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Old 10-25-06, 05:26 PM   #1
DoB
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Some energy calculations for fellow dorks

I would probably be more at home putting this on the "Car Free" forum, but I'm not care free so tough. As an engineer, I was curious about bicycle efficiency, especially after reading some item on the net about how the bicycle is the most efficient transportation mode, even over walking. For a start, I figured I'd take a look at a bike vs. a compact car (something that gets about 30 mpg).

I checked some fitness websites, and most agree that a cyclist doing 15 mph is going to burn about 600 calories (actually 600 kilo-calories which is what your food is actually measured in) per hour. This is 2.508 megajoules / hour in standard units.

As an aside, I realize that if the cyclist is cranking at about 175 watts, this means that 25% of that 2.5 MJ per hour is reaching the cranks, but I figure that this is about right. After all, only about 25% of a 30 mpg car's gallon of gasoline is getting to the drive wheels too. I'm sure you other commuters realize the missing 75% of the energy is what is making you so sweaty.

Anyway, 2.508 million joules per 15 miles means a cyclist is consuming 167,000 joules per mile at 15 mph.

Looking at the car, it burns 0.5 gallons of gasoline containing 60.88 megajoules to go the same 15 miles. This is a simple 4.06 megajoules per mile.

In short, the car needs 24 times as much energy per mile as the cycle (this actually struck me as low.....if I put 5 people in the car it's only using 5X the energy as the cyclist per person delivered). On the other hand, 100% of the car's fuel is non-renewable while the cyclist runs on 100% biomass fuels.

Sorry to bore anybody, but I thought this was interesting and I figured I'd share my quick and dorky calculations
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Old 10-25-06, 05:34 PM   #2
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Originally Posted by DoB
In short, the car needs 24 times as much energy per mile as the cycle (this actually struck me as low.....if I put 5 people in the car it's only using 5X the energy as the cyclist per person delivered). On the other hand, 100% of the car's fuel is non-renewable while the cyclist runs on 100% biomass fuels.
Unfortunately, a considerable amount of non-renewable fuel will go into the steak I'll eat tonight to replenish those spent calories. Petroleum runs the tractor, fertilizes the fields (that grow the corn to feed the cow), powers all the transport and is used to produce much of the packaging.

I try not to think about all that though, because I like meat and couldn't imagine trying to be vegetarian.
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Old 10-25-06, 05:44 PM   #3
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Unfortunately, a considerable amount of non-renewable fuel will go into the steak I'll eat tonight to replenish those spent calories. Petroleum runs the tractor, fertilizes the fields (that grow the corn to feed the cow), powers all the transport and is used to produce much of the packaging.

I try not to think about all that though, because I like meat and couldn't imagine trying to be vegetarian.
The driver of the car also has to eat food, so you're still ahead...
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Old 10-25-06, 06:02 PM   #4
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The driver of the car also has to eat food, so you're still ahead...
The car driver probably only burns about 11,000 joules per mile sitting on his butt compared to the 167,000 joules per mile of the cyclist.
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Old 10-25-06, 06:05 PM   #5
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The car driver probably only burns about 11,000 joules per mile sitting on his butt compared to the 167,000 joules per mile of the cyclist.
And probably eats the same or more; he's just net-positive on caloric intake rather than neutral or net-negative. AKA "wide and growing"
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Old 10-26-06, 08:17 AM   #6
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Well, my steak is out in the pasture and trust me, no oil is being wasted on it. Pure grass fed, no grain. I might raise some corn to fatten it up, but again, no tractor (don't have one) and fertilizer will be from the dairy farm on the other side of the road who don't use tractor, etc either.

If you want to know, most of our fertilizer comes from a nearby volcano... I kid you not.

Besides, you don't really get energy from protein, but from carbs.
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Old 10-26-06, 09:03 AM   #7
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Originally Posted by DoB
Sorry to bore anybody, but I thought this was interesting and I figured I'd share my quick and dorky calculations
Don't worry -- here's one dork who found that interesting!

Edit: I should add that this is a dork who has collected 2-3 years of Beijing air pollution data for fun.

Last edited by gbcb; 10-26-06 at 09:33 AM.
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Old 10-26-06, 09:25 AM   #8
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2 dorks!
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Old 10-26-06, 09:32 AM   #9
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Thanks DoB. I often think about this when I see an oversized SUV accelerating from a stoplight. I weigh roughly the same as the driver and yet they're trying to move a few extra thousand pounds compared to my bike.
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Old 10-26-06, 09:47 AM   #10
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Quote:
Originally Posted by JeffS
Unfortunately, a considerable amount of non-renewable fuel will go into the steak I'll eat tonight to replenish those spent calories. Petroleum runs the tractor, fertilizes the fields (that grow the corn to feed the cow), powers all the transport and is used to produce much of the packaging.

