400 mpg, or so
I was in San Diego two weeks ago when I noticed a Prius license plate that read: 54 mpg. That's pretty efficient, especially by contemporary standards, so it's no wonder that the car's owner was proud.
I was walking at the time, however. And surely, I thought, I'm more efficient than a big, heavy non-evolved car, even a Prius, but by how much?
How many miles per gallon of gas does a human get?
One answer is zero--humans obviously can't convert the potential energy contained in gasoline into kinetic energy. That's a boring answer, though.
How many miles per energy contained in a gallon of gas does a human get?
A gallon of gas contains roughly 132 megajoules of energy, 31,548,757 calories, and 31,548 kilocalories, or food calories, or calories, in common parlance (one food/kilocalorie, remember, actually contains 1,000 calories).
A 155 lb human walking at 3 mph will burn 246 kcal/hour, or 82 kcal/mile. Feed that human one gallon of gas in potential energy--31,548 kcal--and he'll have enough energy to walk for 128 hours. At 3 mph, he'll cover 384 miles; in other words, he'll get 384 mpg at that weight and speed.* That's seven times more efficient than the Prius, and 24 times more efficient than a Hummer H2.
It is worth noting that the Prius, say, can carry five people without seriously compromising its efficiency, and it can go well over 60 mph, and carries a big gas tank, consumes zero gas while out of use, and so on (people drive cars for a reason after all!).
The friend who noted this also wondered how many miles per Big Mac a car gets. (Big Macs, he reasoned, are a "generic unit of consumption." Indeed.)
As it happens, caloric as a Big Mac is, a car still couldn't get too far on such a diet. McDonald's says a Big Mac contains 540 calories, which translates to about 57 Big Macs of energy in a gallon of gas. By this measure, a Prius could get almost a mile per Big Mac, while a H2 could only get a quarter mile, and bit further on a quarter pounder.
* A friend ran the numbers from another source that assumed slightly more efficient walking and got 406 mpg, so 400 is a safe approximation, it seems, though we both concede that we might be way off or missing something key in making these calculations. If any readers try to answer the same question, add a comment to tell us what number(s) you got.
22 comments:
By your calculation the human has 100% efficiency in converting gas into foot work. But this is precisely where the car mainly fails in it's efficiency. For a better comparison you need to take some stab at what the efficiency is for a human to convert 1KJ worth of, say, sugar into foot work. Or to assume the car can convert 100% of the gas energy into piston work. Otherwise it's a silly exercise.
CHEMICAL ENERGY is converted to kinetic energy when gasoline combusts.
not potential
"One answer is zero--human's obviously can't convert the potential energy "
I don't want to be too nit-picky here, but there's a tremendous amount of bad grammar on the internet.
It's an otherwise funny and well-written article, but the "'s" in "human's" denotes either possession or contraction ("Joe's body can't digest gasoline" or "it's more efficient to burn gas in a car than to drink it")
People are going to read your post and pick up the bad habits, and more people will be writing in ways that don't make sense.
Thanks again for a great article!
Another cool fact - of course we humans aren't the most efficient movers - mackeral, albatrosses, perhaps horses, are the sleek champions.
But a human on a bicycle beats them all to become the most efficient self-propelled in the animal kingdom.
(Stolen from a guest speaker on Radio Ecoshock.)
Thanks for the nit, Shaun. I fixed it.
Potential to chemical when gasoline combusts--got it.
I just bought a Prius so I don't want to acknowledge that anything gets better gas mileage! But I do think that a bike gets the best. If only I didn't get so sweaty and smelly after a bike ride... Thanks for the great post!
I'm impressed by the application of your mathematics upon the problem but there are several areas that you are not considering. Efficiency, heat lost, Nike vs. Reebok, etc..
Nice article though.
Assuming your 31,548 Calories / gallon of gas, here is a comparison of walking and bicycling courtesy Google calculator:
Walking 3 mph: 385 mpg
Riding a Bicycle at a moderate pace (12-13.9 mph): 672 mpg
I wish more activities on that linked to HowStuffWorks page were rated in miles per hour to compare, say, snowshoeing to driving an SUV...
Why is not anyone mentioning E=mc^2?
Imagine if you can transform 1kg of water into pure energy and then somehow use it for walking or moving.
To me it sounds the same as feeding the guy a gallon of gas :)
hello,
your math includes the figure of a human burning 246 kcal/hour, but isn't it actually 246 cal/hour?
246 kcal is 246,000 calories! i believed the referenced table clearly states cal, not kcal.
tw
tw--It's confusing, but the standard food calorie that people speak of, including those authoring the referenced table, is actually a kilocalorie and contains 1000 calories. I used kcal units to clarify that I wasn't calculating the smaller of the two caloric units. For instance, a Big Mac that contains 540 calories contains 540 kilocalories, and 540,000 calories.
Here's a source: http://www.nutrition.gov/index.php?mode=faq2&this_faq_entry=119
thank you for the clarification. i think that would be an appropriate footnote for your post.
As long as Shaun is being nit-picky, he might be interested to know that it doesn't make any sense to say "Thanks again for a great article!" when he never said thanks in the first place, but instead wrote his post complaining about bad grammar.
I just thought he should know, since people will be reading his post, and picking up the bad habits.
Well, this is cute but it isn't really right at all. Apples and oranges and all that... When you do the math, human "mpg" is maybe a little better than, but comparable to, the Prius.
The problem is people can't burn gas, and you're ignoring the energy it takes to turn gas into a form humans can use (i.e., food.)
