About Coastdown Testing
Wednesday, August 25, 2010 at 06:40PMThe engineering department at Edison2 is pleased about the informed and civilized discussion on car efficiency that we see on our blog. A constant strand in the discussion is: where does the crossover come between rolling resistance and aero drag?
The procedure used by the automotive industry to determine resistance to motion is the coastdown. There’s an SAE standard for how to do this but, broadly, the car is either driven, pushed or pulled up to a certain speed, usually a little over 70mph, after which it is put into neutral (or released) and coasts until the speed drops below 10mph. Speed and distance during this are recorded very accurately by special purpose instrumentation. Some checks and balances are applied to make sure the results are good: it’s done multiple times in both directions on a straight road to make sure the data is consistent and to cancel out the effects of wind and gradient. Analysis of the recorded speed and distance information yields the car’s drag “fingerprint”.
Although it’s obvious that the car’s speed will decay as it coasts, this does not happen at a constant rate because of multiple different components of the total resistance to motion. The drag that causes the car to slow down fits very precisely an equation of the form
In the equation, drag is in pounds and x is speed in miles per hour. The equation’s A term is a constant value representing static drag: it takes this much to make the car move at all and this drag is always present regardless of the car’s speed. The B term changes linearly with speed and is best thought of as mechanical drag, such as bearing friction. The C term is aero drag and it varies with the square of speed.
Thousands of these tests have been carried out over the years and there is vast accumulated experience showing that a car’s total resistance to motion very closely follows the simple three term equation. In fact, this method and analysis are so well accepted that A, B and C numbers for many current cars are available on the epa.gov website.
The Very Light Car’s A, B and C numbers are 6.31, 0.1862 and 0.00433 respectively. Plotted on a graph against speed, they look like this:
Inspection shows the static drag figure dominates at low speed. Static drag is in large part due to tire deflection caused by car weight and consequently the Very Light Car does extremely well. From numbers Edison2 obtained from the EPA, the corresponding figure for a 2 wheel drive Escalade is 33.61, more than 6 times as much.
The B value, mechanical drag, is similarly low because of the very careful attention we gave the VLC’s mechanical design. The corresponding Escalade figure is 1.0442, more than 5 times as much as ours.
Aero drag (the C term) becomes important above about 40mph and above about 70 it is higher than static and rolling drag combined. That this only happens at such a high speed is testament to the extremely slippery shape devised by Barnaby Wainfan.
It’s interesting that at about 40 mph, static, mechanical and aero drag are close to equal. Even with our particularly light and efficient car, if aero drag were zero, at 40mph there would only be about a 30% reduction in power required and therefore fuel consumption. Low aero drag is important, of course, but so is mechanical efficiency. The VLC has them all.
We would like to emphasize that these numbers are not guesses, estimates, projections or simulations. They are measurements taken in accordance with a recognized SAE standard by experienced and competent people. Edison2 was told by the test facility (Chrysler Proving Grounds, Chelsea, MI) these are the best numbers they have ever seen, by far.
In these measurements are the key to getting acceptable range from an electric car. We’ll be writing more about that in a future post.
Reader Comments (60)
The carbon footprint of electricity is far lower than the carbon footprint of gasoline. No laws of physics are being violated in the EPA or in the X-Prize. They measure the energy used by the car per distance traveled.
If you go back to the generation phase for electricity, then you must also do the same for gasoline -- fair is fair.
Sincerely, Neil
john,
I am talking about a water box the size of about a tenth of the North American continent. Big thinking is needed here if it has any meaning in respect to the volume of CO2 produced by coal fired power plants.
By the way, a demonstration project exists in the 300 mile California Aquaduct that made our California central valley into one of the most productive regions in the world.
Neil,
Just because they say "kWhr" it does not mean they are measuring the energy used.
Compare point of heat production from fuel or go further upstream and charge any reasonable for processing of gasoline or coal, and you still get an answer that is vastly different from the Xprize scoring.
I know it is annoying, but a kWhr is not necessarily a kWhr.
More Spam!
Nobody is arguing for coal -- we simply must move with all possible speed to renewable energies, and set it up like Germany has, with a mix of sources over a wide geographic area, with some storage/buffering capacity.
@Jim a kWh is surely a kWh. How much energy and how much carbon that went into that kWh is what is changeable. Please go back to my earlier post; where I list the major stages for production of petroleum fuels -- there is a lot more embedded energy and carbon "contained" in gasoline/diesel that there is in electricity. Add to that that ICE engines require lubrication maintenance, and all these consumables must be accounted for, when powering a car with an internal combustion engine.
