COMPRESSED AIR ENGINES Vivek S Nath, Saju joseph S6 Mechanical Engineering Mohandas college of Engineering and Technology
Abstract A compressed air engine is primarily an engine that uses the energy stored compressed air to do work. Here the expansion of compressed air stored at high pressure in a storage tank occurs in the engine cylinder to move a piston doing mechanical work. The main application of this engine is in automobile industry where the potential energy of the compressed air is converted into kinetic energy of the linear motion of piston and rotary motion of the crank and the crank shaft. This motion is transferred to the wheels using usual transfer mechanisms .As the working fluid is compressed air there is no requirement of any other fuel other than some amount of electrical energy for compression of air in an electric compressor .The engine is free of emissions at the tailpipe as the only exhaust is air and is environmental friendly. Even though it is below its counterparts in power, comfort and performance, its supporters believe that altered versions of this engine are to dominate the automobile industry in future.
Introduction A Compressed-air engine is a pneumatic actuator that creates useful work by expanding compressed air and converting the potential energy into motion. A pneumatic actuator is a device that converts energy into motion The motion can be rotary or linear, depending on the type of actuator. CAE’s are fueled by compressed air, which is stored in a tank at high pressure such as 30 MPa. The difference between the compressed air engine and IC engine is that instead of mixing fuel with air and burning it to drive pistons with hot expanding gases, compressed air engine use the expansion of previously compressed air to drive their pistons. This technology has been used by many companies like MDI (Motor Development Industry) to develop cars and other vehicles running on compressed air engine.
Parts A basic compressed air engine primarily consists of a source of air under high pressure, means for supplying air from source to engine cylinder, a cylinder system, and an exhaust system. There are also auxiliary parts like the heater which improves the power output and efficiency of the engine.The source is a storage tank where compressed air of pressures as high as 30Mpa is stored. The storage tank is likely to be made of carbon-fiber in order to reduce its weight while
achieving the necessary strength. There is a cylinder having a reciprocating piston .There is also means to supply air from tank to cylinder to drive piston. A crank shaft is coupled to piston and is driven responsive to the reciprocating motion of piston, suitable mechanical arrangement coupled to crank shaft supply power to compressor, and also an independent means to supply power to compressor. Means for supplying air to cylinder comprises: a cylinder head, an auxiliary chamber in cylinder head, conduit means for connecting tank to auxiliary chamber, and input valve operative to periodically admit air from auxiliary chamber into the chamber formed by cylinder head and the top of piston, the periodicity of admission of air being synchronized with the rotation of crank shaft. There is also an inlet valve to allow the entry of air from the surroundings .There are also carbon filters to eliminate dirt, dust, humidity, and other urban air impurities that could hamper the engine’s performance. There is an exhaust valve that lets the expanded air out. A lubricant compartment is provided below the engine cylinder that provides suitable lubrication for the engine. The basic parts of a compressed air engine are illustrated in the figure given below
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That is up to 4 times less than the average vehicle and more than two times less than the cleanest vehicle available today.
Working The compressed air from the storage tank is supplied to the cylinder system by means of supply system. In the cylinder system the air first enters an auxiliary chamber from where it is periodically admitted to the main cylinder. The auxiliary chamber produces some power other than improving the overall efficiency of the engine. The compressed air which expands in the cylinder moves the piston down. When the piston moves up the exhaust valve opens and the expanded air is pushed out. In more evolved systems, the top portion of the main cylinder doubles up as the compressor. the linear up and down motion of the piston is converted to the rotary motion of the crank and crank shaft .This is transferred to the wheels by transfer mechanisms. Parked: It automatically shuts down the engine when the car is stationary. At Lower Speeds: Since the Compressed Air Vehicle is running exclusively on compressed air, it emits only air. The air expelled from the tail pipe is actually cleaner than the air used to fill the tank. This is because before compression, the air is run through carbon filters to eliminate dirt, dust, humidity, and other urban air impurities that could hamper the engine’s performance. At Higher Speeds: At speeds over 35mph the Compressed Air Vehicle uses small amounts of fuel–either gasoline, propane, ethanol or bio fuels–to heat air inside a heating chamber as it enters the engine ( again, to expand volume before entering engine). This process produces emissions of only 0.141lbs of CO2 per mile.
