The Air car

[grimpixie]

I came across this site in my random musings across the Internet: http://www.theaircar.com/index.html

…an engine capable of propelling a car up to 110 Km/h, that can cover a distance of 300 km with one tank refill and a cost of less than a cent per kilometre. Not only providing “Zero pollution” but also purifying the air.

Has anyone heard of this before? Clearly it works, but is it feasable as an alternative to petrol/diesel driven vehicles? Obviously long haul vehicles are out of the question, but within city limits, it has to have a chance…

Incidentally, is there any way of working out the damage to the environment that generating the electricity needed to drive the compressor would do?

Grim

[scotth]

*Originally posted by grimpixie *
Clearly it works

Hmmm… this is the first statement that needs examination.

I would say that it is quite likely not to work.

There is no way the car could be moved 300km on the energy contained in 90 cubic meters (90,000 liters) of air compressed to 300bar.

I have hard time believing it would be enough energy to push a car 10km.

Lets do some real basic and very optimistic guesses about the power available.

1. Assume a compressed air engine of say a half liter displacement using a two stroke cycle (as you would in a compressed air engine)
2. Assume the cylinder has 5 bar of pressure in it at the end of the power stroke. With a half liter displacement that means we use 12.5 liters of air per stroke or revolution.
3. At a modest 2000 rpm, this give the engine power for a whopping 3.6 minutes (actually less).
4. Assume compression ratio of 10 to 1 for a starting pressure of 50 bar in the cylinder and an average pressure over the duration of the stroke of about maybe 13 bar?
5. Assume a “square” layout and you get a piston diameter of 8.6 centimeters and stroke of 8.6 centimeters or 3.39 inches
6. This gives a piston area of 27cm^2 or 4.19 in^2.
7. This gives an average force from the piston (4.19 x 13 x 14.7) of about 800 pounds.
8. This needs to be turned into rotational torque. .5 x .5 x 3.39 x .637x 800 (power for half a rotations x half of the length of the stroke in inches x peek to average conversion factor x average force) gives 432 inch pounds of torque or 36 foot pounds of torque.
9. 36 foot pounds of torque at 2000 rpm delivers 13.7 horsepower.

That is a whopping 13.7 horsepower for 3.6 minutes.

Looks really unlikely to me. I don’t think I could design a car that would make it 300 km on the energy of 13.7 horsepower for 3.6 minutes.

[scotth]

BTW, I assumed 100% efficiency and there is no way their little tricks they are pulling with their design can exceed 100% efficiency.

And phew… that was a lot of work to answer a GQ post.

[scotth]

More grim news about the maximum amount of energy available in the air tanks.

The onboard air compressor can refill the tank in 3.5 hours and uses a maximum of about 4500 watts. This means you have at 100% efficiency going in and going out an absolute maximum of 4500 / 746 (watts per hp) * 3.5 = 21 hp hours.

That means you could get 21 hp for an hour or 11.5 hp for 2 hours. That is much more generous than I have estimated above, but still pitiful.

For comparision 4500x3.5= 15750 watt hours of energy or 55.7k Btu’s. That is about equal to half a gallon of gas.

As I assumed maximum values for everything and 100% efficiency, realistically I would expect the system to use on the 2500 watts at maybe 60% efficiency. Total energy stored about 0.15 gallon of gas equivilent. Or about 6.3 hp hours if the stored energy was used at 100% efficiency.

This still isn’t gonna work.

[toadspittle]

Go to http://www.bellwetherinteractive.com/mdi/index.html

It gives detailed specs on engine CCs, horsepower, etc.

From what I’ve read elsewhere, the main drawback seems to be that the thing is comparatively loud (esp. since the body is necessarily lightweight … the modern vacuum syndrome).

Incidentally, is there any way of working out the damage to the environment that generating the electricity needed to drive the compressor would do?

No different than any ordinary electric car, really. And like one of those, the main advantage is a reduction in local pollution. Other benefits include low fill-up cost ($2 per tankful, I saw on the aircar site) and, as an aside, the company’s decentralized factory model–they’re trying to encourage developing nations to set up lots of small, local factories to build the (comparatively cheap) cars, instead of the big factories most car co.'s have now.

[scotth]

Again, the huge problem is that they “claim” this huge range, but I would put a large sum of money on the fact that they haven’t built one and much less driven one anywhere close to 300 km on a single tank of air.

[toadspittle]

More stuff:

http://www.spacedaily.com/news/020928042702.fkqpd20t.html

Others have doubts; though there appear to have been test-drives, it doesn’t seem that anyone’s actually sat in the thing for a full 120 miles.

What do you make of this, scott?

Underneath the chassis are three long carbon-fibre tubes that contain 300 litres (75 gallons) of air pressurised at 300 bar, or about 150 times the pressure of the average car tyre.

The 800-cc, 25-horsepower piston engine works by taking in outside air and compresses it to 20 bar, which heats it to 400 C (752 F). A squirt of pressurised air is then injected, forcing the piston down and the crankshaft around.

When the car brakes, the kinetic energy from braking is used to drive a pump that helps to restore some of the lost pressure. Unlike conventional cars, the engine does not operate in traffic jams, which thus saves on fuel.