Sincerely, Neil
Neil,
Now we are getting somewhere. A kWhr is not a kWhr, unless it is a measurment of the same form of energy or in certain kinds of conversion.
Specifically, a kWhr of heat will never get you anywhere close to a kWhr of electricity because of the Second Law. Do not confuse the fact that a kWhr of electricity will get you a kWhr of heat when converting from electric energy to heat with the fact that the equal sign does not hold in the other direction.
The heat engine is by far the biggest loss in any system, so it is cheating to try to pretend that the heat engine that is used to generate electricity in a central power plant does not have to be counted. It is such a big cheat that it should be a felony.
Jim,
A kWh doesn't apply to heat, as far as I know. A kWh of electricity is the same anywhere and anytime.
You seem to continue to miss my point: what is being measured by the X-Prize (and the EPA) is the energy used per mile in a particular vehicle. If you insist on including the energy embedded in one case, then you must also include the embedded energy in all cases. That is the scientific way to do it.
You are correct that a kWh of electricity at the plug represents much more energy at the source -- and the point I am trying to make is that a quantity of liquid fuel represents much more energy than it physically contains; and if you include the embedded energy all the way back to the source for one, then you must in all fairness, include the embedded energy all the way back to the source for the other.
Source-to-wheels for one, then source-to-wheels for all.
Sincerely, Neil
I insist on including the input heat energy to either arrangement, expressed in BTU or kWhr. That will change things enormously compared to the way Xprize does it. And BTU is only a "British" speaking unit. We skipped that when we supposedly switched to the metric system. The rest of the world use 'kWhr' without hesitation. And most of them get tricked just like you and the Xprize judges were; not to feel bad, so did the EPA, Argonne, UK DOE and Climate Advisor Dr. David MacKay, Google Chargit Project, Google CEO Eric Schmidt (advisor to US Pres.) John Doerr (apparently), Tesla, Fisker, to mention some off the top of my head. (Some of these probably know better but are ok with sucking in the gullible green public, trying to look good to their admiring wives and daughters, but are fully aware that coal will be the fuel.)
Kevin and I seem to be the last ones standing.
If you want to include heat energy needed to transport and process gasoline, fine, and we will also include the energy needed to transport coal and transport electricity over power lines. This makes the calculation more indefinite, but it will not change things much; the difference from simply including the heat engine is the most important loss, by far.
All the stages of the conversion process count, but the heat conversion far exceeds each of the various losses in the system which are more limited by the First Law of Thermodynamics. The Second Law knocks everything else into the neglible category, well almost. Remember, just converting a BTU from coal into electricity throws away 71% for the USA average and about 60% for natural gas, depending a lot on market price variations from year to year. Both then suffer another throwaway of 7% of the electric power input to the distribution system. To be exact, I should note that the loss in the power plant includes the electrical generating machine loss, which is probably quite small, but I have no data on that. Look at the analysis page at miastrada.com to see a fairly well documented analysis based on hard data from EIA.
You know what guys...I think the endless debating (which I've been a part of) about the XPrize MPGe rule distortion has gone past the point of being productive. I'm confident that I understand the issues pretty well technically and practically, and that the X-Prize did not do the best job on this issue...but it is a done deal There are zealots on many issues that either don't wish to hear "truth" or don't have the technical skills to understand it. I'm hearing people "preaching" to me because they think I'm supporting the status quo, when I was only pointing out that EV's don't (today) get us off of fossil fuels, or even provide better overall energy efficiency or significantly lower CO2. My point is not anti EV, but to suggest that our higher priority should be to get power plants running off of renewables, and in the mean time just get cars as efficient as possible (whether EV or not). The XPrize has been good for this.
Here's someone who really "gets it". I don't agree 100%, but let's say 95%: http://twilightearth.com/environment-archive-2/electric-cars-run-on-coal-are-they-cleaner-than-gas/
Also (Neil) - you've given so much good discussion and friendly support on many XPrize and Edison2 blogs - I wanted to thank you. I started doing a little research on "Carbon Footprint" of gasoline (and other fossil fuels) vs. electric power (using the current grid's energy sources), since that is not a subject I'm very knowledgable on and my intuition told me otherwise. I wasn't sure about the "10X carbon footprint" you mention for gasoline vs. electricity. As it turns out, sorry buddy,10X isn't close to correct. More like 1X to maybe 1.3X, because fossil fuels (coal + natural gas) still represent ~75% of the grid power.
see: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V2W-4RWB115-4&_user=10&_coverDate=04%2F30%2F2008&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_searchStrId=1447055868&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=dc43841143e403219edd315133273469&searchtype=a
and
http://www.yaleclimatemediaforum.org/2009/09/hybrid-all-electric-vehicles/
As you can see - EV's carbon footprint "advantage" is slim enough that it actually can swing positive or negative depending on which state you live in. In other words - close to a wash...but is a bit better.