The compressed air engine works in four different modes according to requirement Mode A: Operating with compressed air from Air Tank only in town less than 30 kph. In this mode, high pressure air from storage tank expands in the cylinder and moves the piston. The linear motion of piston is converted unto rotary motion of crank shaft. Mode B: Operating with compressed air from Air Tank only which is then heated by the heater to expand volume before entering engine. this increases the power output. Mode C: Operating with air from the Intake which is being heated to expand volume before entering engine. This is used on highway over 35 mph. Mode D: Operating as in Mode C but also refilling air Tank while running.
Advantages The principal advantages of an air powered vehicle are: 1) Refueling can be done at home using an air compressor or at service stations. 2) Reduced vehicle weight is the principal efficiency factor of compressed-air cars. Furthermore, they are mechanically more rudimentary than traditional vehicles as many conventional parts of the engine may be omitted. Some plans include motors built into the hubs of each wheel, thereby removing the necessity of a transmission, drive axles and differentials. A four passenger vehicle weighing less than 800 pounds (360 kg) is a reasonable design goal. 3) One manufacturer promises a range of 200 kilometers by the end of the year at a cost of € 1.50 per fill-up. 4) Compressed air engines reduce the initial cost of vehicle production by about 20%, because there is no need to build a cooling system, spark plugs, starter motor, or mufflers. 5) Expansion of the compressed air lowers in temperature; this may be exploited for use as air conditioning. 6) Compressed-air vehicles emit no pollutants. 7) The technology is simple to achieve with low tech materials. This would mean that developing countries,
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and rapidly growing countries like China and India, could easily implement the technology.
Uses
8) The price of fueling air powered vehicles may be significantly cheaper than current fuels. Some estimates project $3.00 for the cost of electricity for filling a tank.
Tools Impact wrenches, drills, die grinders, dental drills and other pneumatic tools use a variety of air engines or motors. These include vane type pumps, turbines and pistons.
9) Reduction or elimination of hazardous chemicals such as gasoline or battery acids/metals
Disadvantages
Torpedoes Most successful early forms of self propelled torpedoes used high pressure compressed air, although this was superseded by internal or external combustion engines, steam engines, or electric motors.
1) The principal disadvantage is the indirect use of energy. Energy is used to compress air, which in turn provides the energy to run the motor. Any conversion of energy between forms results in loss. For compressed air cars, energy is lost when electrical energy is converted to compressed air.
Railways Compressed air engines were used in trams and shunters, and eventually found a successful niche in mining locomotives, although eventually they were replaced by electric trains, underground. Over the years designs increased in complexity, resulting in a triple expansion engine with air to air re-heaters between each stage.
2) When air expands in the engine, it cools significantly and must be heated to desired temperature using a heat exchanger. The cooling is necessary in order to obtain maximum efficiency. The heat exchanger, While it. heats the stored air, the device gets very cold and may ice up in colder climates.
Aircraft Transport category airplanes, such as commercial airliners, use compressed air starters to start the main engines. The air is supplied by the load compressor of the aircraft's auxiliary power unit, or by ground equipment.
3) Refueling the storage tank of compressed air engine using a home or low-end conventional air compressor may take as long as 4 hours though the specialized equipment at service stations may fill the tanks in only 3 minutes. Early tests have demonstrated the limited storage capacity of the tanks; the only published test of a vehicle running on compressed air alone was limited to a range of 7.22 km.
Automotive Main article: Compressed air vehicle There is currently some interest in developing air cars. Several engines have been proposed for these, although none have demonstrated the performance and long life needed for personal transport.
Conclusion With gas prices soaring, as they have over the past two years, it might not be long before many motorists turn to vehicles powered by alternative fuels. Although airpowered vehicles are still behind their gasoline counterparts when it comes to power and performance, they cost less to operate and are arguably more environmentally friendly, which makes them attractive as the future of highway transportation.
References Automobile technology by John Hawkins. Advanced air engine technology by Guy Negre. Air Engines by Franklin Newett New Age Technologies by Wivian Hurly
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