Lower Heating Value (LHV). The
LHV of all kinds fuels has been know for a long time, some dating a couple of
centuries back. As for hydrogen, it has LHV of 10 MJ/m³ at atmospheric
conditions, or 8.5 GJ/kg as liquid.
Expressing "fuel economy" (what a term! - it is un-economy!) in xJ/100 km or
m/J compares engines or vehicles on equal terms: On the efficiency of the
conversion of the potential heat of combustion (LHV) to the distance traveled.
There are, of course, differences in LHV among the same fuels from one vat to
another. As of now, all pumps dispense fuel on volumetric basis, and that
increases the inaccuracy in LHV further. Anyone who tried seriously (that is
correcting for atmospheric conditions) to measure "mileage" knows the
frustration in getting a number with more than two significant digits.
With hydrogen, because volumic measurement is not any easier than mass
measurement, the LHV based dispensing is practical. That makes measuring a heat
("fuel") consumption also practical. It is only a political choice whether we
will pay for mass or for heat (energy) "at the pump."
That, of course, brings us to the bottom line that really matters: How many
dollars, or euros, or .... it will it cost us in "fuel" to get somewhere. The
LHV method is the fairest one of them all - paying for the energy delivered
into the tank. Energy in hydrogen, mainly because it is not a fuel in the sense
that mankind finds it somewhere like oil and also because it is expensive to
transport and store, is and will remain non-competitive for the foreseeable
future, politics aside. Electricity is easier to make, transport and store than
hydrogen.
Stan J.
PS: Concerning mph vs. l/100 km, I believe the mph is a sinister plot of the
oil companies to make it difficult to find cost. Standard Oil focused people on
distance, not cost. Notice how easier it is to figure cost with a "# per
distance" vs. "# per volume."
----- Original Message -----
From: Victor Jockin
To: U.S. Metric Association
Sent: 08 Jul 10, Thursday 11:40
Subject: [USMA:41383] Re: FW: Impressions of a Futurecar - FlowChart
(usnews.com)
I hope the author is wrong about Miles per Pound becoming a standard, but I
suspect, if H2 does ever catch on (which is hardly a sure bet anyway) that
he'll be right. Decisions made thus far have been outside of the broader
public view. If H2 goes mainstream, our banal and parochial legislative
representatives will likely undo any progress initially shepherded by NIST. I
hope I'm wrong, but US history is sobering on that front.
From: Nat Hager III
Sent: 07/10/2008 1:34 AM
To: U.S. Metric Association
Subject: [USMA:41377] FW: Impressions of a Futurecar - FlowChart (usnews.com)
Sorry, missed the most controversial part..
Hydrogen car article in US News...
>>>Kilograms per what? The Hydrogen 7 has one of those onboard computers that
display your fuel efficiency-for both gasoline and hydrogen. Gas mileage, of
course, is measured in the familiar mpg. But hydrogen mileage is flipped around
and expressed in kilograms per 100 kilometers. This took a bit of noodling to
understand, like figuring out the inverse of a currency exchange in a foreign
country. Simply put, it's the metric version of how many gallons it takes to go
100 miles. If you're averaging 4 gallons per 100 miles, for instance, that's
the same as 25 miles per gallon; 3 gallons per 100 miles would be 33 mpg. So in
hydrogen terms, the lower the number, the better.
I started out averaging 3.3 kilograms of hydrogen per 100 kilometers. I tried
to drive gently and see if I could improve on that, and I got it down to 2.9
for a while. Then I hit traffic, and it went back up to 3.2. I studied the
instant efficiency reading, too, which ranged from 0 to 10. When coasting at
about 50 miles per hour-a very efficient speed for most cars-my hydrogen
consumption was less than 2 kilograms. But when pressing hard on the
accelerator to pass somebody, I pegged the meter, burning the maximum 10
kilograms (or more).
If hydrogen catches on, there will have to be standardized metrics for
expressing fuel economy. My guess is that the federal government will adopt
something similar to the mpg construct, while taking account of the fact that
hydrogen is typically measured by weight, not volume. So an Americanized
version might be expressed as miles per pound. If that were the case, my 3.3
kilograms per 100 kilometers would equate to about 8.5 miles per pound of
hydrogen. I think. Or maybe we'll just have to join the rest of the world and
learn the metric system.
>>>
http://www.usnews.com/blogs/flowchart/2008/7/9/impressions-of-a-futurecar.html
Nat