There's a huge amount of energy used in food production. On average it takes 3 calories of energy to produce one calorie of food energy. Add the energy involved in transportation and processing and the total number's closer to 7 to 10 calories of energy use per 1 calorie of useful food energy produced.
Even assuming that 25% of the energy going into food production comes from renewable sources (yeah, right...) this means that your 32,000 Cal of food energy took from 200,000 to 250,000 Cal to produce, or the equivalent of 6 to 8 gallons of gas. So your foot "miles per gallon" rating drops to 50-70 mpg or so. Not bad, but not much better than the Prius mpg.
If you eat a meat-heavy diet you're almost certainly doing worse than the Prius, as it's estimated that grain-fed beef takes 35 Cal of energy per 1 Cal of food energy produced. The same goes if the production process has to use non-renewable sources (you'll never get to 100% renewable with current agricultural tech since many fertilizers are made from petroleum.)
To be fair, it takes about 8,000 Cal to get that gallon of gas out of the ground and into your tank, so really the comparable Prius mileage is closer to 43. Still nowhere near the order-of-magnitude difference the initial post suggests. In any event, the numbers we end up with are close enough to each other that it suggests you really need to do more detailed calculations to see if there is a significant difference IRL.
OTOH, the bicycle still doubles your mileage over walking, which means it's probably a big improvement over the Prius if you stick to a low-energy-impact diet.
I see your point, Opposite, but let's not get carried away here. The premise was: you put gas in a car and it goes x miles per gallon. That gas contains y calories, so how far can a human go on those calories?
My emphasis, in other words, is on how far cars and humans, respectively, can travel once they've been given the energy that propels them. The whole premise is slightly ridiculous, I understand. Evaluating the energy required to create the sources of these inputs--food and fuel--is valid, of course, and perhaps even worthwhile, but it's not what I was looking at.
In any event, yes it's true that there is obviously more to comparing the energy efficiency of humans and cars, so thanks for the comment.
May we reprint a portion of your post in Carbusters magazine letters section? Will provide URL so people can read more and join the debate. Who knows, maybe one of our readers is engaged in trying to answer energy questions such as this. Thank you!
www.carbusters.org
editors (at) carbusters.org
As for the post about the inefficiency of feeding people (opposite's)--I will say this: of course it's easy to make the problem more complicated. For example, if it's raining and you try to walk instead of driving, maybe you will catch pneumonia and die, and the hearse will waste heckof gas. On the plus side, you will return valuable nutrients to the soil, so it's hard to say.
That's a little hyperbole right there. But you have to close the system at some point, and I think the limitations David has imposed are reasonable for the purposes of curiosity. Also, remember that there's some base level of human energy expenditure even when you're not walking. You're going to have to eat that food no matter what. So if you wanted to get really fancy, you'd have to subtract that (but where to draw the line at what energy is going into the walking [i.e. powering your nervous system probably helps] is also ambiguous).
For those of us that dont know our cars average mpg...
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I've run a lot of said calculations myself, and yes, while Opposite does have a point regarding the food production there are things NOT accounted for in that argument which I've heard fairly often in response to my stance as a diehard bike commuter.
I've been active most of my life and everyone agrees that exercise is good general practice. So if you take a form of exhertion that you would do regardless and turn it into useful transportation, it basically nullifies that statement, once again bringing the bike into the effective 600-700mpg range compared to the car.
Also, I've found that most who are truly in this for the environmental and sustainable benefits, usually go to good lengths to buy locally grown unrefined foods that greatly reduce their production energy costs and some like myself *gasp* are even vegetarians since producing meat is a 6-10 fold increase in energy to produce the same caloric amount of food as raw produce.
Point is, that argument is worthy of consideration, but in practice applies much less to those who would generally be active anyway AND go to great lengths to support sustainable practices like local farming.
Anthony
Long Walk to Green
Nice post! Very thought provoking. I think the only thing that I would add, hopefully without getting off the original train of thought, is that humans do alot of other things while they are walking. I think a realistic comparison should include the baseline energy consumption of a human. Personally I have no idea how much that is.
The other thing that I might throw in is a reasonable estimate of the need for rest. I doubt there is a human out there that could walk ~400 miles without rest so maybe best case assume 12 hours of walking per day? Unfortunately, I don't have time to run these numbers now, but hopefully I can get back with something.
I ride a bicycle to work and back. My route is 49 miles roundtrip and I ride 3-5 days per week. I'm over 90 kg and not in the best shape. I also ride a heavy commuter bike with racks, fenders and lights, so I'm no where as effecient as I could be.
During my round trip commute, I burn approximately 2500 Calories according to my highly calibrated heart rate monitor. The heart rate monitor takes into account my age, gender, height, weight, level of activity and the 1,468,800 or so heart beats utilized to get me to work and back.
Just a note of clarification, Calories with a capital 'C', means kilocalories which represent 1,000 calories.
After averaging my gas mileage and consumption for a month, I am confident in saying that my car burns 106,610 Calories in gasoline getting to work and back. This figure is 42 TIMES as many raw Calories as are burned by me on the bike. So, as you can see, a carnivore riding a bicycle is orders of magnitudes more efficient than your beloved Prius. Just think it through logically. If you think a human propelling 115% of their weight is just as effecient as a driving a car, I double-dog dare you to push said Prius to work and back and find out for yourself.
Opposite -- If you're going to figure in the energy cost of food production, then it's only fair to also figure in the energy cost of gasoline production. I'm guessing that alone nullifies your point. But I'll also add that everyone has to eat, and people who walk a lot don't necessarily eat more than people to drive door-to-door everywhere they go and take the escalator (in fact, I'd bet the drivers eat more).
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