It’s got some regenerative braking, which’ll stretch the mileage, but how the heck does the engine itself work? Para. 2 up there throws me. Can you decipher?

[toadspittle]

More (incl. a diagram):

http://www.wheels24.co.za/Wheels24v2/ContentDisplay/w24ThirdLevel/0,5684,1369_1235891,00.html?NewsPath=1

But, what’s the catch? Does the engine actually work, and on what principles? These are the questions on everybody’s lips.

Without going in too many scientific details, it could be explained as follows: (refer to the diagram).

• An air tank, similar in concept to that used by a deep-sea diver, is filled with normal air to a pressure of 300kPa. The pressure inside the tank forces the air into the first piston of the engine, and is allowed to expand.

• The piston then works on the same principle as an internal combustion engine. During the expansion process the pressure is reduced and the volume of air increases.

• This lowers the air temperature to below the atmospheric temperature. A heat exchanger in the connection between the pistons returns the air temperature and the process is repeated in the second and third cylinders.

The MDI e.volution concept manages to keep adding heat to the air by means of heat exchangers, which returns the effectiveness of the exchange process to the ideal.

According to the designer the air is recycled for as long as possible in order to maximally increase the range of the vehicle.

[Ethilrist]

Say, how explosive are those air tanks they expect me to park my butt on? I’d hate to hit a speed bump and have the car disintegrate…

[scotth]

Most of what they say in the “how it works section” is gobblygook that extremely similar to claims made by people selling free energy devices.

The compressed air power car is almost exactly the same as a steam engine powered car. The difference, steam powered cars would make the expansion gas from a liquid with heat. Here, it is just compressed air.

Without even examining the machine, we can rule out greater than 100% efficiency as it defies the laws of physics. The amound of compressed air that the car carries is clearly inadequate to do the job. It simply does not contain enough stored energy. A typical 12 volt car battery (one) would hold a similar amount of energy.

The idea of compressing air into the cylinder before letting more air in from the tank is completely useless. You get back exactly the same amount of energy as the extra gas in cylinder expands as you used compressing it. But, it is very typical of things seen in typical scam devices include to throw off someone not really up on how the world works.

[scotth]

*Originally posted by Ethilrist *
**Say, how explosive are those air tanks they expect me to park my butt on? I’d hate to hit a speed bump and have the car disintegrate… **

The claim their tanks are safe, but I would much rather have a tank of gasoline under my butt.

300 bar is 4410 psi of pressure. A sudden failure will release that pressure explosively even if the tank doesn’t contribute shrapnel.

[scotth]

Really interesting stuff in the article:

"Some of the manufacturers have had a look at the engine, but so far nobody has offered to buy us out.

I can’t imagine why not, could it be because it won’t work?

At present the vehicle is provided with two 300kPa tanks, each supplying two cylinders, and can travel about 200km or 10 hours at an average speed of 53km/h, without any air pollution.

This doesn’t add up… 200km? (the designer says 300km) or 53km/hr for 10 hours (530km’s if I can still multiply)

[scotth]

Hah, here it is: a quote by a physicist in toadspittles second link.

Guenter Hoermandinger, a physicist by training and former consultant on clean cars who is now a transport specialist at the European Commission, says he is a doubter.

Hoermandinger said he wished MDI well, but his personal view was that a range of 200 kms (120 miles) “is out of the question, forget it. If it’s 10 kilometers, it’s a huge success.”

This was based on a “back-of-an-envelope calculation” that the amount of compressed air could never deliver enough energy to drive a car the claimed distance, he said.

This looks just like what I figure in my first post. Even the 10km figure is the same.

[grimpixie]

Thanks for the comments guys - just a pity that it doesn’t seem to live up to its promises…

Grim

[Mangetout]

*Originally posted by scotth *
**The idea of compressing air into the cylinder before letting more air in from the tank is completely useless. You get back exactly the same amount of energy as the extra gas in cylinder expands as you used compressing it. But, it is very typical of things seen in typical scam devices include to throw off someone not really up on how the world works. **

This is actually quite funny - the work required to compress the outside air must be coming from the momentum of the vehicle, so the compression cycle slows the vehicle down then the expansion cycle speeds it up again (but you get back less than you put in because of various inefficiencies).

I have a better idea; my car will be driven by an electric motor; the motor will be powered by a dynamo attached to the driveshaft

[scotth]

*Originally posted by grimpixie *
**Thanks for the comments guys - just a pity that it doesn’t seem to live up to its promises…

Grim **

What’s really a pity is that this is such a poor idea (or outright fraud) and it is obvious that it is sucking in a lot of money from alot of sources.

[Bongmaster]

Great thread here guys, thanks for all your hard work and caculations on this. I see stuff like this a lot, and its great to be able to seperate the wheat from the chaff.

[Mars_Horizon]

Someday they will make their air cars out of gleaming alloy - almost two lanes wide.

[RiverRunner]

*Originally posted by Mars Horizon *
**Someday they will make their air cars out of gleaming alloy - almost two lanes wide. **

You can still leave them stranded at the bridge. You get a great bass solo, though, if I remember correctly.
RR

[Sam_Stone]

Next up… The Moller Skycar!