If my math is right, that only provides a theoretical maximum of about 55 MJ of energy, which is about the same as about 1.5 liters of gasoline.
A combustion engine runs at about 20% efficiency and, if memory serves, compressed air engines at around 30%. But let's give MDI the benefit of the doubt and assume their engines are 50% efficient.
That means this car claims to be able to go 200-300KM (125-185 miles) on the equivalent of about 4 Liters (or 1 gallon) of gasoline.
Now, getting 150 miles/gallon isn't physically impossible, but it seems pretty difficuly and does raise some red flags to me. And that's assuming they have a 50% efficient engine, which is extraordinarily unlikely. Assuming a 30% efficient engine, this car is doing the equivalent of 250 miles/gallon.
Maybe part of the explanation is that it's very light-weight - the car in the photo looks tiny, it says it's made of fiberglass, and my guess is that the engine must also weigh much less than a comparable internal combustion engine (you don't need as much metal to contain the combustion...).
Of course, that probably comes with a heavy price to passenger safety. Fiberglass won't give you any protection in a crush. That may be the reason why Tata is involved and not some European manufacturer. The Euro market has some serious safety regulations.
Fiberglass won't give you any protection in a crush
Damn, just put the tank of air in front. Any wreck will cause a mighty explosion balancing out the various vectors involved. I haven't done (and couldn't do, really) the math, but this seems perfectly reasonable.
But great power density and awful energy density sounds like something an engineer should work with a little before declaring viability: A hybrid design with some other power source having high energy density, even if it can't deliver it very quickly.
I suspect you're underestimating efficiency of both types of engines. Diesels, for instance, are always above 30% and air-compressed should be a lot higher: they don't have an enormous energy loss in a form of heat compared to diesels.
Like your analogy! :) The bigger picture is that we're approaching an energy crisis. That's why gas is getting more expensive, and that is why there is even an interest in these cars to begin with: the message isn't to "separate front end and back end" but to appeal to people who want to save money on energy costs, i.e. gasoline. But it won't work, because gasoline is just the medium, the real beef here is energy, and it's not going to get cheaper because we need more and more of it. It's not only gasoline that's getting expensive, my gas and electric bills are ahead of inflation as well.
"Solutions" covered by the press aren't really solving the problem, they're just shifting it around. "Wow! We have an air-powered car!" - without an explanation where the energy to compress that air is going to come from...
If you assume the problem is "Where will we find energy to sustain our current consumption rate?", then you're right: air-cars aren't a solution. Such a solution might not even exist.
But if you assume the problem is "How can we relieve market pressure on our most constrained resource?", air and electric cars are a solution. The hope is that by diversifying the sources of energy used for transportation, we can continue to use oil for other purposes that we haven't found cost-effective replacements for yet -- like plastics and jet fuel.
So while the cars won't solve the energy crisis by themselves, maybe they'll buy us enough time to reach the "then a miracle occurs" discovery that does solve it. Or, failing that, maybe they'll help civilization make a slow transition to reduced consumption, rather than a sharp crash.
Most likely we wouldn't be burning gasoline to compress air for these cars: gas is expensive. Instead, we'd probably use electricity derived from coal, natural gas, or nuclear, with the possibility of better sources in the future.
"Zero-emission" is a reasonable description, as the car itself doesn't give off emissions while running. If you're going to include the emissions given off in the process of producing the fuel, a gasoline-based solution is suddenly much worse, as you need to include the emission costs of finding, extracting, refining, and transporting the gas.
Looking at this http://www.popularmechanics.com/automotive/new_cars/4217016.... popular mechanics article, this engine uses 340 Liters of air (under STP) compressed at 4350 PSI.
If my math is right, that only provides a theoretical maximum of about 55 MJ of energy, which is about the same as about 1.5 liters of gasoline.
A combustion engine runs at about 20% efficiency and, if memory serves, compressed air engines at around 30%. But let's give MDI the benefit of the doubt and assume their engines are 50% efficient.
That means this car claims to be able to go 200-300KM (125-185 miles) on the equivalent of about 4 Liters (or 1 gallon) of gasoline.
Now, getting 150 miles/gallon isn't physically impossible, but it seems pretty difficuly and does raise some red flags to me. And that's assuming they have a 50% efficient engine, which is extraordinarily unlikely. Assuming a 30% efficient engine, this car is doing the equivalent of 250 miles/gallon.