I try not to think about all that though, because I like meat and couldn't imagine trying to be vegetarian.
Of course the vegans are also consuming a considerable amount of non-renewable pretrol unless they grow the veggies in their own back yard.
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Old 10-26-06, 10:05 AM   #11
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Of course the vegans are also consuming a considerable amount of non-renewable pretrol unless they grow the veggies in their own back yard.
From some more research I find some contentions that a vegetarian eating all commercial food would use 7 calories of fossil fuels for every 1 calorie that the person eats. A non-vegetarian would be closer to 12 calories production for every 1 calorie consumed because meat production is so energy intensive (lots of grain grown per unit of meat).

So if you eat nothing but commerically grown food including meat, your bicycle is only 2X as efficient as a compact car when looking at total use of fossil fuels. A organic home growing vegan would be pretty close to the 24X number I came up with in the first post.

One other complication....Does the energy that the cyclist burns in cycling lead to a 1:1 increase in food consumed compared to the sedentary driver? In theory it should, but what if the sedentary driver is a heavier person due to inactivity? Do fatter individuals have a higher daily (effort neutral) calorie burn rate? Or is it the opposite due to higher muscle mass in the athelete?
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Old 10-26-06, 10:42 AM   #12
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Originally Posted by JeffS
Unfortunately, a considerable amount of non-renewable fuel will go into the steak I'll eat tonight to replenish those spent calories. Petroleum runs the tractor, fertilizes the fields (that grow the corn to feed the cow), powers all the transport and is used to produce much of the packaging.
Petroleum runs the drilling rigs, tankers, refineries, pipelines, and delivery trucks. Cars require a lot more asphalt than bikes, also a petroleum product, that has to be replaced more often as cars pound it to rubble. That's my impression. I'd be interested to know if there are any numbers on prerequisite fuel usage.
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Old 10-26-06, 10:56 AM   #13
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Petroleum runs the drilling rigs, tankers, refineries, pipelines, and delivery trucks. Cars require a lot more asphalt than bikes, also a petroleum product, that has to be replaced more often as cars pound it to rubble. That's my impression. I'd be interested to know if there are any numbers on prerequisite fuel usage.
There was a study posted here a while back that had these numbers. As I recall, petroleum is actually very efficient, costing only about 15% extra to deliver energy from the oilfield to the consumer, including the energy to refine and ship the energy.

Food was very inefficient with it requiring, as I recall, multiple calories of energy to produce, process, and deliver each calorie of food from the farm to the consumer. Of course, we humans have little choice but to consume food for energy so it's basically a moot point.

Almost certainly though, if you are considering transporting yourself from one point to another, it takes less energy to use a bicycle than to use a car since, if nothing else, the car represents nearly 100% of the total weight to be transported versus cycling where your weight is 90% of the total weight. However, if you are transporting 50 cyclists and can use a bus, the weight penalty of the vehicle per person is likely much lower and might require less energy than for everyone to ride - depending on their paceline skills!
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Old 10-26-06, 11:01 AM   #14
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Originally Posted by DoB
One other complication....Does the energy that the cyclist burns in cycling lead to a 1:1 increase in food consumed compared to the sedentary driver? In theory it should, but what if the sedentary driver is a heavier person due to inactivity? Do fatter individuals have a higher daily (effort neutral) calorie burn rate? Or is it the opposite due to higher muscle mass in the athelete?
The BMR formula goes like this:

English BMR Formula

Women: BMR = 655 + ( 4.35 x weight in pounds ) + ( 4.7 x height in inches ) - ( 4.7 x age in years )
Men: BMR = 66 + ( 6.23 x weight in pounds ) + ( 12.7 x height in inches ) - ( 6.8 x age in year )

So, if you weigh more and/or you're taller, you need more kcals. The older you get, the less energy you need. So, heavier people need more calories. The formula above (from http://www.bmi-calculator.net/bmr-ca...mr-formula.php) does not consider a persons lean tissue mass.
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Old 10-26-06, 11:01 AM   #15
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I'm skeptical. While you might be right on, I think a bike is much more efficient than that, compared to a car.

Bikes, at 35 pounds, versus the typical car, at 3500 pounds must be more than 24 times as efficient. I'd guess more like 150 times as efficient.
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Old 10-26-06, 11:53 AM   #16
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I think something that is really missing is how much energy is needed to make that car in the first place. A car starts pretty far in the whole compared to a bike
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Old 10-26-06, 12:10 PM   #17
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You might be interested in this page which shows that an automobile converting calories as efficiently as a human on a bicycle would get about 912 miles per gallon. (I've heard higher numbers, but of course it really varies greatly with weight and wind resistance, among other things.)
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Old 10-26-06, 12:29 PM   #18
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Quote:
Originally Posted by DoB
I checked some fitness websites, and most agree that a cyclist doing 15 mph is going to burn about 600 calories (actually 600 kilo-calories which is what your food is actually measured in) per hour. This is 2.508 megajoules / hour in standard units.

