COMPRESSED AIR ENGINES
Vivek S Nath, Saju joseph
S6 Mechanical Engineering
Mohandas college of Engineering and Technology
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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.
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.
That is up to 4 times less than the average vehicle and
more than two times less than the cleanest vehicle
available today.
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,
and rapidly growing countries like China and
India, could easily implement the technology.
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.
9) Reduction or elimination of hazardous
chemicals such as gasoline or battery
acids/metal
Disadvantages
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.
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.
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.
Uses
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.
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.
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.
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.
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 air-
powered 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
