In the realm of vehicles, there is an advantage to being
wasteful. Let me explain....
You put fuel, usually hydrocarbons like "octane", into the
vehicle. The fuel is chemically combined with oxygen in
the atmosphere. The resulting chemical compounds are
mostly water vapor and carbon dioxide.
Do we store
reaction products in the vehicle? NO! We throw them
away, and there are two reasons why.
First, CO2 and water vapor are gases. There is typically
1000:1 ratio (or higher) between the volumes occupied
a gaseous substance and the liquid form of that same
substance. We could not FIT all that CO2 and water
into any ordinary-sized car!
Second (see link), for every 11 grams of octane we burn,
we need 19.3 grams of oxygen. So if we filled our
with 110 kilograms of pure octane, and retained the
combustion products after burning it, the overall
would get heavier by 193 more kilograms. These days
of the major goals of automakers is to find ways to
REDUCE the weight of cars, not increase it!
This brings us to a Fundamental Problem associated with
battery-powered vehicles. A battery is usually a closed
device; all its "fuel" and "oxidizer" are fully contained.
weight never goes up or down to any notice-able degree
it is energized and drained. The vehicle is ALWAYS
carrying the MAXIMUM weight AND substances-volume
around, associated with
There is an alternative, the "fuel cell". It works like a
battery but is "open" in that it can interact with
atmospheric oxygen, and the reaction-product can be
discarded as waste. This makes the fuel cell much more
Naturally equivalent, weight-wise and volume-wise, to
The main problem with fuel cells is that the ideal fuel
appears to be hydrogen gas. This is very bulky stuff!
is not a good thing, making the vehicle bigger, when we
know that smaller vehicles have less air resistance and
better fuel economy.
SO, WHAT ELSE MIGHT WE TRY?
Is there anything besides Carbon and Hydrogen that we
"burn" and SAFELY throw away the chemical-reaction
There are a number of common substances that, if we
reacted them with oxygen them, we could get a decent
amount of energy from them. Beryllium, for example.
However, practically all beryllium compounds are quite
toxic --it is NOT safe to throw the reaction product,
beryllium oxide, away!
Then there are lithium, sodium, and potassium, more
substances that yield lots of energy when oxidized, but
also are not-so-good to throw away afterward. They
dissolve in rain-water and will make soils too "alkaline"
proper plant growth.
Aluminum is another possibility, but because there are
fears that aluminum might be linked to Alzheimer's
Disease, it might be best to not start dumping lots of
aluminum oxide into the environment, where-ever
That just about leaves us with magnesium and nothing
else. What do we know about its oxide's toxicity?
STUFF. "Milk of magnesia", drinkable, has magnesium
oxide as the main active ingredient.
OK, how does magnesium stack up against octane in
Energy associated with the formation of molecules:
octane . . . . . . . 208.4 kilojoules per mole
carbon dioxide . . 393.5
water . . . . . . . . 241.8 (as vapor)
. . . . . . . . . . . . 285.5 (as liquid; the difference is the
"energy of condensation")
magnesium oxide 601.2
The chemical formula for octane is C8H18. One mole of
octane consists of 8 moles of carbon and 9 moles of
molecular hydrogen. So, 8*393.5 + 9*241.8 = 3139 +
= 5315.2 kilojoules per mole, combusted. Except we
to subtract the energy-of-formation of the octane, 208.4
(because it has to be broken apart in order to burn in
oxygen), and so the actual final result is 5106.8.
If we burned 8 or 9 moles of magnesium (I'll call it 8.5
here) to imitate the quantites above, for carbon and
hydrogen, then 8.5*601.2 = 5110.2 kilojoules/mole. In
terms of pure energy, that's very comparable! But things
are actually better than that, because I'm comparing 8.5
moles of magnesium to 8 moles of carbon PLUS 9 moles
Now for some weight stuff:
carbon . . . 12 grams/mole
hydrogen . . 2 (molecular hydrogen)
octane, C8H18 weighs 12*8 + 9*2 = 96 + 18 = 114
magnesium: 24 grams/mole, so 8.5 times that is 204
grams, roughly twice as much. I'm staying with the 8.5
moles of magnesium because of the "comparable energy"
Some density stuff:
octane . . . . 703 kilograms per cubic meter (1000
so .703 kg/liter
magnesium: 1.738 grams per cubic centimeter (1/1000
liter), so 1.738 kilograms/liter
Calculating density in terms of moles:
octane: 1 mole per 114 grams is 1 mole per .114
Multiply mol/kg * kg/liter to get mol/liter, so this
becomes a division problem, .703/.114 = 6.167
magnesium: 8.5 moles per .204 kg means dividing 1.738
kg/liter by .204 = (8.52 blocks of 8.5 moles)/liter --I'm
still sticking with the energy-comparability thing; a mere
1 mole of magnesium would only occupy about a 1/72 of
Now we get to see the "kicker" that has allowed
hydrocarbon-fueled vehicles to rule the world. Earlier
above we started with 1 mole of octane, and after
translating it into moles of hydrogen and carbon, we
figured that mole of octane has about the same potential
chemical energy as 8.5 moles of magnesium. Now we
see that that single mole of octane occupies about 1/6
of a one-liter fuel tank, and that same tank would be
full of magnesium fuel having the same energy.
Basically, hydrocarbon fuels offer tremendous "energy
density" (but not quite as much as magnesium), and that
--AND because liquids are so much easier to handle than
solids-- is why we'll continue using them even if we have
to build nuclear fusion power plants to get the energy to
synthesize hydrocarbon fuels!
We would have a smaller but heavier fuel tank, for our
magnesium-fueled vehicle. I'm not going to speculate
here about just exactly how we would oxidize the
magnesium to get the energy to power the vehicle. The
purpose of this Idea is simply to explore the possibility
of throwing away the chemical-reaction product, after
obtaining the energy. It looks very feasible.
Of course, someone will immediately build a special
vacuum up the MgO dropped all over the roadways, for
recycling into more magnesium fuel. That's OK, because
the goal here is to give all the OTHER vehicles a route to
smaller size than involves batteries. And, of course, we
don't really want the roads to become clogged with spent
fuel. :) (Note: we are not in any danger of running out of
magnesium soon; it is the 7th most-common element in
the Earth's crust --and I explicitly mentioned "recycling".)