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Is a engine running on compressed air more efficient than a internal combustible engine?
The Tata Air Car vehicle runs on compressed air stored in a carbon fiber tank. The air can be compressed from home in a four hour recharge cycle or on the road using a gas powered compressor. 6000 ZEVs are scheduled to hit the streets of India in 2008.
What is the problem with compressed air technology?
What is the problem with compressed air technology?
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The issue with compressed air technology for powering vehicles (not really a "problem") is the failure to recognize the total cost to compress the air. It takes energy to compress the air to be placed in the car's tank. That means a compressor that is directly driven by a fuel burning engine, or an electric motor that consumes electricity from a fuel burning power plant. There are still green house gas emissions, energy losses (from transmission), infrastructure capital costs, infrastructure operating and maintenance costs, and fuel costs.
This technology is novel in that it can reduce emissions at the location of the car itself, which makes it ideal for crowded urban environments (where there are air quality issues). However, it does not solve the large-scale concerns; it is no silver bullet.
This technology is novel in that it can reduce emissions at the location of the car itself, which makes it ideal for crowded urban environments (where there are air quality issues). However, it does not solve the large-scale concerns; it is no silver bullet.
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The answer to this question is going to be a qualified "it depends".
An internal (gasoline) combustion engine, for the most part, also runs on air, with the addition of around 1 part of gasoline to every 14 parts of air. But this air is freely sucked in from the atmosphere, rather than carrying it around pre-compressed in a tank, and compressed by the inertial spinning of the piston and crank assembly in the engine (Started up by an electric starter, of course), and kept spinning by the continual fuel-air-spark explosions in each cylinder.
If you just measure efficiency by how much fossil fuel you need to carry around and consume while driving either vehicle, the compressed air engine uses no fuel, the internal combustion engine uses gasoline, and thus the compressed air engine is more efficient. But as yagelski points out, it takes energy to compress air into a tank. That energy could be an electric motor powered off the grid, or it could be a fossil-fuel-powered engine. There are various ways to produce electricity, including fuel-powered generators, but also zero-emission solar and wind generators.
The thing about the internal combustion is that it is *really* hard to beat the energy density found in a gallon of gasoline or diesel fuel, in a form that can be easily transported about, which generally means in an unpressurized liquid form at typical Earth-surface temperatures. See the table "Energy densities excluding oxidisers" in the article: http://en.wikipedia.org/wiki/Energy_density
So technically, if you measured efficiency by how far you could go on the same mass of stored energy carried with the vehicle (compressed air -vs- gasoline), you're going to be far more efficient in the gasoline engine, especially when you add in the energy that went into compressing the air. But if you do that, you also need to recognize that it also takes a given amount of energy to extract oil from underground and refine it into gasoline and transport it to a gas station, where compressing air doesn't need to have any of the extraction, purification, and transportation costs (although it still might have some anyway).
You might also consider that there is already a large infrastructure for extracting, refining, and transporting petroleum fuels in huge quantities, where compressed air has far fewer infrastructure resources, and getting a large tank of compressed air delivered to where YOU are might be considerably more expensive for you than stopping at the corner gas station.
So to reiterate: "It depends." -primarily on how you measure efficiency, and what additional energy considerations you wish to include or exclude.
An internal (gasoline) combustion engine, for the most part, also runs on air, with the addition of around 1 part of gasoline to every 14 parts of air. But this air is freely sucked in from the atmosphere, rather than carrying it around pre-compressed in a tank, and compressed by the inertial spinning of the piston and crank assembly in the engine (Started up by an electric starter, of course), and kept spinning by the continual fuel-air-spark explosions in each cylinder.
If you just measure efficiency by how much fossil fuel you need to carry around and consume while driving either vehicle, the compressed air engine uses no fuel, the internal combustion engine uses gasoline, and thus the compressed air engine is more efficient. But as yagelski points out, it takes energy to compress air into a tank. That energy could be an electric motor powered off the grid, or it could be a fossil-fuel-powered engine. There are various ways to produce electricity, including fuel-powered generators, but also zero-emission solar and wind generators.
The thing about the internal combustion is that it is *really* hard to beat the energy density found in a gallon of gasoline or diesel fuel, in a form that can be easily transported about, which generally means in an unpressurized liquid form at typical Earth-surface temperatures. See the table "Energy densities excluding oxidisers" in the article: http://en.wikipedia.org/wiki/Energy_density
So technically, if you measured efficiency by how far you could go on the same mass of stored energy carried with the vehicle (compressed air -vs- gasoline), you're going to be far more efficient in the gasoline engine, especially when you add in the energy that went into compressing the air. But if you do that, you also need to recognize that it also takes a given amount of energy to extract oil from underground and refine it into gasoline and transport it to a gas station, where compressing air doesn't need to have any of the extraction, purification, and transportation costs (although it still might have some anyway).
You might also consider that there is already a large infrastructure for extracting, refining, and transporting petroleum fuels in huge quantities, where compressed air has far fewer infrastructure resources, and getting a large tank of compressed air delivered to where YOU are might be considerably more expensive for you than stopping at the corner gas station.
So to reiterate: "It depends." -primarily on how you measure efficiency, and what additional energy considerations you wish to include or exclude.
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The answer to this is both simple, and difficult. The compressed air is an intermediate fuel, just like the electricity stored in the batteries of an electric car.
Some kind of energy has to be expended to compress the air, and it all depends on where that air comes from (the same as electricity stored in the batteries in electric cars).
If, say, electricity from a nuclear power plant was used to compress the air then it would probably be more efficient than an internal combustion engine. If you used a gas powered generator to compress the air, then it would be less efficient.
Some kind of energy has to be expended to compress the air, and it all depends on where that air comes from (the same as electricity stored in the batteries in electric cars).
If, say, electricity from a nuclear power plant was used to compress the air then it would probably be more efficient than an internal combustion engine. If you used a gas powered generator to compress the air, then it would be less efficient.
As a consumer, your only concerned about your direct cost. The cost of the electricity, the cost of the fuel, and the cost of the vehicle. Your not concerned about the cost of government regulation, infrastructure investment, or innovation risk. The innovators and business carry the burden of the bring the product to market. The consumer carries the risk of buy a technology not widely used.
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Are you an proponent of Fuel Cells propulsion?
Systems like compressed air and electric vehicles look good on an experimental or small-scale basis, but when you consider what it takes to scale this technology up for say, millions of cars, the overall fiscal and environmental costs to our society become staggering.
For compressed air as an example, start at the electric generation facility; a 50% heat rate is considered good. Now transmit that electricity to the market area; lose another third to one half. Now convert that electric power to mechanical motion to compress air; lose another third to one half. Now place that compresses air in a tank for use in the Tata; its own conversion of compressed air to mechanical motion will lose another third to one half. If you can efficiently consume the fuel directly in the ultra-low emission or hybrid vehicle, you prevent a whole lot of losses.
The same goes for fuel cells. I get really excited about the efficiency and performance of hydrogen fuel cells. However, the fiscal and environment costs to create sufficient hydrogen to meet the demand of the transportation market are huge. Current technology requires more energy to create a unit of hydrogen than what can be derived from that same unit of hydrogen. That is not a good thing.
Both compressed air and hydrogen (or any other fuel cell catalyst) will require the development of an entirely new retail distribution network. There are a number of hurdles associated with that as well.
I'm all for alternate fuels and supporting the environmental concerns, but I am also realistic about what is reasonably possible. I am a much bigger proponent of conservation efforts (i.e. mass transit) which provide a much bigger bang for the buck.
For anyone else reading this, thanks for your indulgence. I will cease my commentary with this comment.
Peace.