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I have some questions about fuel efficiency and units!

Discussion in 'Science & Technology' started by LucyDuke, Jul 21, 2008.

  1. LucyDuke

    LucyDuke staring at the clock

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    This is gonna come out like a babble 'cause I'm not really sure what I'm talking about.

    Cars' fuel efficiency is measured in miles per gallon. But how much energy are they using? Is that what horsepower has to do with? How much energy is there in a gallon of gasoline? How much of that energy does a car use? What are the different efficiencies of different fuels, like can you get 40% of gasoline's energy but only 25% of ethanol's, maybe? I imagine that that depends on different engines too?

    What are the different units of energy? I'm aware of the existence of calorie and joule but how much are those?

    What about human fuel? Is the calorie count of a given food a measure of all the potential calories in it, or the measure of how many a human can get out of it? Because there are things we can't digest, right, that a cow would be able to get energy from. So do we have different efficiencies for different foods? Are there things other than protein and carbohydrates and fats that we can get energy from? (I think we get calories out of alcohol, don't we? Anything else?)

    How many cheeseburgers would my Saab have to eat to drive two miles?

    I realize this is an incoherent babble but even just a couple of partial answers would probably cover a lot of babble. I have a lot more babbling to do but I bet a few explanations will save me a lot of babble. :) Thanks!
     
  2. GoodGame

    GoodGame Red, White, & Blue, baby!

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    Thanks to the conservation of energy, all of energy is equivalent. That's why you can talk about a car having 'horsepower'. A calorie is a calorie, a joule is a joule, however it's made; 1 calorie = 4.18400 joules (according to Google). The Joule is the SI unit, just like the meter, so technically there are microJoules, milliJoules, etc... http://en.wikipedia.org/wiki/Joule

    Engine efficiency is a separate discussion. It's actually a ratio, basically energy input vs. work output, and will depend upon the engine's design, materials, etc...
    http://en.wikipedia.org/wiki/Engine_efficiency

    Fuel efficiency in theory is the engine efficiency converted to miles/kilometers, but obviously other real world factors affect it like surface friction, tire pressure, traffic patterns, road conditions, number of stoplights, how efficiently the engine goes from stop to cruise speed, if the car can recoup braking energy in a hybrid-battery system, etc..
    http://en.wikipedia.org/wiki/Fuel_efficiency

    I suppose there is a net fuel+engine efficiency calculation for different cars, but I'm not an expert. This all like about Physics 101, I suspect.

    In the human body, you can get energy from whatever your body metabolizes, which is primarily protein (amino acids), fats, and carbohydrates (simple like sugars, but also the complex carbs). I suppose nucleotides fit in there as well. Alcohol takes energy to metabolize, IIRC. Pretty much the major biological material that we can't metabolize is cellulose (plant cell wall tissue) which would be a great boon if we could, if only for the conversion to fuel alcohols.
     
  3. uppi

    uppi Chieftain

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    These are the units of energy commonly used on this scale:
    - Joule (J): The SI unit for energy 1 J = 1 Nm
    - Calorie (cal): Old unit for energy, use is discouraged but it is still widely used for food, 1 cal = 4.19 J
    - Watt hours (Wh): 1 Wh = 3600 J

    These all can have SI prefixes, relevant for energy efficieny are kilo (k, 1000) and Mega (M, 1000000).

    For other scales there can be other units of energy like electron Volt (eV) or tons of TNT.

    Related to energy units are the power units, power means energy divided by time:

    - Watt (W): basic SI power unit: 1 W = 1 J/s
    - horse power (hp): old unit for power: 1 hp = 0.735 kW

    There are no efficiencies of different fuels, theoretically you should be able to get all the energy out of every kind of fuel. Efficiency is a property of the engine. Different fuels, however, might lead to different engine designs with different typical efficiencies.

    The power rating of an engine (hp or kW) give the maximum power for that engine. However a car engine won't be run on maximum power all the time so this isn't a useful figure for fuel efficiency.

    The miles per gallon (or liters per 100km) rating is the typical amount of fuel per distance a car consumes. This doesn't just depend on the engine but also on other factors like aerodynamic profile or weight of the car.


    The calorie count (note that what is commonly called "calories" in food is usually "kilocalories") is defined as the energy released by burning that food minus the (estimated) energy in the excrements after the food has been digested. This means that everything that can be digested is counted and everything that can not be digested is subtracted because it will be in the excrements

    That would really depend on how efficient an cheeseburger engine for your Saab would be.
     
  4. Perfection

    Perfection The Great Head.

