Air Motors for Automobiles

Hello PAPADOC,

The standard gasoline motor is basically a piston air pump/motor that makes its air pressure in the cylinder. As inefficient as it is the power density is hard to beat.

An air motor to get around the neighborhood would be fun but the cost to compress the air to run it is very prohibitive. Your standard shop 60 gallon shop compressor would have a hard time getting a golf cart around the block.

Your gas cylinder on the vehicle would either be huge or a heavy bomb.

The whole idea is doable but not cost effective. Just not enough power to be useful. I did entertain the idea of using explosions to pressurise my cylinder (External combustion) but still not cost effective (or really safe)

Brad

Papadoc,

Energy/volume @ 3000 psi (tank included): air, battery, hydrogen, CNG is 1:10:20:100

Energy/mass @ 3000 psi (tank included): 1: 10:160: 800

This tells you that hydrogen is 2x-16xtime better than a battery.

A 16 kw-hr battery (like that on a GM Volt) has .44 gallons of gasoline energy (16kw-hr/36 kw-hr/gallon=.44gallons). The volt will go about 120 mile/gallon, or about 40-50 miles on that charge. That battery weighs over 400 lbs. 1 gallon of liquid gasoline weighs about 8.5 lbs if you include the tank. CNG is 12 lbs/gallon. Get the picture?

As you can see a battery is low/volume and very bad/mass, but compressed air is...off the chart low.

Basically CNG is the only thing gaseous that works until you can get the engines up to a constant 60%, instead of the typical 15% (4xs ratio). After the 60% point CNG and hydrogen start to make sense and cents.

Moreover, staging of gas is very difficult from 3000 psi to ambient. Volumentric loses will kill you, along with adiabatic. There needs to be an isothermal exspansion as any rate. Figure that out and you might be on your way to an air motor of sorts...

seaplaneguy

Papadoc,

In my narrow view of life, EVERY car will be an "air motor" as every IC engine NOW is an "air motor" of sorts. Combustion requires compressed air to work. If you notice the fine details of the MDI car glossed over by the media reports (read their patents...), you will notice them adding heat to the mix.

The key to air motors is isothermal expansion and compression within the system at some point depending on the power needed, type of heat source, temperature of the heat source and relative temperatures of the working gas (air). The MDI car captures ambient heat by the fact that air will cool some amount when it expands, and this limits the (peak) power delivery to inefficient use.

We as engineer are mostly realizing that any engine system MUST have a leveling capacitance with some ability to deliver large AND small amounts of power efficiently. The realities of the needs of combustion need to be considered. A Prius does this by running the engine where it if most efficient and storing it in batteries. Production of energy and delivery of energy MUST be decoupled, as each has different needs and time realities. It is a matter of duty cycle.

When you follow the logic, you will find that air as a storage device for energy is needed if you want to work in the thermal combustion domain. The Prius does not consider these requirements and therefore is limited. Cars are thermal envelopes that require energy to maintain room temperature where a person is comfortable. Electrics have a large disadvantage because they are not in the thermal domain, and total energy, when environmental control inside the car is considered, will be larger for electric hybrids than for other "hybrid" engine systems.

Take the time to understand the thermal loads needed to keep a person comfortable in a car in extremes of heat and cold, while keeping the air fresh and the windows unfogged. When you make a low weight car like an Aptera (www.aptera.com) you will find that thermal loads are 50% of load. You need an air conditioner...which is a thermal cycle. You end up adding in an "engine" subsystem just to make the cockpit comfortable, or using thermoelectrics which are typcially 10% efficient (ouch). If you stay in the thermal domain, you can combine the engine and cooling into one system, and thereby lower weight...and be ahead of electrics...hence using air...

An air motor can be many times lighter per power than an electric motor, and be made of simple materials without complex computers. When a battery weighs 400 lbs and only has .44 gallons of fuel in it, ...we have a problem...a big fat one.

To move to alternative energy supplies, and "save the planet" as global warming religionist would have you believe, we need LARGER ranges to cars, and the more "alternative" the fuel is, the longer the range needs to be. Why? Time! Time is money...

CNG cars should have at least 800-1000 miles of range because people don't have the time to go out of their way to find a station in some back alley way every day. Also, to get across the USA you need at least a 550 miles range. 200 miles found in a typical CNG car is a joke! If you want to run hydrogen, you need 1500-2000 miles range because you have to go out of your way eveny more than CNG by 10 times usually to find it and the time costs will kill you, along with the fact that hydrogen will leak out over time..., but since hydrogen takes five times the volume as CNG...again we have a problem here...

In short it will be interesting to see how it all plays out. Air will be part of it...

And, there are air motor cars being built today.

That's a little misleading. There are prototypes, but no production vehicles. Although MDI has claimed 120 mile range, they have never demonstrated range beyond 4 miles. I am one of "those who discount the air motor" as a realistic means to propel a car, partly because, in industry, air power is considered the least efficient way to power things, with the most obvious problem being the physics of compressing air. The heat given off by a compressor is waste heat. The other obvious problem is that air power adds to the number of energy conversions, and each energy conversion has losses. So one would be far better off to use the energy that drives the compressor more directly, rather than incurring those losses. Liquifying air adds even more losses, with additional energy required to refrigerate the air.

