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The Footprint of Air Pressure: CO2 Comparison

Since Tata Motors announced plans to build an air-powered car, the world has been buzzing about this environmentally friendly, innovative way to propel a vehicle. But is using the grid to compress air and, in turn, run an engine, any more efficient than, say, a hybrid's gas-electric powertrain or even a conventional internal-combustion engine? We crunched the numbers on six vehicles that already have solid environmental bragging rights to find out.

When Tata Motors teasedthe market with a car powered by compressed air, the buzz around the Web was palpable. The momentum continued when Zero Pollution Motors told PM that it expects to sell the world's first air-powered car in the U.S. by early 2010. Now the AirPod, ZPM's three-person working prototype, has made global headlines. The company plans to market the vehicle to tourist destinations in the U.S. So what is all the fuss about an air-powered car? CO2 emissions. Or more precisely—a lack thereof.

The company's name, Zero Pollution Motors, alludes to the perception that its cars run on air and nothing but air—and wisely so, as that's its big sell. A tiny car powered by compressed air probably isn't all that highway friendly, and the AirPod isn't likely to get you much farther than the distance of the average commute (if that). But the prospect of a radically low-emission vehicle is appealing to automakers, given increasing political pressure to reduce emissions of carbon dioxide in a sector that accounts for about 28 percent of all greenhouse gases in the United States. (Only the electric power industry produces more emissions than transportation.) Of course, air needs to be compressed and stored before it can be used to power a car—and that requires electricity. And right now, in the U.S., that electricity comes predominantly from coal.

So is using the grid to compress air and, in turn, run an engine any more efficient than a hybrid's gas-electric powertrain or even a conventional internal-combustion engine? We crunched the numbers on six vehicles that already have solid environmental bragging rights to find out. Note that this is, in no way, a cradle-to-grave comparison. It takes an enormous amount of energy to build a car and all of its components as well as transport that car to its dealer—not to mention to dispose of it at the end of its lifetime. And, let's be clear, the vehicles with conventional powertrains are vastly superior in terms of real-world capability. But there is evidence that the air car, should it make production, might be a superstar when it comes to reduced CO2 emissions. Let's take a closer look.

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According to the Environmental Protection Agency, CO2 emissions from a gallon of gasoline can be calculated by taking the gasoline carbon content per gallon (2421 grams) and multiplying this figure by 0.99 (99 percent of the carbon in the fuel is eventually oxidized, while 1 percent remains unoxidized). Multiply that by the molecular weight of CO2 over carbon (44/12) and you get 8788 grams, or 8.8 kilograms per gallon, equivalent to 19.4 pounds of CO2 per gallon of gasoline.


The CO2 emissions from a gallon of diesel are calculated using the same method as above. Take 2778 grams (diesel carbon content per gallon), multiply that by 0.99 (oxidation rate) times 44/12 (molecular weight of CO2/carbon), and you get 10,084 grams or 10.1 kg per gallon, equivalent to 22.2 pounds CO2 per gallon of diesel fuel

According to the EPA's Emissions & Generation Resource Integrated Database (eGRID), electricity coming from the U.S. grid produces, on average, 1.58 pounds (7.18 x 10-4 metric tons) CO2 per kilowatt-hour.

For simplicity, we assume a 50/50 highway-and-city-mile mix for a 100-mile trip.

The Vehicle:
Zero Pollution Motors plans to launch the air-powered revolution with the AirPod, a tiny three-seater with a 135-mile range. The vehicle has been proposed for use in Paris with its first U.S. appearance in tourist locales. The company is also investigating the feasibility of an on-the-fly rental business plan they call aircar2go (similar to Daimler's car2go rental concept in Austin, Texas). Sure, the AirPod is no replacement for the conventional car, but it's not much stranger than GM and Segway's P.U.M.A. and, right now, it may be the best bet for a consumer air-powered vehicle that we've seen.

CO2 Created:
Unlike Zero Pollution's earlier plans for a combination gas or electric and air vehicle, the AirPod guzzles air only: 4500 psi for a 46-gallon tank. Air-compressed and stored in the high-pressure tank is used to drive the pistons in a compressed air engine.

Based on a typical air compressor from Max-Air, one of the leading scuba compressor manufacturers, it would take 45 seconds to fill up the AirPod's 46-gallon or 6.15-cubic-foot tank. Multiply that by 300 (4500 psi is about 300 atmospheres) and you get 225 minutes. The Max-Air compressor runs on 7.5 hp (5.5 kw). So, 5.5 kw x 3.75 hours = 20.63 kwh. One tank, the company claims, can take the AirPod 136 miles. 20.63 x 100 miles / 136 = 15.17 kwh x 1.583 lbs CO2 per kwh, or
24.014 pounds CO2 for 100 miles.

Bottom Line:
The AirPod is CO2-light, even for a tiny three-seater (although we wonder if the mileage goes down with three people onboard, which effectively doubles the vehicle's weight). But zero pollution? Not so much.

The Vehicle: The Smart ForTwo was long an urban dream machine for those with European sensibilities—and then, in 2007, this easy-to-park, microcar came stateside. The Brabus is a leaner, meaner iteration of the Daimler's Fortwo, starting at $18,000. The Brabus features upgraded interior trim, a more aggressive exterior appearance and improved handling. But the powertrain remains untouched—a 70-hp three-cylinder gasoline engine.