As an aside, I realize that if the cyclist is cranking at about 175 watts, this means that 25% of that 2.5 MJ per hour is reaching the cranks, but I figure that this is about right. After all, only about 25% of a 30 mpg car's gallon of gasoline is getting to the drive wheels too. I'm sure you other commuters realize the missing 75% of the energy is what is making you so sweaty.

Anyway, 2.508 million joules per 15 miles means a cyclist is consuming 167,000 joules per mile at 15 mph.

Looking at the car, it burns 0.5 gallons of gasoline containing 60.88 megajoules to go the same 15 miles. This is a simple 4.06 megajoules per mile.

In short, the car needs 24 times as much energy per mile as the cycle (this actually struck me as low.....if I put 5 people in the car it's only using 5X the energy as the cyclist per person delivered). On the other hand, 100% of the car's fuel is non-renewable while the cyclist runs on 100% biomass fuels.

Sorry to bore anybody, but I thought this was interesting and I figured I'd share my quick and dorky calculations
Just looking at the raw caloric value, a cyclist uses around 35 Kcal per mile at 15 mph. A compact car uses 1033 kcal/mile (no need to take into account the efficiencies since the fuel is burned in either case). I make that out to be 30 times as much energy per mile.

One thing to consider on the placing more people in the car is that the amount of energy will increase since the car will require more fuel/mile to move that many people.

For a long time, I've explained the difference in energy usage in terms that most people can understand. Rather then using calories or joules, use jelly donuts! Your car requires 4 jelly donuts (assuming 250 Kcal/donut) to move the car per mile while the cyclist uses 0.14 jelly donuts/mile. To go 15 miles, the cyclist uses, roughly, 2 jelly donuts while the car uses 60 We'd be a lot more concerned about energy usage if we had to cram 60 jelly donuts into the tank to go 15 miles
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Old 10-26-06, 12:32 PM   #19
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Of course the vegans are also consuming a considerable amount of non-renewable pretrol unless they grow the veggies in their own back yard.
Plus their plastic shoes and belts.

We have a half of a locally pastured cow in our freezer (mostly powered by hydro I think, the bane of my own fish guy existence but what are you gonna do).
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Old 10-26-06, 01:22 PM   #20
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As a number-cruncher myself, I enjoy this sort of back-of-the-envelope stuff.
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Old 10-26-06, 02:03 PM   #21
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Originally Posted by cyccommute
For a long time, I've explained the difference in energy usage in terms that most people can understand. Rather then using calories or joules, use jelly donuts! Your car requires 4 jelly donuts (assuming 250 Kcal/donut) to move the car per mile while the cyclist uses 0.14 jelly donuts/mile. To go 15 miles, the cyclist uses, roughly, 2 jelly donuts while the car uses 60 We'd be a lot more concerned about energy usage if we had to cram 60 jelly donuts into the tank to go 15 miles
Thank you! This information just went up on the outside wall of my cubicle, with an attractive picture of a donut.
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Old 10-26-06, 08:11 PM   #22
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Originally Posted by JeffS
Unfortunately, a considerable amount of non-renewable fuel will go into the steak I'll eat tonight to replenish those spent calories. Petroleum runs the tractor, fertilizes the fields (that grow the corn to feed the cow), powers all the transport and is used to produce much of the packaging.

I try not to think about all that though, because I like meat and couldn't imagine trying to be vegetarian.
This increases the yield and efficiency of food production. If this was not so we would be living in a vastly different world with a lot fewer people where basic needs would require a lot more of our effort. On the other hand we have plenty to do to cut down unnecessarily long trips in oversized vehicles from oversized houses before we worry about how much fuel we use on necessities.
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Old 10-26-06, 10:17 PM   #23
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All this talk about effeciency, and it's using a barndoor for a bicycle. Think of how effecient a bike is that repositions the rider for a minimal frontal profile. If you want effieciency, these guys live and breath it.
81mph for a bicycle on flat ground without a tailwind, that's efficiency. Of course, there are more practical every day bikes that are efficient like my Corsa which takes about 250ish watts to push it along at 25 on the flats, and 175 will have me around 20-21, at least that's what the power tap tells me. I was amazed at how the change in rider position could make such a huge difference when I first got it, 19-20mph effort had me doing 23-25.

Aerodynamics has a huge impact on the energy required to go a certain speed. Who's most effiecient here:

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Old 10-27-06, 05:06 AM   #24
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Aerodynamics has a huge impact on the energy required to go a certain speed. Who's most effiecient here:


I guess you want us to say the guy using his crotch for a parachute. (just kidding)
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Old 10-27-06, 05:12 AM   #25
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Ok, now someone convert it into dollars/mile!
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