Several years ago, Tata Motors announced an air powered car. The car was powered by a Tata compressed air engine. The concept sounds very cool...literally, The exhaust air from the engine is at a temperature of 0-15 degrees below zero! The car appears to be on the way to market.
The Tata car is fiberglass, and fully electronic. The car has not passed western safety tests however. The car claims a range of 300km/186miles per charged air tank. Top speed of the Air Tata is 105kph/65mph. And for all that, the one liter of vegetable based engine oil only needs changing once every 50,000 km or 31,000 miles.
Tata says that once developed, compressed air stations will be able to re-fill the tank in 2-3 minutes. The cars now come with their own air compressor capable of refilling the tank in 3-4 hours.
Compressed Air Reality
Compressed air has many environmental advantages to gasoline, and many uses. However, compressed air is just that, a secondary energy unit. It takes energy to compress the air. To store the equivalent of 14.3 kWh @300 bar in 300 liter (90 m3 @ 1 bar) reservoirs, you need around 93 kWh of energy just to run the air compressor.
The Nissan Leaf for instance stores 24 kWh in its Lithium Ion battery pack.
In terms of energy density, air has a lower value than other sources. Here are some comparative energy energy densities:
Gasoline - about 9.7 kWh per liter
Lithium-Ion Battery - around 0.5 kWh per liter (varies)
Compressed Air - about 0.05 kWh per liter
Hybrid Compressed Air Vehicles
So, compressed air does not contain much in the way of energy per volume or mass. However, if you are generating the compressed air as you go as a by product of a wasted energy process like vehicular braking, then the energy density is not that big of a deal.
This is the latest thinking with the air hybrid. If you have a vehicle that routinely makes a lot of starts and stops, then you can capture and re-use a good fraction of the otherwise lost energy.
In fact, Researchers at Lund University in Sweden say that they have a compressed air hybrid engine that will power buses and cut fuel use in half. That is some saving.
The trick is in the tech. The piston bus engine is designed to also act as an air compressor. Step on the brakes and fill the air tank. Next stop: fill it up!
Not to skip the air powered bike. There have been DIY air powered bikes that go about 18 mph and get around 7 miles per tank of compressed air. Now, a start up called O2 is releasing a pre-production 60 mph 125cc based prototype. Change tanks at an air-swap station!
The compressed air powered car has been around a long time, and is unlikely to overtake the electric car anytime soon. However, there appears to be a place for the air powered hybrid. The Tata Motors MiniCAT air-hybrid may be on the market in the summer of 2012. The car can be refueled with for around $2 which will take the air powered rig 180 miles.
Compressed Air In Action
Compressed air was first used to power vehicles back in the 1920s. The low energy density of compressed air has prevented them from the main stream of auto development. However, recently manufacturers have developed compressed air systems to store otherwise lost braking energy. Once
released from the tank, the expanded air provided on demand power.
Compressed air has a relatively low energy density, and is less efficient than chemical battery power. This among other things limits the use of the air powered car. Tata has developed an air-hybrid to improve performance of the compressed air only car.
The air car hybrid is a car with an internal combustion engine and air compressor, tank, and other gear needed to store braking energy in the tank. The air is compressed to high pressure: up to 30 MPa (4500 psi), or 300 bar. That is a lot of pressure! The air holding tank may be steel or more recently carbon fiber.