CO2 Created:
33/41 miles per gallon from gasoline, giving an average of 37 mpg.
100 miles / 37 mpg = 2.702 gallons of gasoline x 19.4, or
52.42 pounds of CO2 for 100 miles

Bottom Line:
The Smart is tiny. So we'd guess (and it's only a guess), fewer materials needed to build this small car might mean it has an equally small cradle-to-grave footprint. However, when it comes to on-road emissions, the Smart's carbon footprint is the third largest here.

Reader Comments

12. RE: The Footprint of Air Pressure: CO2 Comparison
Well rob Roberts, you are probably correct, you will not see anything like this in your lifetime. But not for the reasons you think. You think it is because of car and oil companies, when the reality of the situatuion is it's physics that makes these cars impossible. All you need to do is come up with a way to defy the laws of physics, and you can have your car any tmie you want.

11. Air Car Math
#8 -- a gallon of gasoline weighs about 6.25 pounds and has around 170 megajoules of energy (About 47 kilowatt hours). A gallon of air at 300 bar (4500 psi) weighs about 3 pounds (half the weight of gasoline, but has energy of only 0.65 megajoules or 0.18 kilowatt-hours (about 1/250th of the energy content of a gallon of gasoline). #7 -- you are correct, except the AirPod has even less power than you assumed -- various MDI publications have the engine horsepower being somewhere between 4hp and 10hp. The 46 gallon air tank at 4500psi/300 bar holds less energy than does a pint of gasoline. That's why although the MDI/ZPM claims are fabulous, they are also incredible as in "not believable" --- until they actually deliver a product. They have claim each year since 2000 to have a car going into production "next year". The credibility is getting low.

10. RE: The Footprint of Air Pressure: CO2 Comparison
I understand that the compressed air car does not meet its specifications but if it could it would do amazing things environmentally. Electricity can be harvested from almost anything so can mechanical energy. I'm from B.C. and most out power comes from hydroelectric dams. Windmills, waterfalls, rain, all these things can power a pump to compress air. No battery means no big chemical waste pile. The car could be made to be mostly recyclable (like most cars now). But it would put millions of people out of work. It would have most of the advantages of an electric car without the worst part, the battery.

9. RE: The Footprint of Air Pressure: CO2 Comparison
What I worry about is the air conditioning effect as more compressed air is released into the engine, absorbing more and more heat and lowering efficiency as the engine reaches freezing. I'd like a compressed air engine for a bicycle, but not for a car.

8. RE: The Footprint of Air Pressure: CO2 Comparison
How much does a gallon of gas weigh?!?

7. RE: The Footprint of Air Pressure: CO2 Comparison
Think about this carefully and compare apples with apples: imagine you take the dangerously ultra-light, slow AirPod and replace the frankly terrifying 46 gallon compressed-air bomb with a small fuel tank and a petrol or diesel engine with similar horsepower. I'm assuming it's lighter and much slower than the Smart, so let's say 30hp. It's clearly only got scooter tires and two of them are nearly touching so it won't be safe above 50mph anyway. You would be doing much better in terms of emissions by replacing air power with internal combustion! It is the rest of the car which saves the environment, not the air power, but a 30hp diesel micro-car would not sell well.

6. RE: The Footprint of Air Pressure: CO2 Comparison
It's sad that you don't give these guys enough credit. They are taking a bold new step in the automotive world. What about the energy it takes to get oil out of the ground, then refine it into gasoline, or diesel, and the finally to transport it, resell it, and finally pump it into your should include that energy also, for a true comparison!

5. RE: The Footprint of Air Pressure: CO2 Comparison
The better way to make compressed air could be to use windmills. They really do work well for compressing air.

4. RE: The Footprint of Air Pressure: CO2 Comparison
Hybrids with batteries make me laugh. Pure battery powered cars make me laugh even more. Batteries are nasty to manufacturer, I know this first hand because I have had several friends that have made lead-acid, Ni-Cd, NiMH and Li-ion. The environmental impact of those factories alone was massive. I used to work at a facility that made Ni-Cd and NiMH and it was a superfund site. For those of us who use power tools and laptops we freakin’ hate batteries. They lose capacity and will not fully charge after some months, regardless of how well you take care of them. I have been in manufacturing all of my career and contrary to popular belief any company will change it’s product line if the new product is more cost effective and profitable when the change over costs are factored in. Any thought to the contrary is just lunacy.

3. RE: The Footprint of Air Pressure: CO2 Comparison
Add availability of aftermarket solar panels to power small compressors. Brillant. Can't wait to own one.

2. RE: The Footprint of Air Pressure: CO2 Comparison
The most glaring error in the analysis is the assumption that the range of the AirPod is indeed 136 miles. So far the only demo run of an MDI/ZPM car had the car running out of air after only 7.22km. That was a CityCat car rated for 120+ miles / 200+ km. If you want to calculate just using paper specifications that have never been tested, use the Magnetic Air Car or the Air4Zero specs. They claim infinite range and 100,000 mile range. Just like MDI/ZPM, their prototypes have never been independently tested.

1. RE: The Footprint of Air Pressure: CO2 Comparison
We can continue to invent all types of hybrids over an over again. The problem is the auto makers and there dealers. Everyone is afraid to change because is will put so many people out of business.....such-as parts....after market suppliers....and even salvage yards. Its those people that also refuse to change. We should be driving cpu cars by now.....with gas only being used for heavy equipment....but with all these people refusing to change....we are stuck in the 1980's with oil the most powerful market....and every follower down the line. I love to see a new hybrid come-out....but at age 47 in 2009....I know I'll never get to own one in my life time. Rob Roberts Roanoke Va
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