Built on Facts

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Physics of fuelling a car

May 24th, 2008 · 5 Comments

Energy has to come from somewhere. The law of conservation of energy prevents energy from being created out of nothing, so any energy that we use has to be acquired from some source. The energy that fuels our bodies comes from plants (and animals that eat plants), which get their energy from the sun.

A little more indirectly, our cars also get energy from the sun. Gasoline is refined from oil, which was formed by heat and pressure applied to dead plants in ages past. But as you can see from every price sign on gas stations, there’s a problem. That oil is expensive and getting more so. There’s a lot of reasons. The much-maligned oil companies make huge profits, but that’s mainly a result of how much they sell. Typical profits are in the $0.10/gallon range. The government takes a much larger share in taxes, sometimes up to $0.60 or more. But of course the majority of the cost comes from the cost of the crude oil - right now contributing more than $3.00 to the cost of a gallon of gasoline.

But cars have to have energy. A half-ton vehicle at interstate speeds has a kinetic energy of several million joules. Much worse, friction losses to the road, wind, and moving vehicle parts eats tremendous energy simply to keep the kinetic energy at a constant level. A gallon of gasoline has around 130 million joules of energy, but gasoline is definitely not the only thing that can hold energy for use by a car. Eventually, the price of gasoline will be higher than whatever energy storage technology is the next least expensive.

What technology will that be? I have no idea, but likely it will be one of three strong technologies.

  • Electricity
  • Hydrogen
  • Gasoline - Wait, replace gasoline with gasoline? Actually, it may turn out to be feasible to generate gasoline from leftovers in the meat processing industry. As a bonus, the fuel itself adds no net carbon to the atmosphere since it was generated from plants and animals whose carbon came from the atmosphere in the first place. It’s thus much more environmentally neutral than fossil-fuel derived gasoline.

Fundamentally the question is the intersection of physics and economics. What can deliver energy, but do so easily and at low cost?

As you and I cringe when the price display counts upward when we pump gas, at least we can take comfort in this: the more the price goes up, the sooner we can scrap fossil fuels: that antique, dirty, and geopolitically corrupt method of maintaining our cars’ energy of motion.

Tags: Physical Concepts

5 responses so far ↓

  • 1 meichenl // May 25, 2008 at 12:14 pm

    it’s only half the story. the air in my room here is 300K - full of energy!! nitpicky, maybe, but i think there’s a big gap in the public’s knowledge of the important difference between “energy” and “free energy”. car’s don’t use up energy, after all, seeing as energy is conserved. so why can’t i just take the energy coming out the tailpipe and recycle it back to my gastank. i think it’s a reasonable question for the average joe to ask.
    on the other hand, god help you if you start to talk about “free energy” without explaining yourself very, very carefully

    Matt replies: Definitely! If I ever say free energy on this blog, you can be assured that I mean ECONOMICALLY free (or at least very cheap) energy. Or Gibbs free energy and similar, but that would be said specifically. When I post again in more detail about this, I’ll be sure to clarify that we’re after energy in a form that the car can turn into forward motion - unfortunately plain old heat without a temperature gradient won’t do us any good!

  • 2 Uncle Al // May 25, 2008 at 1:10 pm

    Theoretical minimim mi^2 for 1 GW electrical,

    Area Modality
    ==============
    1000 biomass
    300 wind
    60 solar
    0.3 nuclear

    On 21 July 2006 California statewide consumpton peaked at 49 GW. In the entire history of mankind, aft and fore, has there been/will there be produced 3000 mi^2 of solar cells (plus contingency)?

    Total US 2002 cropland was 678,375 mi^2. Powering California alone would require 50,000 mi^2 or 7%. When corn is burned for fuel there is neither food nor fuel. (2008 is a record low corn yield year given late wet weather from Global Warming desiccation.) Assuming zero inputs and losses, 371,875 mi^2 of corn fermented would replace imported petroleum. Corn requires 150 lbs nitrogen/acre. Who will donate 18 million pounds of fixed nitrogen/year, for free? Maize pimp Archer-Daniels-Midland? The Bush Faery?

    Biodiesel
    Crop gal/acre Iodine #(big=bad)
    ===========================
    Corn 18 125
    opium poppy 124 117
    avocado 282 85
    oil palm 635 37-54

    Who wants Louisiana fueling Amerrica? Not Texas.

    Matt replies: I don’t think solar will ever be able to generate base-load power for most areas. However for areas with a lot of natural sun it may well be helpful in dealing with peak demand - especially since air conditioning and the sun tend to be happening at the same time. Biofuels are a mess right now, but largely due to political meddling. Corn-derived ethanol is particular is a debacle on an epic scale. Cellulosic ethanol may well turn out to be another story.

  • 3 Carl Brannen // May 25, 2008 at 6:45 pm

    Oil algae (which is converted to biodiesel) is likely to be the eventual solution for vehicles. Hydrogen is probably not going to be workable. Electricity is great but you still have to make it somewhere and it’s difficult to store.

    How many cars exist on the planet? Something like a few billion, after the rest of the planet reaches 1st world standards of living. Each requires a few hundred pounds of lead to make battery out of, so lead requirements are about a hundred million tons. That’s at least conceivable, but world production of lead is around 4 million tons per year so it would take some time.

  • 4 Paul Murray // May 29, 2008 at 8:06 pm

    Civilisation simply will not be able to continue using enery at the current rate. This means that we will change the way we live - no more two hour commutes in individual SUVs. I ride a 8-cubic-inch motor scooter, and am shocked everyday by the lines of people driving individual light trucks to commute to work.

    Perhaps we will even resume rising at dawn and retiring at dusk. Its nature’s way. I’ve done it while on a long road trip, and it’s strangely wonderful.

    Oh - and no more out-of-season fruit and veg.

    Matt replies: I think I have to disagree. If civilization will not continue to use energy at the same rate, civilization is going to have to give up a lot more than big cars and distantly grown food. Even just by the rubric of CO2 emission transportation isn’t actually anything approaching a majority share of human resource use. We’d have to revert nearly to the pre-industrial stage.

    But if civilization can develop nearly-unlimited cheap and clean energy, we need not give up the health and prosperity that civilization and energy have brought. Nuclear (fission and fusion), wind, solar, geothermal, biomass, and several other fuels show great promise in that regard. We’re not nearly there yet, and won’t be for a long time. But we’re taking the first steps down that road.

  • 5 Thomas Salinas // Nov 12, 2008 at 6:59 pm

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