Again - not to detract from EV's....but we shouldn't fool ourselves that they are the solution to our energy crisis and global warming. Until the power plants are greened-up, EV's only help a little bit, but have some advantages and some strong niche markets. Once the grid is greened up (or if you charge off of your own solar panels) - then EV's become very attractive indeed.
Kevin
I fully agree that EV's are not a panacea -- but they are lower carbon footprint, even with the ~52% coal generation we have on average here in the US. The key thing about EV's is that they make it possible to go even greener over time. And as individual cars, they are by far the most efficient.
And since the pollution is concentrated in far fewer places, their pollution can be cleaned more easily. This also concentrates the responsibility on all of us as a group; and the more we are aware of our consumption and the more we take responsibility for it, the better.
We need to not let perfection be the enemy of the good.
I'll take this moment to mention the most important book I have read, and I hope you can all read it: "Eaarth" by Bill McKibben. We live on a planet together, and together we have already changed it significantly, and the best outcome we can achieve is to limit the damage we have already wrought -- which has already started the momentum of damaging effects.
http://www.billmckibben.com/eaarth/eaarthbook.html
Put it on reserve at your local library, or buy it and then pass it around to your friends; or donate it to your library.
Sincerely, Neil
Kevin and Neil,
I guess the consensus here is that electric vehicles might be good some day, and that we should drag the coal companies out for a good flogging so as to speed along the day when the developed world is running on windmills and mirrors, or stuff like that. That seems to be the level of Hansen and McKibben, well intentioned that they clearly are.
Well, I am in on the first part only. Giving up the developed world seems like a hard task, and it looks to me like scaling up the ideal ways to produce energy is not going to even come close to meeting our needs. Not only do I see a problem with CO2, I also understand, I think, that the present developed world is about to become the colony of the emerging new economic powers. And the economic trend may happen faster than the GHG problem. So making foolish decisions about energy might lead us to a situation where the economy really fails, and then all the talk about the environment will certainly fall flat. So I look for better answers, and standing in the way are some really wrong answers.
Right answers include high aerodynamic efficiency for cars and trucks, efficient wheels, cogeneration based on vehicle engines, insulation, and massive scale water distribution.
Wrong answers are energy guzzling electric vehicles, central power plants, bluff body vehicles carrying double wide seating, drivel sustainable solutions that require subsidies to even get a few examples built, waste of natural gas in low level uses like running trucks and generating electric power, and of course, the absurdly expensive 'carbon' capture concepts now in the works.
And here we can't even get to the point that electric power will come from coal as long as that is the cheapest alternative from available capacity, somewhere.
I think electric vehicles are plenty good, right now. They will/can get better in the future. We can conserve enough to more than offset the energy to run the initial EV's, and lower our demand for electricity, to boot.
Sincerely, Neil
Neil,
You do not conserve energy because you move the heat engine away from the car with an electric system.
You might get our government to make the rest of us pay for electric energy for you to use, but that does not really qualify as conserving energy.
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Hi Jim,
There is lots of heat and energy added to gasoline "away from the car" that is not actually contained in it.
And who do you think heavily subsidizes the whole petroleum system? Who' military spending and who's foreign policy is heavily involved with supporting and paying many hidden costs of the oil system?
http://neilblanchard.blogspot.com/2010/09/oil-is-finite-electricity-is-infinite.html
Sincerely, Neil
PS Note to site admin: the Spammers are always at it! :-(
Yup, Neil, there is a bigger picture but that is not called energy efficiency. But these are appropriate to consider separately. It is not helpful to claim that definitions known in the field of physics should be distorted to reflect all the big picture issues. And MPGE pretensiously is used as representing fundamental energy efficiency.
If you are proposing that we would close down enough of the military establishment to make room in the budget for wind and solar, we might consider that, but I suspect this would not get very far.
Jim,
Why is electricity generation and distribution different from gasoline production? They are both source-to-car energy. The energy stored in the vehicle is the energy stored in the vehicle.
