A Compressed-air engine is a pneumatic actuator that creates useful work by expanding compressed air. A compressed-air vehicle is powered by an air engine, using compressed air, which is stored in a tank. Instead of mixing fuel with air and burning it in the engine to drive pistons with hot expanding gases,compressed air vehicles (CAV) use the expansion of compressed air to drive their pistons.They have existed in many forms over the past two centuries, ranging in size from hand held turbines up to several hundred horsepower. For example, the first mechanically-powered submarine, the 1863 Plongeur, used a compressed air engine.

The laws of physics dictate that unconstrained gases will fill any given space.The easiest way to see this in action is to inflate a balloon. The elastic skin of the balloon holds the air tightly inside, but the moment you use a pin to create a hole in the balloon's surface, the air expands outward with so much energy that the balloon explodes. Compressing a gas into a small space is a way to store energy.When the gas expands again, that energy is released to do work. That's the basic principle behind what makes an air car go.
1.3 Applications:

The compressed air engine can be used in many vehicles. Some of its applications to be used as engine for vehicles are:

a) Mopeds

JemStansfield, an English inventor has been able to convert a regular scooter to a compressed air moped. This has been done by equipping the scooter with a compressed air engine and air tank.

b) Buses

MDI makes MultiCATs vehicle that can be used as buses or trucks. RATPhas also already expressed an interest in the compressed-air pollution-free bus.

c) c) Locomotives

Compressed air locomotives have been historically used as mining locomotives and in various areas.

d) Trams

Various compressed-air-powered trams were trailed, starting in 1876 andhas been successfully implemented in some cases.

e) Watercraft and aircraft

Currently, no water or air vehicles exist that make use of the air engine.Historically compressed air engines propelled certain torpedoes.

The model designed by me is a small scale working model of the compressed air engine. When scaled to higher level it can be used for driving automobiles independently or combined (hybrid) with other engines like I.C. engines.Main advantages of Compressed Air Engine (C.A.E.) are:

1. Zero emission.
2. Use of renewable fuel.
3. Zero fuel cost (the cost is involved only in the compression of air).

But the Compressed Air Engine (C.A.E.) has some disadvantages, which are:

1. Less power output
2. High pressure of compressed air may lead to bursting of storage tank.
3. Probability of air leakage.