There is nothing extraordinary about the MDI car, (other than the fact that the company has gone on for so long without any demonstration that their claims are valid). Air vehicles were used in the underground mining industry before explosion proof electrical equipment became common place, but have been replaced by electric vehicles in the interests of efficiency and simplicity.

The fundamental challenges with making air cars works efficiently are:

• Compressing air without incurring very large losses
• The ultra-low energy density of compressed air.
  
• The dangers of handling and storing air at the ultra high pressures required for anything remotely close to reasonable range.
  
• The large number of energy conversions: If power comes from the grid, then you might as well charge batteries, given that you've already lost about 70% of the original coal (grid mix) energy, rather than incurring further very high losses. If the compressor is powered by gasoline or diesel, then you'd be many miles ahead to just use that engine directly: use a gasoline of diesel engine to power the car.
  

Most of these problems have to do with fundamental physics that are not likely to change. If you are aware of an internet site which promotes the air car concept and which deals specifically with each of these issues (and which, for example, provides the actual energy content of the air in the tanks in units that provide for easy comparison) then I'd be interested in links to those sites.

This link is to an earlier thread, and contains a link to a yet earlier one.

I agree with a great deal (in fact virtually all) of what you say.

I am fundamentally an optimist (and partly as a result of that, have been an entrepreneur for most of my life). I find that I am becoming increasingly skeptical, however, particularly regarding energy usage issues. The plug-in hybrid I am developing obviously is intended to run on electricity from the grid most of the time, and gasoline only when you need extra range. Neither energy source is fundamentally evil, nor spotlessly clean. The current spin on electric vehicles, however, is that they are "zero emission" and "green." This is only partly true: The nationwide average efficiency in generating electricity is roughly 33 - 38%, not including line losses. (This figure varies with information source, but it is in that general range.) There are many, many, facile counter "answers" to the contention that electric vehicles are not a panacea. For instance, many people maintain that controlling pollution at a single source is easier and more effective than controlling it at many. Such statements get repeated so many times that people begin to believe them without thinking. Regarding the regulated car emissions, HC, NOx, and CO, there really is nothing difficult about cleaning up these emissions at each car, and in fact, the NOx limits are lower for gasoline-powered cars than for for electric cars fed by fossil-fuel power plants (in other words, an electric car of equal mass creates slightly more NOx at the power plant than a gasoline-powered vehicle does at its tailpipe. The story on CO2 emissions is fairly similar, with a Prius -- which, granted, has an unusually efficient engine -- generating somewhat lower CO2 emissions per mile than a comparable electric car does at the power plant).

None of which means that electric cars are not certainly the way to go. But the knee jerk "electric vehicles good, gasoline vehicles bad" is the same sort of thinking that got us (in the US) into spending lots of money on hydrogen-powered cars and ethanol-powered cars before really thinking about their overall energy usage.

Air car hype is similar but more severe. They are portrayed as being environmentally sound (they run on "clean" air) but of course, they do not run on air, they run on (in this country) coal, and they make unusually poor use of that coal, in every respect: resource depletion, pollution generated, CO2 generated. They are much less efficient than an electric car, for the simple reason that the electric car suffers far fewer losses -- there are fewer energy conversions. Both are powered by coal, and the air car uses more of that per mile.

Electric car enthusiasts bring up the fact that more electricity will be generated by renewable sources, which is true. However, the DOE projects that the percentage generated by renewables will not change through 2030. If we start to think of electric cars as pristeen, this scenario looks even bleaker. If people believe that their cars are creating no pollution, then there is no reason for them to press for wind and solar.

Just a couple years ago, nuclear energy was a scourge on the earth according to environmental hype. Now, it is a poster child for "green" thinking. It is neither. The issue of what to do with the waste still remains, as do safety issues.

We now have 20 mpg SUVs advertised as 150 MPGe. While such ads are not technically fraud, they are certainly misleading, because they suggest that the vehicle is efficient, when it is not. It is true that, given a particular driving routine a plug-in hybrid will use less gasoline, but instead it uses electricity of an energy value appropriate for a 20 mpg vehicle. By simply making a tiny change in the driving routine and no change to the vehicle this SUV could just as easily be rated 1140 MPGe. (See my feedback at the linked article for details.)

Obviously, given that I am promoting a plug-in hybrid, I think they are viable, and have the potential for being highy efficient and environmentally sound. (Just the fact that my prototytype is a series hybrid alone roughly doubles its fuel efficiency in the urban cycle, even if it is never plugged in). But I think the way to promote them is truthfully, and with eyes wide open, not by completely ignoring the environmental consequences of electricity.

Interesting that the knee-jerk "green" thought tends to be that incandescent bulbs are "bad" (because they use more electricity than CF bulbs, which therefore generates more CO2). Electric cars, however, are viewed as pristeen (despite the fact that one charge uses more electricity than a half a year's worth of lighting in an average house). I think of myself as something of a militant environmentalist, but some of the hype around this stuff is hard to swallow.

Ultimately, such hype can lead to bad policy decisions. (The CAFE law, for example, has a 6.67 multiplier for any alternative fuel, so an electric car or an ethanol car is considered to get 6.67 times better mileage - per well-to-wheels BTU consumed - than it really does. In practice, that means that manufacturers crank out flex fuel vehicles with the sole intent of avoiding CAFE fines. Is ethanol really better -- not 6.67 times better -- but even slightly better? That is a question that has been answered in different ways by different serious researchers.)