Just a quick note about drag. You don't really care so much about the coefficient of drag (Cd), but the drag force (Fd) which is proportional to the Cd times the projected frontal area.
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Tdave
Member
This is why the Roadster has about the same drag as a Prius, even though the Cd for the Roadster is so much worse -- it's pretty bad, I was surprised to learn.
Wikipedia puts the efficiency of the human body at 18-26 percent, which seems to be about the efficiency of an ICE. Maybe we need to be electrified, too!
I would tend to agree with the point that we have to take into account a number of different variables, including the cost and "carbon foot print" of being in poor health and overweight. Although I plan on getting the Model S, and I am really rather supportive of the Tesla electric car, we can probably generally surmise that the more complicated a mechanical, electric or electronic device or machine, the greater the carbon foot print, thus the favor goes to the bicycle. Yet, I have to often think how my health may be suffering by inhaling the small molecular particles produced by non-electric vehicles? That is why I hope for a day when most vehicles on the road are all electric.
Yes, of course. I even said that in the initial post. However, you've have to go farther and consider the end-of-life costs and the footprint of building the much larger road that the car takes. It all pretty much favors the bike.
However, I was trying to compute the *marginal* footprint of bike-vs-drive. That is, you've got a Roadster and a bike in the garage, what's the difference between taking one and the other. The fact that it was even close was really the point of the post, not that in the long run you should abandon your bikes and drive electric to save the planet (you shouldn't; you should telecommute and ride the bus).
No, we can actually quantify an estimate of the difference. It's way better than surmising, or at least it would be if you started with decent numbers rather than the guesses I pulled from the web.
Anyway, I can provide a counterexample to your point: a big old diesel bus. It's huge, it has tons of complicated moving parts, it belches CO2 and spews particulates, and its carbon footprint per passenger mile is way less than the Roadster (or for that matter the bike). So, big & complicated does not always mean worse.
I came across this a few days ago:
Fat Knowledge: Food CO2 and Land Footprints
CO2/cal is much lower than your initial assumptions. Last year I was riding up to 400 mi/week and consuming up to 4500 kcal/day. Most of it was carbs from processed junk foods. I eat beef perhaps 1-2 times a year. Despite riding so much, none of the miles were for utilitarian purposes. I still commuted in my hybrid and was content to separate work from recreation unless gas hits $20/gal.
Fat Knowledge: Food CO2 and Land Footprints
CO2/cal is much lower than your initial assumptions. Last year I was riding up to 400 mi/week and consuming up to 4500 kcal/day. Most of it was carbs from processed junk foods. I eat beef perhaps 1-2 times a year. Despite riding so much, none of the miles were for utilitarian purposes. I still commuted in my hybrid and was content to separate work from recreation unless gas hits $20/gal.
I think you need to step back from the equations and think about what "Carbon footprint" is trying to measure -- the increase of atmospheric CO2.
Burning coal raises atmospheric CO2 because it is returning Carbon to the atmosphere which was sequestered millions of years ago. When this is done on a worldwide industrial scale it changes the atmosphere because we are burning thousands or millions of years worth of CO2 sequestration in the course of a single year.
Metabolizing a banana releases CO2, but the Carbon in the banana was only just recently removed from the atmosphere when the plant took in CO2. So this represents a fairly "closed loop" and there is no net increase in atmospheric CO2.
Of course, this doesn't take into account any fossil fuels burned in the growing, processing or transportation of the food. But human metabolism of CO2 shouldn't be part of the equation, imho.
That's exactly what I was trying to do. The numbers I used for carbon footprint of food are (I think) based entirely on the production and distribution of the food, not the carbon in the food itself.
I've just had a rather unsuccessful try at semi-defending bolosky's numbers on a discussion over here:
http://hembrow.blogspot.com/2010/09/tank-full-of-petrol-gasoline-if-you.html
The site's author claims he was able to ride his Mango Sinner 235 km in six hours (avg. speed around 24.4 mph) and burn through just 3700 (extra) calories, or about 4.2kWh. (He happens to be a vegan, and can make a lower emissions per calorie claim.) If my math is right, the Roadster, at equivalent speed, would use about 150 wH/mile, 94 wH / km, or about 22 kWh for the same distance. I'm not sure how one can get equivalent emissions even for meat-eating cyclists if the Roadster uses four times the energy for the same distance.