So, either you consider just the energy carried in the vehicle, or you include all of the steps it takes to produce the energy. You cannot be fair if you include the source-to-wheels for one, and only vehicle-to-wheels for the other.
The MPGe includes energy carried on board the vehicle to the wheels for any type of energy. It is completely fair and logical, and it breaks no laws of physics.
I am not saying that we would divert money from the military budget to pay for renewable. I'm saying that a fair part of the military is a hidden cost that should be included in the embedded energy of OIL.
Sincerely, Neil
To give some perspective on the huge CO2 problem, and why we need to switch to zero carbon EV solutions:
Every year the United States emits a 33.14cm (1.1foot) high blanket of carbon dioxide over its land area.
A New Beetle, driven 12,000 miles, will create enough CO2 emissions to fill the volume of the Washington Monument three times --- That monument was among the tallest buildings in the world at the time it was built, and I believe it still is the tallest masonry building.
A VLC with an IC engine might burn only one third as much gas, but its CO2 emissions would still fill the entire volume of the Washington Monument in just 12,000 miles of driving.
With global CO2 levels of 392ppm and climbing, we need to go to zero CO2 emissions, and plant billions of trees to recapture carbon and bring CO2 levels back down to 350ppm or lower.
GW Climate disruption is not the only worry. Ocean warming has reduced phytoplankton about 1% per year, for a total 40% loss since 1950. Phytoplankton is the first rung of the ocean food chain, and it also produces half the oxygen we breath.
The following is from the CBE CarbonDatabase:
CO2 Volume Calculation
Volume calculation of one metric ton CO2
One ton = 1000kg
One cubic meter = 1000liters
One mole CO2 = 44.0g (CO2 = 12.0g + 32.0g = 44.0g)
One ton contains 22730 moles of CO2 (1,000,000g / 44.0g/mole)
One mole is 24.47L (Boyle's law at 25°C and 1 atmosphere pressure)
Volume of one ton CO2 = 22730moles × 24.47L/mole = 556200L = 556.2m³
One ton of CO2 occupies 556.2m³ of volume.
Height of CO2 on US land surface
US 1997 CO2 production = 5,456,000,000ton
US volume of CO2 production = 5,456,000,000ton × 556.2m³/ton = 3,035,000,000,000m³
US land surface area 9,158,960km²
Height of CO2 on US land surface = CO2 volume / surface area = 3,035,000,000,000m³ / 9,158,960km²= 33.14cm (about 1.1 feet high)
Every year the United States emits a 33.14cm high blanket of carbon dioxide over its land area.
Volume CO2 from one gallon of conventional gasoline
Gasoline density = 2791grams/gallon
Percent carbon by mass = 85.5%
Mass of CO2 from 1 gal of gas = 2.791kg/gal × 85.5% × (44.0g CO2 / 12.0g C) = 8.750kg
Volume of CO2 from one gallon of gas = 8.750kg × 556.2m³/ton = 4.867m³
The combustion of each gallon of conventional gasoline produces 4.867m³ or 171.88ft3 of CO2.
CO2 and the Washington Monument
Volume of the Washington Monument = 22026ft³ = 623.7m³
Gallons of gasoline needed to fill the Washington Monument with CO2 = 623.7m³ / (4.867m³/gallon) = 128 gallons
The New Beetle can run about 4000 miles on 128 gallons.
A New Beetle, driving 12,000 miles, will create enough CO2 emissions to fill up the Washington Monument three times
CO2 emission data from USEPA and land area data from CIA
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Battery charging is never 100% efficient.
Charging efficiency is typically around 80%, even worse in the case of a fast charge.
For any efficiency contest, this internal charging loss is a part of the total energy system ( from the coal mine to the end use at motor) and should be included in the energy conversion calculation.
Yes, I agree Bill - but gasoline doesn't appear out of thin air, either. In fact, it takes about as much electricity to make a gallon of gasoline as it would take to just drive an EV as far as an average car could go on that gallon. It also takes a lot of natural gas and a lot of water for many methods of getting oil and gas out of the ground, so the energy used to get those *also* needs to be added to making gasoline. And ethanol is possibly a negative energy net.
Electricity *can* come from several different renewable sources, so the carbon overhead for this is very low and will get lower and lower as time goes on; because you use renewable energy to make each generation of renewable energy system.
A typical plug-to-wheel efficiency is ~85%, which still is at least 2X as efficient as the best ICE is pump-to-wheel, and is about 3X more than a typical car.
Neil