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    And the size of the cheeseburgers. We talking about Hardee's Monster Thickburgers here, or White Castle Sliders?
     
  5. Brighteye

    Brighteye intuitively Bayesian

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    I've not much to add, but a car's fuel efficiency is very low. I'd be surprised if many engines were more than 1% efficient.
    Similarly, the amount of energy in petrol is vast. A quick Google search suggests that one litre of petrol has 32MJ of energy.
    This compares with an average of 380KC for a Burger King cheeseburger. At 4.2J per calorie, that's 1.6MJ, or 20 cheeseburgers to a litre of petrol. Each cheeseburger can take a person 4 miles (at 4mph), for a total of 80 miles per litre. At 4.5 litres to the gallon, that's 360mpg for walking.
     
  6. Brighteye

    Brighteye intuitively Bayesian

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    If we give your Saab an efficiency of 20mpg, we want 0.1 gallons, or half a litre, which is 10 cheeseburgers, 20 times as many as you need to walk the same distance.

    We can metabolise many things; alcohol may require a small energy investment to metabolise, but it gives more energy than protein or carbohydrate, weight for weight; it's about half-way between these two and fat.
     
  7. Zelig

    Zelig Beep Boop

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    To answer the questions that uppi didn't, in order:

    Gasoline contains from about 34-40 megajoules per litre (9.67 kWh/l), depending on grade. You can figure out the amount of energy a car burns by working from the fuel efficiency rating. (litres/100 km)

    Somewhat more than in a litre, depending on the type of gallon.

    Efficiency is mostly depending on engines, the fuel is only important in as much as it takes different kinds of engines. See here from some efficiency examples: http://en.wikipedia.org/wiki/Brake_specific_fuel_consumption

    Body also gets energy from organic acids and polyols. wiki link

    Your Saab would have to eat about 40 cheesburgers to drive 2 miles.

    FWIW, bicycles kill pretty much every other form of transportation in terms of energy spent per distance per passenger.
     
  8. uppi

    uppi Chieftain

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    Then consider yourself surprised: The typical effciency of a petrol engine is about 20-30%. Diesel engines are even more efficient.
     
  9. Brighteye

    Brighteye intuitively Bayesian

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    Sorry then. How much of that 25% efficiency is lost due to air resistance and friction when actually driving a car?

    I was under the impression that driving was only about 0.5% efficient, walking perhaps 40% and cycling 95% (counting the energy input as energy put into motion, not consumed by the person; that would decrease efficiency a lot, since our muscles give off a lot of heat; perhaps 90% of the energy they burn, at a guess).
     
  10. linfeixb27

    linfeixb27 Chieftain

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    If the roads become more rough, I mean if the coefficent of friction of the roads increase, will the efficiency decrease? What about the car is getting heavier, say more men on it?
     
  11. Brighteye

    Brighteye intuitively Bayesian

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    Let's redo my calculations then, taking 36MJ per litre for petrol. That makes 22 cheeseburgers, which gives walking 410mpg, which we can round to 400.
    Using US gallons, it's 340mpg.

    To travel 2 miles, you'd need half a cheeseburger, whereas your car, apparently, requires 40 cheeseburgers (according to Zelig). That means that you need one eightieth of your car's fuel allowance.

    This link here: http://en.wikipedia.org/wiki/Fuel_efficiency_in_transportation
    suggests that fuel does indeed only contain 32MJ; diesel has more, at 36.4MJ/l. However, it also combines running and walking into one mode of transport when the two are not the same.

    1 horsepower, by the way, is 746W, which is J/s. Thus a 300HP engine is giving 0.224MJ/s.
     
  12. uppi

    uppi Chieftain

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    All of it. You see, there is no energy associated with moving a certain distance. Unless you move up or down a hill, the final state (You being at point B) has the same energy has the initial state (You being at point A). If there was no friction, going from A to B would cost next to no energy. If yoo go ice-skating for example it a tiny amount of energy will get you quite a distance. All of the energy that is used accelerating somthing will be lost either due to air resistance or friction (and most of that friction is intentionally created for braking)

    Efficiency figueres in percent are not very useful for transportation efficiencies, because as explained above they would be all 0%. It only makes sense for the engine (the fraction of the energy that is converted into mechanical energy instead of going straight to heat).

    Therefore the efficiency of transportation can only be given as energy consumed per distance. If one wants to fairly compare efficiencies of different ways of transportation one should include speed and transported mass as well. For example moving 40 tons of goods by bike would probably cost way more energy than doing it with a 40 ton truck.
     