http://hembrow.blogspot.com/2010/09/tank-full-of-petrol-gasoline-if-you.html
The site's author claims he was able to ride his Mango Sinner 235 km in six hours (avg. speed around 24.4 mph) and burn through just 3700 (extra) calories, or about 4.2kWh. (He happens to be a vegan, and can make a lower emissions per calorie claim.) If my math is right, the Roadster, at equivalent speed, would use about 150 wH/mile, 94 wH / km, or about 22 kWh for the same distance. I'm not sure how one can get equivalent emissions even for meat-eating cyclists if the Roadster uses four times the energy for the same distance.
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stopcrazypp
Well-Known Member
Keep in mind, even with the same efficiency, emissions may be different. Bolosky's approach is to measure food production and distribution, so you have to find the relevant emissions for 3700 calories of food. However, I still find it unlikely that the Roadster will do better, since you are doing significantly more work in the Roadster (moving a ~3000lb car vs moving a bike).
You don't do any work at all by merely moving a car, you work when you accelerate it, push it uphill, overcome rolling resistance, and push it through the air. The latter is independent of mass, and you don't necessarily do much of the two former. Also, regeneration can recuperate much of the energy spent on acceleration and differences in elevation, further reducing the impact of mass.
Yes, this may be my mistake. I think bolosky said something about food production emitting roughly 4 times more carbon than electrical production, which could account for the gap. That's a different claim than efficiency of one transport over another.
stopcrazypp
Well-Known Member
It is true for EVs the weight contribution is reduced (and work done overall for a car vs a bike doesn't scale directly by weight), although for regen I think the max you recuperate is ~30%. For the air part, you do more work simply due to significantly more frontal area (although at the speeds a bike travels, the air resistance doesn't play as big a role, and the Cd of a biker probably is a lot worse than a car).
However, at bike speeds, weight does count even when maintaining speed, given a significant part of the work is rolling resistance from tires (chart below from http://www.teslamotors.com/blog/roadster-efficiency-and-range).
I seem remember that cyclists use 80 percent of their energy to move through the air at 20 mph, and 90 percent at 30 mph. But I think you're making a different point.
stopcrazypp
Well-Known Member
I'm talking about the car, but I assumed it might be the same for the cyclist, which doesn't seem to be the case. So it seems drivetrain and tire losses dominate at 20-30 mph for the car, while aero dominates for the cyclist at those speeds.
The construction of the bike may have something to do with it: since the tires are much skinner and there is less weight on them, rolling resistance is likely significantly lower, which lowers the contribution.
The construction of the bike may have something to do with it: since the tires are much skinner and there is less weight on them, rolling resistance is likely significantly lower, which lowers the contribution.
Assume efficiency of 87%. You lose 13% of the chemical energy when accelerating, and 13% of the kinetic energy when regenerating for a "regen efficiency" of 76%. Charging might be slightly less efficient than discharging, but the difference is very small.
So regen removes around three quarters of the acceleration and elevation related disadvantage of the heavier vehicle.
The drag coefficient of a bicycle is very, very poor. This goes for motorcycles too. I have no exact numbers, but I would not be surprised to learn that a very streamlined car had a drag approaching that of a bicycle, even with its much larger frontal area. The Roadster isn't all that streamlined, so I believe you're right in this particular case.
Wow, the rolling resistance is far higher than I thought. You're absolutely right. Interesting. I was going to ask wether rolling resistance is linear with mass, but I checked it up myself, and it is (or rather, linear with the normal force, which amounts to the same thing).
This is what I'm getting at: Lots of people seem to think that if vehicle A is heavier than vehicle B, then A will use more energy. That is incorrect, drag plays a huge role. The frontal areas of the two vehicles may be the same, and the larger vehicle might actually have a lower drag coefficient. The lighter one will obviously be at an advantage in stop-and-go traffic, but with regen, the difference is reduced a lot. Mass is a poor indicator of efficiency in an EV.
This may seem to contradict earlier postings by me where I talk about wanting to conserve energy by coasting. But the drag coefficient is fixed, you can't do much about it besides buying another car or drive slowly, which is boring. The mass related losses, however, can be reduced by the driver, by trying to conserve momentum. You might be able to get 10% more range than you would have gotten if you didn't think about it.