  13. Brighteye

    Brighteye intuitively Bayesian

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    That was rather my point: that we should look not at engine efficiencies, but car efficiencies.
    I would argue that moving 40 tonnes by bike would be more efficient. We'd just need more than one person towing it by bike.

    Efficiency figures wouldn't be 0%. We'd have a total amount of energy in the fuel, and a total amount of energy consumed, and a theoretical amount of energy required simply for acceleration. The efficiency for muscle is 15-30%, but that hasn't accounted for wastage in consumption, digestion or inefficient movement.
    Similarly, in as far as I had thought about it carefully at all, I meant that I doubted that the efficiency of a car, including all similar factors, would be more than 1%. The engine itself, removed from the car, may give off relatively little heat. I don't doubt that.


    I agree that energy consumed per passenger distance works best. The Wikipedia article lists such figures.
     
  14. Zelig

    Zelig Beep Boop

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    They're extremely close though, IIRC air resistance differences play a larger role than mechanical differences.
     
  15. uppi

    uppi Chieftain

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    I doubt that. The reason why bikes get such a good energy/distance ratio is because a bike provides efficient moving while having little mass. The problem with many people towing 40 tons would be that you would many people which would have to be accelerated as well. If everybody could tow his own body weight, then 80 tons would have to be accelerated and taht would be way more than the mass of a loaded 40 ton truck.

    As I said, giving efficiency percentages for transportation doesn't work and your proposal doesn't change that: A vehicle going at a constant speed doesn't need to accelerate and thus the energy required for acceleration doesn't change although it covers distance and consumes fuel.
     
  16. Mise

    Mise isle of lucy

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    Uppi's right, except I'd clarify that it doesn't need to accelerate in the "change in velocity / time" sense, but DOES need to accelerate in the "F=ma" sense. F obviously being the various drag forces involved.
     
  17. LucyDuke

    LucyDuke staring at the clock

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    You guys are awesome. :)

    What kinds of things have energy that we can't digest that other things can? For example I imagine that if I ate some jellied gasoline then I wouldn't get any energy from it, but an airplane might be able to. What about the stuff that cows digest that we can't digest? Is there a lot of this stuff in food? Does lettuce actually have a lot of energy but we can't digest it to get at it? I mean bunnies would starve if it didn't, right?

    I would like answers in several different units of cheeseburger measurement! How many Baconators? How many Big Macs? How many of those stupid mini burgers like Burger King has been pimping? (That would be an excellent standard unit.)

    I wonder what the volume of the cheeseburger in question is, so we can get miles per cheeseburger-gallon for a person. :)

    How is that 360 figure accounting for your 40% walking efficiency figure?
     
  18. Mise

    Mise isle of lucy

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    So for every cheeseburger I eat, I have to walk 4 miles, or else I'd put on weight?

    And the less efficient my body is at converting Cheeseburgers into Walking Energy, the less distance I have to walk?
     
  19. Brighteye

    Brighteye intuitively Bayesian

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    That's fair enough. I still think it'd be better in many ways, because we can grow more food, but not more petrol.

    It would work, but all efficiencies would be very small, and would vary with distance. They certainly wouldn't be 0%, although they'd tend towards it as distance increased. The figures would basically reflect fuel consumption.

    If we can calculate efficiency in a percentage at all, as a measured output divided by some sort of input, which apparently we can, I don't see why it should be something like an engine on its own when such a figure is meaningless without also knowing the build quality of drive shafts, tyres and so on which also transfer the energy into movements.
    Furthermore, for a car we have a lot of 'wasted' energy that transports the extra mass of the car; this is less for walking or cycling.
    Human muscles produce maybe 25% of the energy they consume as movement, but it's important to know how much of this is converted into fighting resistance and how much is wasted in excess movement; a good walker might be lucky to get 60%, I think.

    I wasn't trying to justify using percentages any more than you; merely trying to explain why the figure I had remembered of 0.5% might have been so much lower than the engine efficiencies currently quoted.
    Which I never denied...
     
  20. Brighteye

    Brighteye intuitively Bayesian

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    The 40% figure is one I vaguely remember from some time ago, rather than one I have calculated. If muscle efficiency varies from 15-30%, and 60% efficiency is respectable in walking, then walking varies from 9-18%, not 40 at all. The 40% figure probably comes from older studies of muscle efficiency.

    The cheeseburger is a Burger King one, so it's the volume of a cylinder: I'd guess pix25x5 cm^3, which is 393cm cubed. That means that there are almost 26 cheeseburgers to a litre.
     

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